When small things matter. Small Field Dosimetry Application Guide

Transcription

1 R A D I AT I O N T H E R A P Y When small things matter. Small Field Dosimetry Application Guide

2 Contents 1 Introduction 1 Introduction 2 2 The Physics of Small Fields 3 3 Detector Types 10 4 Detector Selection Guide 11 Overview: Field Size Range 18 Overview: Additional Selection Criteria 19 5 Absolute Dose Measurements with PTW Small Field Detectors 21 6 Frequently Asked Questions 22 7 Detector Overview 25 Dose determination in small photon fields is an important and challenging task. Small photon fields are used in stereotactic radiosurgery as well as in IMRT and IMAT, where mini or micro MLCs create fields of 1 cm x 1 cm or smaller. Current dosimetry protocols such as [IAEA 398, AAPM TG51, DIN ] describe procedures for absolute dose measurements based on ionization chambers at field sizes of typically 10 cm x 10 cm. No advice is given as to appropriate procedures and detectors for field sizes of 1 cm x 1 cm. Presently, national and international committees are working on dedicated dosimetry protocols for small field dosimetry, see e.g. [Alfonso2008] or [DIN6809-8_draft]. 8 References and Further Reading 34 2

3 2 The Physics of Small Fields 2.1 Under which conditions can you consider a field as small? } If the field is smaller than approximately 4 cm x 4 cm. } If the focus is partially hidden by the collimators. } If lateral electron equilibrium is not given in the center of the field. 2.2 The dose volume effect When the dose changes noticeably across the detector, the signal is subject to the volume effect. As a consequence of the volume effect, the dose in the field is underestimated and the width of the penumbra is overestimated. In Figure 1 you can see a size comparison of some small field detectors against a Gaussian shaped field of FWHM cm x 1.4 cm. From the figure it is apparent that a diode is probably small enough to characterize such a field but a Semiflex cm³ chamber is not. In Figure 2 the effect of a too large detector is described in more detail, experimental results are shown in Figure 3. 1 Full width at half maximum, this is the same as the width of the 50 % isodose Dose [%] Figure 1 Size comparison of a 1.4 cm x 1.4 cm FWHM Gaussian shaped field with some small field detectors. 3

4 a b 4

5 c Figure 2 Viewgraph showing the origin of the volume effect. In part a) you can see the size of a Semiflex cm³ chamber against a FWHM 1.4 cm x 1.4 cm Gaussian field. Clearly, the chamber seems to be too big to characterize that field. In part b) you can see what that chamber will actually do: it will average the dose across its sensitive volume, depicted as a blue box. When you move the chamber through the field, it will always average across its volume at every measurement position. The result is shown in part c). The blue curve shows the signal after averaging. The CAX ² value of the dose is underestimated, and the penumbra is broadened. 2 CAX stands for central axis. 5

6 a b Dose normalized to Diamond CAX [%] Output factor 6

7 c 100 Signal [mm] Semiflex T31010 T Position [mm] Figure 3 Experimental verification of the volume effect. In part a) the output factors 3 for small square fields are shown. For the 1 cm x 1 cm field, the reduction of the measured dose for the Semiflex and the PinPoint chamber is clearly visible. Part b) shows a profile measurement in an 1 cm x 1 cm field. Again, the dose reduction of the Semiflex chamber in the field center is apparent. In part c) the penumbra broadening of the Semiflex chamber in a 10 cm x 10 cm field can be seen. Note that the field width (50 % isodose) is measured correctly. This is always the case when there is no volume effect in the field center. 3 Synonyms for output factor: relative dose factor and total scatter factor. 7

8 Additional effects due to CAX normalization Usually, profiles are evaluated after performing a CAX 4 normalization, i.e. all profiles are normalized such that their CAX-value corresponds to 100 %. For the example in Figure 3 b, this corresponds to multiplying the entire blue curve by This includes the penumbra of the measurement and the out-of-field part. Hence if you combine the volume effect with a CAX normalization, the out-of-field dose and the penumbra dose will be slightly overestimated. This can be seen in Figure 4 where the data is taken from Figure 3 b) and normalized to the respective CAX values of the curves. The increase of the penumbra data leads to an increase of the apparent field width (i.e. the FWHM is broadened). A similar effect can happen with percentage depth dose curves (PDDs) if there is a strong volume effect present. As the volume effect depends on field size and the field size depends on depth, the volume effect at the normalization point (at maximum dose) is different compared to positions deep in the water. A PDD subject to this effect will overestimate the dose deep in the water. Figure 4 Profiles of a 6 MV 1 cm x 1 cm field measured with a Diode E (similar to Diode SRS), a TM60003 Diamond and a Semiflex chamber after CAX normalization. The data is the same as in Figure 3 b). In addition to penumbra broadening two more effects are visible, indicated by arrows. (i) The FWHM of the Semiflex measurement seems larger than that of the other detectors. This is in contrast to the original measurement without CAX normalization shown in Figure 3 b). (ii) The dose in the out-of-field region is overestimated. 4 CAX stands for central axis. 8

9 2.3 Low energy response Low energy scattered radiation hardly plays any role in small fields. In large fields (roughly above 10 cm x 10 cm) there is a large dose contribution due to lowenergy scattered radiation. In small fields, the dose contribution by this radiation is comparatively small. Consequently, the low-energy response (response to photons in the kev range) does not play a large role in small fields. What about the out-of-field region? In the out-of-field region, the radiation consists only of scattered photons. For small fields this radiation contains a low-energy part but it is less important than for large fields. Hence, for small fields: } Silicon diode detectors can be used. } Shielding of the silicon diode is not necessary. } In very small fields, shielding will lead to an overestimated dose due to the density perturbation effect. 2.4 Other effects in small fields } The alignment of beam and detector is much more important compared to large field sizes. } Often, an irradiation is composed of many small fields. To correctly add these up, the penumbras of the fields must be determined very accurately. } For small fields the field size must not equal the set collimator value due to partial occlusion of the focus by the collimators and penumbra overlap. } In field sizes below roughly 2 cm x 2 cm, lack of lateral electron equilibrium leads to the density perturbation effect, see e.g. [Fenwick2013]. We recommend to thoroughly study small field literature before working in such small fields. } Some small field systems are flattening filter free linacs. Summary: } If your detector is larger than roughly 1/4th of the lateral field dimension, you should watch out for a volume effect of several percent. } kev scattered photon radiation is less important in small fields. Unshielded silicon diodes can be used. } If the volume effect is present, The dose in the field center will be underestimated; The penumbra appears wider than it is. } If in addition to the volume effect you perform a CAX normalization in a small field, The field (50 % isodose) will appear wider than it is; The dose in the out-of-field region will be overestimated; The dose of PDDs at large depths can be overestimated. } [IPEM 103] recommends to use more than one detector to perform a high quality characterization. } For a thorough introduction see, e.g., [Wuerfel2013] 9

