in partnership with Phantoms in Medical Physics (RT) U. Oelfke Division of Radiotherapy & Imaging uwe.oelfke@icr.ac.uk Making the discoveries that defeat cancer
1. Introduction
What is a phantom? Wiki: Latin Phantasmas Phantasy A phantom is a device which mimics a limited number of aspects of reality.
What is a phantom used for? Step 1: Basic hypothesis validation A phantom is used to provide answers to a simplified problem, mostly to validate a hypothesis, often with high accuracy Outcome: Real success, wonderful!
What is a phantom used for? Step 1: Basic hypothesis validation A phantom is used to provide answers to a simplified problem, mostly to validate a hypothesis often with high accuracy Outcome: Our favourite but then
Step 2: Extrapolation of the validity of the hypothesis to a more complex situation not covered by the phantom Dangerous: Scientist become believers Classical example: MC dose calculations for inhomogeneous patient geometries One cannot disprove it s validity, but one also cannot prove it
2. What do we verify with phantoms? Quality - Geometrical accuracy (images, dose distributions, equipment alignment, ) - Dose algorithm accuracy calibration of algorithms - Image quality (contrast, spatial resolution, geometrical accuracy) - Machine QA (dose delivery) - Patient QA - Workflow, Irradiation techniques end to end testing - Calibrations of measurements to required physical quantities -.
3. Material Phantoms
a) Water and Solid Water
Water phantom 10 Automated water phantom Source: PTW web page
Water phantom 11 Source: PTW web page
Solid Water 12
Solid water phantoms 13 Source: PTW web page
Hounsfield number calibration phantom 14 Source: SunNuclear web page
Mimicking patient anatomy for dosimetry and imaging or end to end a)static a)dynamic
Image quality/igrt irradiation geometries
CBCT Image quality assessment 17 Contrast Spatial resolution Imaging dose (CT, ) Geometrical accuracy (MRI) Source: phantomlab.com web page cirsinc.com web page
EPID phantoms IGRT calibrations 18 Source: PTW.com web page standardimaging.com
W. Mao et al.
Determination of 9 parameters Source pos. Detector trans. Detector rot.
Non-ideal projection geometry Real world: projection geometry is non-ideal due to gravitational sag of the imaging hardware determine projection matrix experimentally: calibration phantom alignment at the isocentre sample projection
Contrast/resolution phantom Geometrical calibration calibrated Not calibrated
kv/mv - Image Quality Images: examples Images: artifacts Images: doses
kv-cbct: Contrast phantom 1cGy 2cGy 440 projections over 220 degrees Estimated dose at the isocenter
Image Artifacts Courtesy of Jeffrey Siewerdsen Rings & flex Streaks Shading de Frise Metal Lag Truncation Motion
In room 3D-imaging MV/kV kv CBCT (cone beam, electron energy: 70-140 kev,fpi) In room kv-ct (Spiral CT (fan), 60-140 KeV, ion-chamber) MV CBCT (Cone beam, 6 MeV,FPI) MV-CT (Fan beam, tomo, 3.5 MeV,FPI) IBL ( inline kview, conebeam, 3.5 MeV, C-target,FPI)
Siemens Cone beam phantom Contrast slices I,II,III, spatial resolution slice, noise & scaling slice, MTF slice
Example: Image quality and dose
Geometrical distortions in MRI 31 Source: cirsinc.com phantomlab.com modusqa.com
Phantoms in RT 32 Multi modality phantoms (e.g. US/kV/MRI) Source: cirsinc.com web page
4. Mimicking patient anatomy for dosimetry
Anatomical Phantoms - Patient-sections - Organs (OARS) Geometrical shape (Internal Structure) Patient/Organ materials - e-density, Z, A Detector-compatible Mobility/Elasticity - Deformations - Organ movement - Un-/correlated movement of organs Connectivity to physiological processes - airflow - blood circulation Tumour Models?
Alderson Phantom 35 Source: rsdphantoms.com web page
Static Lung Phantoms 36 Attempt 1: Source: cirsinc.com web page
Attempt 2: Features of the phantom 50 mm Density: 0.27 g cm -3 Randomly filled 0.5 mm 3 structures Built using 3D-printer 50 mm 25 mm x 2
Attempt 3: Lung tissue 38
Pos 1 Pos 2
Experimental Results 90% +117% 90% +52% 10% 10% Phantom leads to degradation of Bragg peak
Patient lung? Physiological structure (not just e density) Motion means also change in physiology
Dynamic phantoms 42 Source: cirsinc.com modusqa.com
Phantoms in RT 43 MR compatible lung phantoms Source: cirsinc.com modusqa.com
Realistic lung phantoms? 44 Source: artichest.com web page PSI
Patient QA phantoms?? 45 Source: scandidos.com sunnuclear.com ptw.com
End to End Phantoms?? 46 Source: cirsinc.com
Conclusions Phantoms are an integral and extremely essential component of QA and R&D in medical physics All phantoms represent only limited aspects of reality and the relevance of achieved results needs to be assessed with outmost care Phantom development is an important area of research to advance the development and clinical integration of new imaging and treatment technologies
Conclusions So far, (almost) all phantoms are dead no standardized biological or physiological processes are accounted for
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