3D Brachytherapy with Afterloading Machines 3D Brachytherapy/MS Page 1
Introduction 3D-Brachytherapy refers to the case when the planning is performed based on a set of CT, MR or UltraSound (US) images. Since the treatments can be done using either afterloading machines like the MicroSelectron from Nucletron or seeds, the section of the manual devoted to 3d-brachytherapy will be divided into two individual sections: one for actual machines and the other for seeds. This is furthermore justified since the planning procedures for these two modalities follow different paths: In the case of a machine like the microselectron, empty catheters are placed inside the patient. For prostate or perineal implants, a template may be used at the time of the implantation to guide the positioning of the catheters. Once the catheters are in place, a set of CT or MR images is taken and the catheters are reconstructed on these images, independently of the templates used for their positioning. Following the catheter reconstruction, the source positions and dwell times are planned and once the plan is optimized the treatment follows. In the case of seeds, like may be the case for a prostate implant with I-125 seeds, there is a preplan. This preplan is normally done by means of US images, perpendicular to the US transducer and showing the grid points of the template. Needles or vectors are later on inserted only through these grid points. Since the needles are perpendicular to all images, the needle positions are marked only in one image and propagated in straight line from the first to the last image. The points of seed depositions are then selected along each needle to give the best dose distribution. This however, is not the actual treatment the patient is going to receive: the purpose of this preplan is basically to know the number of seeds which will be needed for the treatment. Once the seeds are orderer and received, some 2 or 3 weeks later, the patient is recalled and a new US imaging procedure and planning follows prior to the actual implant process to ascertain of the reproducibility of the previous preplan. Normally some minor modifications are required in this second planning due to the fact that the tumor itself may not be exactly the same as before, or also it could be that the positioning of the US guide and the patient itself is not quite the same as in the preplan. A second and final plan is thus prepared and then seeds inserted according to it. Depending on whether stranded or loose seeds are used, one may assume them to be at regular intervals o not. Furthermore, in the case of a prostate implant, a third plan is done based on CT images a month after the implantation. This is just to verify, once the oedema has disappeared, how good (or bad!) the implantation was actually done. There are enough differences between the two techniques to justify a separate treatment of the subject: permanent implant vs. temporary, fixed times for permanent vs- variable programable time for afterloaders, etc. 3D Brachytherapy/MS Page 2
Brachy Operations. To do Brachytherapy with CT, MRI or UltraSound images, click on the 3D Brachytherapy icon in the main screen and proceed to read in the patient's images as described in the previous section "Starting a Planning session". At this point we have then a set of raw images to be read and the processing needed will depend on the nature of these images (CT, MR or UltraSound), the way in which the acquisition was done (direct transfer from the CT/MR, or digitized gary scale images) as well as the type of brachytherapy treatment to be done (nonpermanent implants with afterloaders, ribbons or wires, versus permanent implants with seeds). Therefore, the description of the raw image processing will be spit along these lines. The most important factor here is that of determining a suitable coordinate system of reference in the absence of absolute references, as is the case with stereotactic planning (fiducial markers). Therefore we will rely on the couch index to establish an absolute scale in the direction perpendicular to the slice plane. Furthermore, if the images were digitized through the use of video or a flatbed scanner, they will not be, in general, properly aligned with each other and therefore we will also need a reference common to all images, like a ruler in the case of CT or MR, or the template grid in the case of UltraSound images. This ruler or grid will also be used to determine the horizontal and vertical scales in the plane of the image. In the case of UltraSound images, the ruler can be substituted by a line between two grid points. The first step thus is to prepare the raw images so we have a suitable system of coordinates. 3D Brachytherapy/MS Page 3
Processing the raw images. After selecting the option Read Images from Disk, the window will dissapear and a new file selection window will open letting the user navigate until the raw images are found (they are called raw images since no processing as yet been done on them). Notice that the folder containing these images may reside anywhere on the disk, and may have any arbitrary name, as well as the images themselves. In reality, images from scanners will have very criptic names which do not relate at all with the patient name; unless the actual file format is known, SIMUPLAN does not have a way to know the imaging modality used for these images, short of asking the user, something which is done through another selection window as shown below: The user must select the imaging modality for those images and click OK to dismiss the window. 3D Brachytherapy/MS Page 4