10 3 Detector Types The following section presents a quick introduction into the various types of single detectors used for dose measurements in a water phantom. 3.1 Medium-size vented ionization chambers Gold standard for dose measurements are vented ionization chambers as specified in IEC The sensitive volume of such chambers is usually between 0.1 cm³ and 1.0 cm³. Their only disadvantage is the relatively large size. When used in small fields, large detectors can be subject to the dose volume effect, see chapter Small-size vented ionization chambers Small-size vented ion chambers (PinPoint chambers) have a sensitive volume in the order of 0.01 cm³. They can typically be used for dose measurements in fields down to 2 cm x 2 cm. Care must be taken if PinPoint chambers are used in very large fields when stem and cable effects become important. Make sure that the chamber you use does not have a steel electrode. 3.3 Diamond detectors Diamond detectors are solid state detectors combining small size and high response. In addition, their response is almost independent upon energy, i.e. they are very much water equivalent. They also feature a very good directional response. Diamond detectors can be constructed as solid state ionization chambers (TM60003 diamond) or as diodes (T60019 ). 3.4 Silicon diodes Silicon diode detectors feature the highest response per volume of all common detector types. Hence their sensitive volume is usually small enough to avoid dose volume effects down to very small fields. However, the density perturbation effect is still present. The directional response of sillicon diodes is not ideal, as well as the response to low-energy scattered photons. To reduce the latter effect, diodes exist in a shielded design where the shield reduces the signal from these photons. In small fields the low-energy scatter contribution is low, hence diode shielding is not needed and unshielded diodes are recommended for small fields [IPEM 103]. 10

11 4 Detector Selection Guide Which detector is best suited for my application? 11

12 Detector Selection Tree Minimum field size required 1 cm x 1 cm MAXIMUM field size (cm) required: 10 x x 20 Type of measurement: Absolute dose 1 & output factors Profiles & PDDs Absolute dose & output factors Profiles & PDDs Suitable detectors: Diode E Diode SRS Diode E Diode SRS Recommended detectors: Diode E or SRS Diode E or SRS Remarks 1 In small fields absolute dose measurement often requires cross calibration, see chapter 5. 12

13 30 x x 40 Absolute dose & output factors Profiles & PDDs Absolute dose & output factors Profiles & PDDs for fields smaller than 20 cm x 20 cm for fields smaller than 20 cm x 20 cm for fields smaller than 20 cm x 20 cm for fields smaller than 20 cm x 20 cm Semiflex 3D for larger fields Semiflex 3D for larger fields Semiflex 3D for larger fields Semiflex 3D for larger fields Both and are well suited for the entire field size range from 1 cm x 1 cm up to 40 cm x 40 cm. But if you are aiming for utmost accuracy in large fields, a medium sized air-filled ionization chamber will be better than any solid state detector. If you can choose between and, take the. 13

14 Detector Selection Tree Minimum field size required 2 cm x 2 cm MAXIMUM field size (cm) required: 10 x x 20 Type of measurement: Absolute dose 1 & output factors Profiles & PDDs Absolute dose & output factors Profiles & PDDs Suitable detectors: Diode E Diode SRS PinPoint PinPoint 0.03 PinPoint 3D Diode E Diode SRS PinPoint PinPoint 0.03 PinPoint PinPoint 0.03 PinPoint 3D PinPoint PinPoint 0.03 PinPoint 3D Recommended detectors: PinPoint 3D (new) PinPoint 3D (new) Remarks 1 In small fields absolute dose measurement often requires cross calibration, see chapter 5. The detector is suitable for absolute dose and output factor measurements. However, whereas the must be cross-calibrated, the PinPoint 3D (new, T31022) chamber can be directly applied according to IAEA 398 and DIN

15 30 x x 40 Absolute dose & output factors Profiles & PDDs Absolute dose & output factors Profiles & PDDs PinPoint PinPoint 0.03 PinPoint 3D PinPoint PinPoint 0.03 PinPoint 3D for fields smaller than 20 cm x 20 cm for fields smaller than 20 cm x 20 cm for fields smaller than 20 cm x 20 cm for fields smaller than 20 cm x 20 cm Semiflex 3D for larger fields Semiflex 3D for larger fields Semiflex 3D for larger fields Semiflex 3D for larger fields Both and are well suited for the entire field size range from 1 cm x 1 cm up to 40 cm x 40 cm. But if you are aiming for utmost accuracy in large fields, a medium sized air-filled ionization chamber will be better than any solid state detector. If you can choose between and, take the. 15

16 Detector Selection Tree Minimum field size required 3 cm x 3 cm MAXIMUM field size (cm) required: 10 x x 20 Type of measurement: Absolute dose 1 & output factors Profiles & PDDs Absolute dose & output factors Profiles & PDDs Suitable detectors: Diode E Diode SRS PinPoint PinPoint 0.03 PinPoint 3D Semiflex 3D Semiflex Diode E Diode SRS PinPoint PinPoint 0.03 PinPoint 3D PinPoint PinPoint 0.03 PinPoint 3D Semiflex 3D Semiflex PinPoint PinPoint 0.03 PinPoint 3D Recommended detectors: Semiflex 3D Semiflex 3D Remarks The Semiflex 3D is best suited for absolute dose measurements as it does not need to be cross-calibrated. 1 In small fields absolute dose measurement often requires cross calibration, see chapter 5. The detector is suitable for absolute dose and output factor measurements. However, whereas the must be cross-calibrated, the Semiflex 3D chamber can be directly applied according to IAEA 398 and DIN

17 30 x x 40 Absolute dose & output factors Profiles & PDDs Absolute dose & output factors Profiles & PDDs PinPoint PinPoint 0.03 PinPoint 3D Semiflex 3D Semiflex PinPoint PinPoint 0.03 PinPoint 3D Semiflex 3D Semiflex Semiflex 3D for fields smaller than 20 cm x 20 cm Semiflex 3D for fields smaller than 20 cm x 20 cm Semiflex 3D for fields larger than 20 cm x 20 cm Semiflex 3D for fields larger than 20 cm x 20 cm Though the PinPoint chambers, the and the are well suited for measurements over the entire range from 3 cm x 3 cm to 30 cm x 30 cm, we recommend using a combination of two detectors for the most accurate profile and PDD measurements. Both and are well suited for the entire field size range from 1 cm x 1 cm up to 40 cm x 40 cm. But if you are aiming for utmost accuracy in large fields, a medium sized air-filled ionization chamber will be better than any solid state detector. If you can choose between and, take the. For accurate penumbra measurements in fields smaller or equal than 20 cm x 20 cm, a detector smaller than the Semiflex should be used. 17

18 Overview: Field Size Range Field size range of PTW small field detectors. Data is taken from [DETECTORS] and valid for output factor measurements. 18

19 Overview: Additional Selection Criteria Detectors Additional Selection Criteria Penumbra Accuracy Out-Of-Field Dose Accuracy Dose Stability Dose Rate Independence Energy Response (MeV) Energy Response (kev) Fast Measurement 1 Diode E, unshielded Diode SRS, unshielded , shielded Detector PinPoint Chamber, cm³, axial orientation PinPoint Chamber, cm³, radial orientation PinPoint Chamber, 0.03 cm³, radial orientation PinPoint Chamber 3D, cm³, radial orientation Semiflex 3D Chamber, 0.07 cm³, radial or axial orientation Semiflex Chamber, cm³, radial orientation excellent +++ very good ++ good + OK 1 see Fast measurement on next page 2 only <_ 6MV 3 can be corrected, see e.g. [DIN6800-2] 4 can be corrected, k Q available in [DIN ] and [IAEA 398] 5 can be corrected, k Q available from PTW technical support 6 can be corrected, k Q available in [DETECTORS] 19

20 Why is it relevant? Penumbra accuracy In IMRT and IMAT treatments, many small fields are superimposed to get the full dose. To make this work, the penumbra should be known to a high accuracy. Dose stability When the dose stability is good, you seldom have to recalibrate your detector. A bad dose stability requires more frequent recalibrations. Dose rate independence A possible dose rate dependence of the detector will be part of the measurement uncertainty. The better the dose rate independence, the higher the accuracy of the measurement. Energy response (kev) The kev energy response is important when the beam contains a lot of scattered radiation. This is the case for large fields (more than 10 cm x 10 cm), especially in the out-of-field region. In small fields (below 5 cm x 5 cm), the effect is not important within the field and of medium importance outside of the field. Energy response (MeV) A good MeV energy response corresponds to a quality correction factor k Q close to 1 for all energies above 60 Co. For air-filled ionization chambers, k Q is known, for other detectors this is not the case. Hence, the better the energy response, the smaller is the induced uncertainty. Note, the mean energy of a beam can slightly change over a beam cross section or with depth in the water. Out-of-field dose accuracy In IMRT and IMAT treatments, many small fields are superimposed to get the full dose. The out-of-field dose can be several percent of the central dose and will add up to a background dose. In addition, it is a main contribution to the dose in the surrounding healthy tissue. Fast measurement A good signal to noise ratio (SNR) is preferable for profile and PDD measurements. The better the SNR, the faster the measurement can be performed. Every detector is subject to quantum noise of the radiation. The following quantities influence quantum noise: (i) the number of quanta of the primary radiation, (ii) the material of the detector (i.e. air, silicon, diamond, ) and (iii) the size of the detector (large detector: better SNR). Hence, depending on your detector, the signal to noise ratio will be different. We have classified detectors with a high SNR as fast detectors. Note, the SNR is mainly a material property of the detector, it is not a function of the response of the detector. Rule of thumb: if you use a high quality electrometer, the smallest air-filled ionization chamber (PinPoint cm³) will have a better SNR than any diode, even though the response is a lot lower. 20

21 5 Absolute Dose Measurements with PTW Small Field Detectors } Fields < 2 cm 2 cm Cross-calibrate your small field detector for each radiation quality in a 4 cm x 4 cm or 5 cm x 5 cm field against a Semiflex 3D or Semiflex cm³ ionization chamber. } Fields 2 cm 2 cm 4 cm x 4 cm Use a PinPoint ionization chamber directly or cross-calibrate your small field detector for each radiation quality in a 4 cm x 4 cm or 5 cm x 5 cm field against a Semiflex 3D or Semiflex cm³ ionization chamber. } Fields >_ 4 cm x 4 cm Use a Semiflex 3D or Semiflex cm³ ionization chamber. } Detector orientation Perform a new cross-calibration if you change the detector orientation. Note: when you use an ionization chamber directly, follow one of the international or national dosimetry protocols, e.g. [IAEA 398, AAPM TG51, DIN ]. Additional k Q correction factors for the PinPoint chambers are given in [Muir2011], [DETECTORS], or can be obtained from PTW technical support. 5.1 How to perform the crosscalibration For absolute dose measurements, all small field detectors except air-filled ionization chambers must be cross-calibrated against a mediumsize ionization chamber such as a Semiflex 3D or Semiflex The cross-calibration is done in a phantom for each radiation quality. It should be performed in two steps in a field of 4 cm x 4 cm or 5 cm x 5 cm: 1. Use a medium-size vented ionization chamber, e.g. a Semiflex 3D or a Semiflex chamber, to determine the dose D ref for the radiation quality and depth of interest. Use one of the international or national dosimetry protocols, e.g. [IAEA 398, AAPM TG51, DIN ]. 2. Replace the medium-size ionization chamber by the small-size detector to be cross-calibrated. Make sure the effective points of measurement are located at the same depth. Apply the same number of monitor units as before and determine the reading M small of the small-size detector. The cross-calibration factor for the small-size detector is the ratio D ref /M small. After cross-calibration, the small-size detector can be used in fields smaller than the cross-calibration field and at different depths, but always at the same radiation quality and detector orientation. In literature, this approach is sometimes addressed as daisy chaining. 21