The program poceeds to display the first raw image as well as an additional window to gather information about the operations to be performed on these images. Using this additional window, named "Image Analysis Mode", the user can enter the apropriate image information such as: Image type: Image section: Image view: As one can see we have the option of using Fiducials or Couch index + rulers as the method to obtain absolute coordinates. If we are doing stereotactic brachytherapy with a stereotactic frame, then we will select Fiducials in the Coordinates section otherwise we have to select Couch index + rulers. As soon as Couch index+ rulers is chosen, the frame type remains locked indicating No Frame. Additionally, the Pat outline option is dimmed since the user will have a diferent selection for the patient s contour once the next window opens. 3D Brachytherapy/MS Page 5
In the case of stereotatcic brachytherapy, we would have selected "Fiducials" as the coordinate determination method and the Frame type item would be active with a pop-up menu showing the available supported frames. After all the selections have been done, click OK to go back to the image operations. The program will open immediately the Couch Indexing control window; a red ruler and an orange oval line with red square points will also be drawn on the raw image Coordinates and patient's contour. Working on the first image These two operations, coordinates definition and Patient contouring, are done simultaneously. In the case of 3D Brachytherapy where the actual patient contour is not needed, but it may or may not be available, we need some extra contouring options. If the actual patient contour can be seen in the images and is going to be used, refer to the chapter Fiducial and Patient Contouring for the contouring details. This section will describe only the case when the actual patient contour is not used, which is the common case: even if the actual patient contour could be available, its use would reduce the size of the area of interest to an inconveniently small size. After observing the actual parameters of the image, the user can now proceed to modify the default settings shown in the Couch Indexing control window. Enter the couch index as read on the film. By comparing with the rest of the images, determine the direction in which the index increases, i.e., SUP-INF or INF-SUP, and click on the appropriate selection button. As an option, if the slice interval is constant, enter it so the program will calculate the couch index for successive images, assuming of course they are entered in consecutive order. If only one ruler is to be used, un-check the other one, and proceed to enter the length of the ruler. At this time, move the ruler shown on the image over the proper scale by grabbing one end at a time with the mouse. Now select the kind and shape of the contour. The options are: elliptical, rectangular with rounded corners or a real contour. For this case, the most appropriate would be the elliptical contour. 3D Brachytherapy/MS Page 6
Since the ellipse drawn on the image will not, in general, delineate the region of interest, it can be moved anywhere by pressing the mouse inside and dragging it to the new position. The size can be changed as well by grabbing the red handles and displacing them. Notice that these handles operate symmetrically relative to the centre of the shape. Once the ruler position and the contour are acceptable, clicking on the Save and Next button in the Couch Indexing control window will save this image and bring in the next raw image. What operations have been done with just this simple click? 3D Brachytherapy/MS Page 7
Operations performed before saving the image. a) on the first image, the whole image is rotated, including the ruler and the contour, so the ruler orientation is perfectly vertical if the selected ruler was a vertical ruler, or perfectly horizontal if the selected ruler was horizontal. In consecutive images, the image and ruler will be rotated and translated as needed to make the ruler, which may have been moved by the user, coincident with the ruler of the first image. b) the couch index is taken as the basis for the determination of 3 points which will define the equation of the plane of the image. c) the image is "packed", i.e., the portion of the image inside the contour is retained and written to disk, inside the "packed folder" in a special format suitable for future operations. At the same time, a set of data known as the "image index" is also written to the same "packed" folder. This index will be rewritten every time a new raw image is processed and saved, to include the relevant numerical data for the new image d) the next raw image will be picked up from the list of raw images, read and displayed, with the ruler and the contour in their