22 6 Frequently Asked Questions How can I tell whether my detector is too big for my field size? As a crude rule of thumb: if the dimension of your detector is more than 25 % of the field width, you might observe a volume effect of several %. To make sure, cross-calibrate a smaller detector against yours in a 4 cm x 4 cm or 5 cm x 5 cm field and compare their respective signals in the targeted small field. If the measured doses clearly deviate, you are probably experiencing a volume effect. Do I need special detectors to perform dosimetry in small fields? Yes. There exists no detector that is suitable to perform high accuracy measurements in very small as well as very large fields. In larger fields highest accuracy is reached with ionization chambers, especially the semiflex chambers. In small fields, small field detectors should be used. Is film dosimetry the best solution for small fields? No. The main advantage of film dosimetry is the very good spatial resolution. Unfortunately this is the only advantage. Silver films exhibit a very bad energy response in the kev-energy range and their quality depends a lot on the development process. [IPEM 103] recommends not to use that type of film. Radiochromic films have a better energy dependence, but require a high dose for development, their result depends on handling, i.e. on staff, they darken by a few percent after exposure, their response can vary by several percent over the area of the film, and there are batch-to-batch variations [IPEM 103]. Is a scintillation detector the best solution for small fields? Theoretically, a scintillator has a good waterequivalence because it can be built from plastic. In practice a dosemeter also needs good dosimetric properties. Scintillators can be subject to LET-, dose rate- and temperature dependence. Because of the low optical signal output, which even reduces with accumulated dose, scintillation detectors cannot be built as small as solid state detectors and they feature a fairly high quantum noise. The optical signal transfer (if performed in a PMMA light guide) leads to very strong stem- and cable-irradiation effects. If you correct for these effects using a two-color-channel method, measurements have to be performed without a reference signal. In addition, this correction method is very prone to handling-errors. All in all, using scintillation detectors is similar to using gafchromic film: if you want accurate results, you need to know exactly what you are doing. Can I use any detector to perform absolute dosimetry? Usually absolute dosimetry is performed in 10 cm x 10 cm fields and according to international dosimetry protocols. Currently there is no such protocol describing dose measurements in small fields. Hence, in small fields we recommend to cross-calibrate the to-be-used detector against a Semiflex 3D or Semiflex chamber in a 4 cm x 4 cm or 5 cm x 5 cm field. Any detector can be cross-calibrated as long as it is stable during the entire measurement. We recommend to perform the cross-calibration before each measurement session to check for detector dose and temperature stability (this is especially important when using silicon diodes or scintillators) and to check for the reproducibility of the calibration procedure. 22

23 My field is smaller than 1 cm x 1 cm. Which detector can I use? If you need to measure smaller field sizes, we recommend to use non-shielded detectors with a small cross-section perpendicular to the beam. These are the Diode E (T60017) for all photon energies and the Diode SRS (T60018) for photon energies of 6 MV and below. You can also consider the T60019 for such measurements. For any detector we recomend to look up correction factors for very small fields in scientific literature. My field is not square. Which detector is suitable? There are formulas to calculate an equivalent square field size for non-square field shapes. The aim of these calculations is to predict the output factor of an irregular field. To estimate whether a detector will be prone to the volume effect, these formulas cannot be used. Instead, the smallest dimension of the field plays the central role. For rectangular fields, this is the small edge. For example, if your field size is 2 cm x 10 cm, take a detector that is suited for a 2 cm x 2 cm field. For circular fields, the vendor of your irradiation unit will in most cases recommend a detector for the measurements. As rule of thumb: to measure output factors, i.e. when measuring in the center of the field, take the diameter of the field as smallest dimension. For example, to measure the output factor in a 3 cm diameter field, take a detector that is suited for a 3 cm x 3 cm field. For profile measurements, it is difficult to give a precise recommendation. If you are unsure which detector to use, take the smaller one. What is the advantage of silicon diodes over air-filled ionization chambers? Due to the higher density of atoms in silicon compared to air, a diode detector can be constructed very small and still have a good response. Hence in high-gradient regions, such as the penumbra, a diode detector will be more accurate. The detector combines the advantages of silicon diodes and air-filled ionization chambers, but its crosssection in the beam is slightly larger than for the PTW silicon diode detectors. What is the advantage of air-filled ionization chambers over silicon diodes? In contrast to silicon diodes the response of air-filled ionization chambers to low-energy scattered radiation is excellent, except if they have a steel central electrode. For this reason, they are suited to precisely deduce the dose in large fields and in the out-of-field region. In addition, air-filled ionization chambers are perfectly suited to deduce the absolute dose according to international dosimetry protocols. Air-filled ionization chambers do not suffer any response degradation due to irradiation. When do I use a shielded diode? In shielded diodes, the over-response to kev-energy scattered radiation which is mainly present in fields >_ 10 cm x 10 cm is compensated by a metal shield absorbing that type of radiation. Due to this combination, shielded diodes can be used in the entire field size range from 1 cm x 1 cm up to 40 cm x 40 cm. Nevertheless one must keep in mind that this large field size range does not come free of costs. Shielded diodes are a compromise. They can be used for small and large fields, but if you want to increase the accuracy, we 23

24 recommend to use a instead of a. For highest accuracy use a small field detector for small fields (e.g. an unshielded silicon diode or a ) and an air-filled ionization chamber for large fields. How can I check if my detector is accurately positioned in the field? The option CenterCheck of the MEPHYSTO package allows you to check the positioning and alignment of your detector in the beam. In addition, you can improve reproducibility and ease of use by mounting your detectors using the TRUFIX system. It is important to check the positioning at shallow and large depths in the water. Use the PTW technical note D to optimize CenterCheck for small fields. How can I tell the effective point of measurement and water equivalent window thickness of PTW solid state detectors? Each PTW solid state detector has a colored ring which is situated at the water equivalent depth of the effective point of measurement of the detector. To find the zero water position, make the ring level with the water surface and define this as zero water level. The detector should be used in axial orientation for this procedure. If you are using TRUFIX and the stop thimble corresponding to your detector, the detector will directly be positioned in the correct depth. This, of course, requires that you first have correctly set the zero position with TRUFIX. Where do I place the reference detector in a small field? Placing a reference detector in a very small field without disturbing the main detector is not feasible. Simply placing the reference detector outside the field border is not a very good solution either, because the signal of the reference will then be very noisy and will lead to a noisy measurement (i.e. the curves will not be flat). There are several options what you could do: } You can use the PTW T-REF chamber. This is a very thin transmission chamber providing a strong and very low noise reference signal } If you are very sure that your linac is very stable, measure without reference } You can use a very large ionization chamber, e.g. a Bragg-Peak chamber or a 100 mm CT-chamber as reference right next to the beam. The larger the chamber the better, a Farmer chamber is still better than a semiflex chamber. Note, that this technique will increase the noise of your measurement. Do not use a diode as reference as diodes exhibit strong quantum noise } You can increase your integration time. Four times longer integration time leads to half the noise } You can measure the PDD, profile, etc. several times. If several curves coincide, the linac was stable } You can measure step by step irradiating a fixed number of MUs for each data point If you use a reference chamber outside of the beam, remember to pre-irradiate it if it has not been in the beam before. A more thorough description including measured data can be found in [Wuerfel2013]. 24

25 7 Detector Overview Dimensions, specs Radiation Quality T cm³ radius of sensitive 60 Co 50 MV photons Semiflex 3D Chamber volume 2.4 mm, (9 45) MeV electrons length 4.8 mm T cm³ radius of sensitive 66 kv 50 MV photons Semiflex Chamber volume 2.75 mm, (10 45) MeV electrons length 6.5 mm (50 270) MeV protons T cm³ radius of sensitive 60 Co 50 MV photons PinPoint Chamber volume 1 mm, length 5 mm T cm³ radius of sensitive 60 Co 50 MV photons PinPoint Chamber volume 1.45 mm, length 5 mm T cm³ radius of sensitive 60 Co 25 MV photons PinPoint 3D Chamber volume 1.45 mm, length 2.9 mm T60019 sensitive volume 100 kv 25 MV photons Detector mm³, (6 25) MeV electrons radius 1.1 mm, (70 230) MeV protons thickness mm ( ) MeV/u carbon ions T60016 Dosimetry sensitive volume 60 Co 25 MV photons 0.03 mm³, radius of sensitive volume 0.6 mm, shielded T60017 Dosimetry Diode E sensitive volume 60 Co 25 MV photons 0.03 mm³, radius of (6 25) MeV electrons sensitive volume 0.6 mm, unshielded T60018 Dosimetry Diode SRS sensitive volume 60 Co 6 MV photons 0.3 mm³, radius of sensitive volume 0.6 mm, unshielded, high response 25

26 0.07 cm 3 Semiflex 3D Chamber Type Standard therapy chamber with excellent 3D characteristics for scanning systems and for absolute dosimetry Features Waterproof, semiflexible design for easy mounting in scanning water phantoms Excellent 3D characteristics Sensitive volume of 0.07 cm 3 Outperforms all requirements of IEC and AAPM TG-51 Designed for axial and radial irradiation The Semiflex 3D chamber is ideal for dose measurements in small fields as encountered e.g. in IORT, IMRT and stereotactic beams as well as for dose measurements in standard fields up to 40 x 40 cm 2. Relative dose distribution can be measured with high spatial resolution in any direction. The waterproof, fully guarded chamber can be used in air, solid state phantoms and in water. Specification Type of product Application Measuring quantities Reference radiation quality vented cylindrical ionization chamber absolute dosimetry in radiotherapy beams absorbed dose to water, air kerma, exposure 60Co Nominal sensitive 0.07 cm 3 volume Design waterproof, vented, fully guarded Reference point on chamber axis, 3.45 mm from chamber tip Direction of incidence axial, radial Nominal response 2 nc/gy Long-term stability 0.3 % over 2 years Chamber voltage 400 V nominal ± 500 V maximal Polarity effect at 60 Co photons ± 0.8 % electrons ± 1 % Directional response in water Leakage current Cable leakage ± 0.5 % for rotation around the chamber axis ± 1 % for tilting of the axis up to ± 90 ± 4 fa 100 fc/(gy cm) Materials and measures: Wall of sensitive volume 0.57 mm PMMA, 1.19 g/cm mm graphite, 1.85 g/cm 3 Total wall area density 84 mg/cm 2 Dimension of sensitive radius 2.4 mm volume length 4.8 mm Central electrode Al 99.98, diameter 0.8 mm Build-up cap PMMA, thickness 3 mm Ion collection efficiency at nominal voltage: Ion collection time 118 µs Max. dose rate for 99.5 % saturation 6.7 Gy/s 99.0 % saturation 13.4 Gy/s Max. dose per pulse for 99.5 % saturation 0.68 mgy 99.0 % saturation 1.42 mgy Useful ranges: Chamber voltage Radiation quality ± ( ) V 60Co MV photons ( ) MeV electrons Field size (2.5 x 2.5) cm 2... (40 x 40) cm 2 (3.0 x 3.0) cm 2... (40 x 40) cm 2 18 MV Temperature ( ) C ( ) F Humidity ( ) %, max 20 g/m 3 Air pressure ( ) hpa Ordering Information TN31021 Semiflex 3D chamber 0.07 cm 3, connecting system BNT TW31021 Semiflex 3D chamber 0.07 cm 3, connecting system TNC TM31021 Semiflex 3D chamber 0.07 cm 3, connecting system M Options T48012 Radioactive check device 90 Sr T Chamber holding device for check device 26