original aligned positions, i.e, in the positions resulting after step a). Working on the rest of the images The ruler length is locked after the first image. In order to allign all images with the first one, the user can grab either end or the body of the ruler to move it around but its length will remain unchanged. Grabbing the ruler by its body will displace it in parallel while grabbing either end will rotate it. The user can thus align the ruler with the reference common to all images. After the Save&Next button is pressed the program will align the image as explained in the section above "Operations performed before saving the image". If for any reason (a bad image or an image which does not belong to the set like a scout image, etc) the user does not want to save the image, then pressing the button "No save & Next" will skip the packing operations and will proceed to read the next image in the raw folder. If there are no more images to be processed or the user clicked on the No More Images button, the program will stop processing the raw images. From here on, all planning continues using the packed images only. 3D Brachytherapy/MS Page 8
Working with the packed images Although we can continue from here, lets stop and look at the status of the planning at this point. To do this we exit the program completely by selecting Quit from the pop-up menu to go back to the main window and pressing the button GoodBye. If we look inside the SIMUPLAN folder we will see the newly created folder, Pat folder, for the new patient, with the Info file and the CT folder containing the packed and raw images folders. Notice that the old folder Pat Images which originally contained the raw images and was placed in the SIMUPLAN folder has also disappeared. 3D Brachytherapy/MS Page 9
Defining the target volume and other structures. Please refer to the chapter on Contouring Structures for these operations. Editing the Applicators. Selecting the Treatment Machine. Definition of the applicators is started by selecting Applicator Edit from the 3D Brachy Operations submenu in the pop-up menu. If the applicators have not been defined in a previous session, all the items in the 3D Brachy Operations menu will be dimmed except for Applicator Edit. Selecting this option will open a window showing the available treatment machines (i.e., those which have been incorporated into the brachy machines library with the Brachy Machine Editor), as well as the standard "machines" or techniques: ribbons, seeds and wires. A machine or technique is selected by clicking into its icon. 3D Brachytherapy/MS Page 10
In the case of a regular machine a second window, the Treatment Machine data window, will be displayed showing the relevant data of the selected machine, as recorded in the machine library, including also the source activity at treatment date. If everything is correct, pressing the OK button will continue ahead to the next step. Otherwise, click Cancel and the machine selection window will reopen. Particular attention should be paid to the calibration data shown in this window, since it could happen that after a recent change of source or recalibration, the affected machine data was not updated. These windows are not shown again if the Applicator Edit menu item is selected later on to add or delete some applicator. Defining the Applicators. Once the Treatment Machine has been selected, an Applicator Editor control window opens. This window shows in its upper half the controls to manipulate the applicator creation and deletion, and in its lower half some extra controls related to the contours and the way the catheters should be loaded. So for instance we could apply a margin to the target contours and request the program to load the source positions which fall inside these expanded contours. 3D Brachytherapy/MS Page 11
Since the loading of the catheters is done after exiting the Edit Mode, it is recommended to apply the desired target margin before entering the edit mode. To enter the Edit Mode just click on the Edit button which will be hilighted to indicate its state. Applicator definition should be started on the most inferior slice. Applicators are defined by positioning the mouse at the desired position and clicking the mouse button with the Command key pressed down. New applicators will be numbered starting at 1 and up until the maximum number of channels is reached, at which time the program will not allow the creation of any more applicators. The maximum number of channels is that of the machine selected. If an existing applicator is clicked on with the Command key pressed, then we can move this applicator. Pressing the right arrow key will show the next image with a copy of the applicators drawn on it, at the position they had in the previous image. The applicators can be moved individually or as whole. To move a single applicator just grab it with the mouse while keping the Command key down. If we hold down at the same time the Shift and Comm and keys while grabbing any applicator, then all of them will follow the mouse. The normal procedure will be first to move them all until they are close to their new positions and next tune up each one individually. 3D Brachytherapy/MS Page 12