27 0.125 cm 3 Semiflex Chamber Type Standard therapy chamber for scanning systems and for absolute dosimetry Features Waterproof, semiflexible design for easy mounting in scanning water phantoms Minimized directional response Sensitive volume cm 3, vented to air Radioactive check device (option) The semiflexible chamber is the ideal compromise between small size for reasonable spatial resolution and large sensitive volume for precise dose measurements. This makes the chamber to one of the most commonly used chambers in scanning water phantom systems. The chamber volume of cm 3 gives enough signal to use the chamber also for high precision absolute dose measurements. The sensitive volume is approxima tely spherical resulting in a flat angular response and a uniform spatial resolution along all three axes of a water phantom. Specification Type of product Application Measuring quantities Reference radiation quality vented cylindrical ionization chamber absolute dosimetry in radiotherapy beams absorbed dose to water, air kerma, exposure 60Co Nominal sensitive cm 3 volume Design waterproof, vented, fully guarded Reference point on chamber axis, 4.5 mm from chamber tip Direction of incidence radial Nominal response 3.3 nc/gy Long-term stability 1 % per year Chamber voltage 400 V nominal ± 500 V maximal Polarity effect at 60 Co < 2 % Photon energy response ± 2 % (140 kv kv) ± 4 % (140 kv Co) Directional response in water Leakage current Cable leakage ± 0.5 % for rotation around the chamber axis and for tilting of the axis up to ± 10 ± 4 fa 1 pc/(gy cm) Materials and measures: Wall of sensitive volume 0.55 mm PMMA, 1.19 g/cm mm graphite, 0.82 g/cm 3 Total wall area density 78 mg/cm 2 Dimension of sensitive radius 2.75 mm volume length 6.5 mm Central electrode Al 99.98, diameter 1.1 mm Build-up cap PMMA, thickness 3 mm Ion collection efficiency at nominal voltage: Ion collection time 121 µs Max. dose rate for 99.5 % saturation 6 Gy/s 99.0 % saturation 12 Gy/s Max. dose per pulse for 99.5 % saturation 0.5 mgy 99.0 % saturation 1.0 mgy Useful ranges: Chamber voltage ± ( ) V Radiation quality 140 kv MV photons ( ) MeV electrons ( ) MeV protons Field size (3 x 3) cm 2... (40 x 40) cm 2 Temperature ( ) C ( ) F Humidity ( ) %, max 20 g/m 3 Air pressure ( ) hpa Ordering Information TN31010 Semiflex chamber cm 3, connecting system BNT TW31010 Semiflex chamber cm 3, connecting system TNC TM31010 Semiflex chamber cm 3, connecting system M Options T48012 Radioactive check device 90 Sr T Chamber holding device for check device 27

28 PinPoint Chambers Type 31014, Ultra small-sized therapy chambers for dosimetry in high-energy photon beams Features Small-sized sensitive volumes of only cm 3 and 0.03 cm 3, 2 mm and 2.9 mm in diameter, vented to air Very high spatial resolution when used for scans perpendicular to the chamber axis Aluminum central electrode Radioactive check device (option) The PinPoint chambers are ideal for dose measurements in small fields as encountered e.g. in IORT, IMRT and stereotactic beams. Relative dose distributions can be measured with very high spatial resolution when the chambers are moved perpendicular to the chamber axis. The waterproof, fully guarded chambers can be used in air, solid state phantoms and in water. Specification Type of products Application Measuring quantities Reference radiation quality vented cylindrical ionization chambers dosimetry in high-energy photon beams with high spatial resolution absorbed dose to water, air kerma, exposure 60Co Nominal sensitive cm 3, 0.03 cm 3 volume Design waterproof, vented, fully guarded Reference point on chamber axis, 3.4 mm from chamber tip Direction of incidence radial, axial (31014) Pre-irradiation dose 2 Gy Nominal response 400 pc/gy, 800 pc/gy Long-term stability 1 % per year Chamber voltage 400 V nominal ± 500 V maximal Polarity effect ± 2 % Directional response in water Leakage current Cable leakage ± 0.5 % for rotation around the chamber axis, ± 1 % for tilting of the axis up to ± 20 (radial incidence) ± 15 (axial incidence) ± 4 fa 1 pc/(gy cm) Materials and measures: Wall of sensitive volume 0.57 mm PMMA, 1.19 g/cm mm graphite, 1.85 g/cm 3 Total wall area density 85 mg/cm 2 Dimensions of sensitive radius 1 mm, 1.45 mm volume length 5 mm Central electrode Al 99.98, diameter 0.3 mm Build-up cap PMMA, thickness 3 mm Ion collection efficiency at nominal voltage: Ion collection time 20 µs, 50 µs Max. dose rate for 99.5 % saturation 265 Gy/s, 29 Gy/s 99.0 % saturation 580 Gy/s, 55 Gy/s Max. dose per pulse for 99.5 % saturation 3.5 mgy, 1.2 mgy 99.0 % saturation 7 mgy, 2.3 mgy Useful ranges: Chamber voltage ± ( ) V Radiation quality 60Co MV photons Field size (2 x 2) cm 2... (30 x 30) cm 2 Temperature ( ) C ( ) F Humidity ( ) %, max 20 g/m 3 Air pressure ( ) hpa Ordering Information TN31014 PinPoint chamber cm 3, connecting system BNT TW31014 PinPoint chamber cm 3, connecting system TNC TM31014 PinPoint chamber cm 3, connecting system M TN31015 PinPoint chamber 0.03 cm 3, connecting system BNT TW31015 PinPoint chamber 0.03 cm 3, connecting system TNC TM31015 PinPoint chamber 0.03 cm 3, connecting system M Options T48012 Radioactive check device 90 Sr T Chamber holding device for check device 28

29 PinPoint 3D Chamber Type New ultra small-sized therapy chamber with 3D characteristics for dosimetry in high-energy photon beams Features Small polarity effect Minimal cable irradiation effect Short ion collection time Large field size range The new PinPoint 3D chamber is ideal for meas - urements in small fields but can also be used for measurements in large fields. Designed for radial beam orientation, the small-sized chamber shows excellent 3D characteristics. Relative dose distributions can be measured with high spatial resolution in any direction. It is waterproof and fully guarded, thus it can be used in air, solid state phantoms and in water. Specification Type of product Application Measuring quantities Reference radiation quality vented cylindrical ionization chamber dosimetry in photon beams absorbed dose to water, air kerma, exposure 60Co Nominal sensitive cm 3 volume Design waterproof, vented, guarded Reference point on chamber axis, 2.4 mm from chamber tip Direction of incidence radial Pre-irradiation dose 1 Gy Nominal response 400 pc/gy Long-term stability 1 % per year Chamber voltage 300 V nominal ± 500 V maximal Polarity effect ± 0.8 % Directional response in water Leakage current Cable leakage ± 0.5 % for rotation around the chamber axis, ± 1 % for tilting of the axis up to ± 10 ± 4 fa 100 fc/(gy cm) Materials and measures: Wall of sensitive 0.57 mm PMMA, volume 1.19 g/cm mm graphite, 1.85 g/cm 3 Total wall area density 84 mg/cm 2 Dimensions of sensitive radius 1.45 mm volume length 2.9 mm Central electrode Al 99.98, diameter 0.6 mm Build-up cap PMMA, thickness 3 mm Ion collection efficiency at nominal voltage: Ion collection time 45 µs Max. dose rate for 99.5 % saturation 46 Gy/s 99.0 % saturation 91 Gy/s Max. dose per pulse for 99.5 % saturation 0.8 mgy 99.0 % saturation 2.2 mgy Useful ranges: Chamber voltage ± ( ) V Radiation quality 60Co MV photons Field size (2 x 2) cm 2 (40 x 40) cm 2 Temperature ( ) C ( ) F Humidity ( ) %, max 20 g/m 3 Air pressure ( ) hpa Ordering Information TN31022 PinPoint 3D chamber cm 3, connecting system BNT TW31022 PinPoint 3D chamber cm 3, connecting system TNC TM31022 PinPoint 3D chamber cm 3, connecting system M Options T48012 Radioactive check device 90 Sr T Chamber holding device for check device 29

30 Type Diamond Detector for dosimetry in high-energy photon and electron beams, especially useful for small field dosimetry Features Small sensitive volume of mm 3 Excellent radiation hardness and temperature independence Near tissue-equivalence Operates without high voltage All connecting systems available (BNT, TNC, M) The new detector is a synthetic single crystal diamond detector (SCDD), based on a unique fabrication process [1, 2]. Significant advantages of the synthetic production are standardised assembly and consequently a high reproducibility of the dosimetric properties and good availability of the detector. Specification Type of product Application Measuring quantitiy Reference radiation quality synthetic single crystal diamond detector dosimetry in radiotherapy beams absorbed dose to water 60Co Nominal sensitive mm 3 volume Design waterproof, disk-shaped, sensitive volume perpendi - cular to detector axis Reference point on detector axis, 1 mm from detector tip, marked by ring Direction of axial incidence Pre-irradiation dose 5 Gy Nominal response 1 nc/gy Long-term stability 0.5 % per year Dose stability < 0.25 % / kgy at 18 MV Temperature 0.08 % / K response Energy response ± 13 % (100 kev Co) Bias voltage 0 V Signal polarity positive Directional 1 % for tilting ± 40 response in water Leakage current 1 20 fa Cable leakage 200 fc / (Gy cm) Materials and measures: Entrance window 0.3 mm RW3 0.6 mm Epoxy 0.01 mm Al 99.5 Total window 101 mg/cm 2 area density Water-equivalent 1.0 mm window thickness Sensitive volume radius 1.1 mm, circular, thickness 1 µm Outer dimensions diameter 7 mm, length 45.5 mm Useful ranges: Radiation quality 100 kev MV photons ( ) MeV electrons (70 230) MeV protons ( ) MeV/u carbon ions Field size 2 (1 x 1) cm 2... (40 x 40) cm 2 Temperature ( ) C, ( ) F Humidity range ( ) %, max 20 g/m 3 Ordering Information TN60019 Detector, connecting system BNT TW60019 Detector, connecting system TNC TM60019 Detector, connecting system M The detector is realized in collaboration with Marco Marinelli and Gianluca Verona-Rinati and their team, Industrial Engineering Department of Rome Tor Vergata University, Italy. [1] I. Ciancaglioni, M. Marinelli, E. Milani, G. Prestopino, C. Verona, G. Verona-Rinati, R. Consorti, A. Petrucci and F. De Notaristefani, Dosimetric characterization of a synthetic single crystal diamond detector in clinical radiation therapy small photon beams, Med. Phys. 39 (2012), 4493 [2] C. Di Venanzio, M. Marinelli, E. Milani, G. Prestopino, C. Verona, G. Verona-Rinati, M. D. Falco, P. Bagalà, R. Santoni and M. Pimpinella, Characterization of a synthetic single crystal diamond Schottky diode for radiotherapy electron beam dosimetry, Med. Phys. 40 (2013), At the high end of the temperature range, higher leakage currents may occur. 2 This detector is well suited for measurements in field sizes smaller than 1 cm x 1 cm. Depending on the accuracy required by the user, correction factors may be necessary as described in international scientific publications. This applies to any detector used in very small fields. 30

31 Dosimetry Type Waterproof silicon detector for dosi metry in high-energy photon beams up to field size 40 cm x 40 cm Features Useful for measurements in small and large photon fields Excellent spatial resolution Minimized energy response for field size independent measurements up to 40 cm x 40 cm The Dosimetry is ideal for dose measurements in small photon fields as encountered in IORT, IMRT and stereotactic beams. The excellent spatial resolution makes it possible to measure very precisely beam profiles even in the penumbra region of small fields. The superior energy response enables the user to perform accurate percentage depth dose measurements which are field size independent up to field sizes of (40 x 40) cm 2. The waterproof detector can be used in air, solid state phantoms and in water. Specification Type of product Application Measuring quantity Reference radiation quality p-type silicon diode dosimetry in radiotherapy beams absorbed dose to water 60Co Nominal sensitive 0.03 mm 3 volume Design waterproof, disk-shaped sensitive volume perpendi - cular to detector axis Reference point on detector axis, 2.42 mm from detector tip Direction of incidence axial Nominal response 9 nc/gy Dose stability 0.5 %/kgy at 6 MV 1 %/kgy at 15 MV 0.5 %/kgy at 5 MeV 4 %/kgy at 21 MeV Temperature response 0.4 %/K Energy response at higher depths than d max, the percentage depth dose curves match curves measured with ionization chambers within ± 0.5 % Bias voltage 0 V Signal polarity negative Directional response in water Leakage current Cable leakage ± 0.5 % for rotation around the chamber axis, ± 1 % for tilting ± 40 ± 50 fa 1 pc/(gy cm) Materials and measures: Entrance window 1 mm RW3, g/cm 3 1 mm epoxy Total window area density 250 mg/cm 2 Water-equivalent 2.42 mm window thickness Sensitive volume 1 mm 2 circular 30 µm thick Outer dimensions diameter 7 mm, length 47 mm Useful ranges: Radiation quality 60Co MV photons Field size (1 x 1) cm 2... (40 x 40) cm 2 Temperature ( ) C ( ) F Humidity ( ) %, max 20 g/m 3 Ordering Information TN60016 Dosimetry, connecting system BNT TW60016 Dosimetry, connecting system TNC TM60016 Dosimetry, connecting system M 31

32 Dosimetry Diode E Type Waterproof silicon detector for dosimetry in high-energy electron and photon beams Features Useful for measurements in all electron fields and for small photon fields Excellent spatial resolution Minimized energy response Thin entrance window for measurements in the vicinity of surfaces and interfaces The Dosimetry Diode E is ideal for dose measurements in small electron and photon fields as encountered in IORT, IMRT and stereotactic beams. The excellent spatial resolution makes it possible to measure very precisely beam profiles even in the penumbra region of small fields. The Dosimetry Diode E is recommended for dose measurements in all electron fields and for photon fields up to (10 x 10) cm 2. The waterproof detector can be used in air, solid state phantoms and in water. Specification Type of product Application Measuring quantity Reference radiation quality p-type silicon diode dosimetry in radiotherapy beams absorbed dose to water 60Co Nominal sensitive 0.03 mm 3 volume Design waterproof, disk-shaped sensitive volume perpendi - cular to detector axis Reference point on detector axis, 1.33 mm from detector tip Direction of incidence axial Nominal response 9 nc/gy Dose stability 0.5 %/kgy at 6 MV 1 %/kgy at 15 MV 0.5 %/kgy at 5 MeV 4 %/kgy at 21 MeV Temperature response 0.4 %/K Energy response at higher depths than d max, the percentage depth dose curves match curves measured with ionization chambers within ± 0.5 % Bias voltage 0 V Signal polarity negative Directional response in water Leakage current Cable leakage ± 0.5 % for rotation around the chamber axis, ± 1 % for tilting ± 20 ± 50 fa 1 pc/(gy cm) Materials and measures: Entrance window 0.3 mm RW3, g/cm mm epoxy Total window area density 140 mg/cm 2 Water-equivalent 1.33 mm window thickness Sensitive volume 1 mm 2 circular 30 µm thick Outer dimensions diameter 7 mm, length 45.5 mm Useful ranges: Radiation quality ( ) MeV electrons 60Co MV photons Field size 1 (1 x 1) cm 2... (40 x 40) cm 2 for electrons (1 x 1) cm 2... (10 x 10) cm 2 for photons Temperature ( ) C ( ) F Humidity ( ) %, max 20 g/m 3 Ordering Information TN60017 Dosimetry Diode E, connecting system BNT TW60017 Dosimetry Diode E, connecting system TNC TM60017 Dosimetry Diode E, connecting system M 1 This detector is well suited for measurements in field sizes smaller than 1 cm x 1 cm. Depending on the accuracy required by the user, correction factors may be necessary as described in international scientific publications. This applies to any detector used in very small fields. 32

33 Dosimetry Diode SRS Type Waterproof silicon detector for dosimetry in 6 MV photon beams up to field size 10 cm x 10 cm Features Designed for measurements in small photon fields with maximum 6 MV Excellent spatial resolution High response Very low noise Thin entrance window for measurements in the vicinity of surfaces and interfaces The Dosimetry Diode SRS is ideal for dose measurements in photon fields with a maximum field size of 10 cm x 10 cm and with a maximum energy of 6 MV. The very high response of this detector allows to measure beam profiles with a very high resolution and very short dwell time. Typical use is beam profile measurement for stereotactic radio surgery (SRS). Specification Type of product Application Measuring quantity Reference radiation quality p-type silicon diode dosimetry in radiotherapy beams absorbed dose to water 60Co Nominal sensitive 0.3 mm 3 volume Design waterproof, disk-shaped sensitive volume perpendi - cular to detector axis Reference point on detector axis, 1.31 mm from detector tip Direction of incidence axial Nominal response 175 nc/gy Dose stability 0.8 %/kgy at 6 MV Temperature response (0.1 ± 0.05) %/K Energy response at higher depths than d max, the percentage depth dose curves match curves measured with ionization chambers within ± 0.5 % Bias voltage 0 V Signal polarity negative Directional response in water Leakage current Cable leakage ± 0.5 % for rotation around the chamber axis, ± 1 % for tilting ± 20 ± 50 fa 1 pc/(gy cm) Materials and measures: Entrance window 0.3 mm RW3, 0.27 mm epoxy Total window area density 140 mg/cm 2 Water-equivalent 1.31 mm window thickness Sensitive volume 1 mm 2 circular 250 µm thick Outer dimensions diameter 7 mm, length 45.5 mm Useful ranges: Radiation quality 60Co... 6 MV photons Field size 1 (1 x 1) cm 2... (10 x 10) cm 2 Temperature ( ) C ( ) F Humidity ( ) %, max 20 g/m 3 Ordering Information TN60018 Dosimetry Diode SRS, connecting system BNT TW60018 Dosimetry Diode SRS, connecting system TNC TM60018 Dosimetry Diode SRS, connecting system M 1 This detector is well suited for measurements in field sizes smaller than 1 cm x 1 cm. Depending on the accuracy required by the user, correction factors may be necessary as described in international scientific publications. This applies to any detector used in very small fields. 33