Breast Treatment System REFERENCE MANUAL

Transcription

1 Breast Treatment System REFERENCE MANUAL This manual may not be reproduced in whole or in part, by mimeograph or any other means, without the written permission of Diacor, Inc. The MammoRx Breast Treatment System is intended for use only by physicians qualified in radiation oncology and experienced in breast cancer treatment, or by technologists at the specific direction of such a qualified physician. It is the sole responsibility of the physician to judge whether the use of the MammoRx Breast Treatment System is clinically appropriate, whether the initial treatment plan is adequate to achieve his or her clinical goals, and whether the daily treatments are administered as prescribed. Diacor and MammoRx are trademarks of Diacor, Inc. Portalcast is a registered trademark of Diacor, Inc. Poloroid is a registered trademark of Poloroid Corporation. Copyright 2000 Diacor, Inc. DIACOR, 3191 SOUTH 3300 EAST, SALT LAKE CITY, UTAH / FAX

2 MammoRx Breast Treatment System Components A. MammoRx Positioning Board with arm supports and head holder. B. MammoRx Caliper with inclinometer. C. MammoRx Breast Setup Calculator, Printer and Case. D. MammoRx Simulation and Treatment Video. E. MammoRx Reference Manuals. F. MammoRx Miscellaneous Parts.

3 CONTENTS INTRODUCTION FOR THE PHYSICIAN ii PART 1. OVERVIEW 1 PART 2. SETUP PROCEDURES POSITIONING THE PATIENT FOR SETUP SETUP FOR TANGENT AND SUPRACLAVICULAR FIELDS Setup for Supraclavicular Fields Setup for Tangent Fields SETUP FOR TANGENT-ONLY FIELDS 15 PART 3. TREATMENT PROCEDURES POSITIONING THE PATIENT FOR TREATMENT TREATING THE PATIENT USING SUPRACLAVICULAR AND TANGENT FIELDS TREATING THE PATIENT USING TANGENT-ONLY FIELDS 21 APPENDIX A WORKSHEETS 22 A.1 BREAST SIMULATION WORKSHEET 23 A.2 PATIENT SETUP DIAGRAM - LEFT 24 A.3 PATIENT SETUP DIAGRAM - RIGHT 25 APPENDIX B ASSUMPTIONS 26 B.1 SETUP ASSUMPTIONS 26 B.2 GANTRY ANGLE CONVENTIONS 27 i

4 INTRODUCTION FOR THE PHYSICIAN Certain clinical decisions must be made well before the initial setup for treatment begins. Once you have decided to irradiate the breast, you must then decide whether it is necessary to also irradiate the internal mammary chain, the axillary apical nodes, and the supraclavicular nodes. It is of great value to ask the surgeons, with whom you work to place a metallic clip at the apex of their axillary dissections to assist you in designing the coverage of the axillary apex, when such coverage is required. If, in your judgment, it is not necessary to irradiate the supraclavicular nodes and the apex of the axilla, use the two-field technique (tangents only). This method not only is far quicker for both the patient and the technologist, but it also eliminates many of the potential sources of error and morbidity. If you have decided to treat the internal mammary (IM) nodes, you must choose whether this will be done as part of the tangential fields or as a separate direct anterior field treated with electrons and/or photons. Localization of these nodes always involves an element of uncertainty, and their treatment may add to the morbidity of the treatment, primarily depending upon how much lung and heart must be included in the field. The position of the sternum on the simulator films may be sufficient to give assurance of adequate coverage (Figure 1). A planning CT scan may be used for more accurate determination of the anatomy and to visualize enlarged nodes. If lymphoscintigraphy is available, it can be used to localize the AP projection of the nodes and their depth below the skin surface. Remember that, if the margin of coverage around the IM nodes is critical, the final determination of adequate dose delivery must be made from an appropriate isodose distribution with representation of relevant anatomic structures, especially heart and lung. ii

5 Note: The internal mammary nodes may or may not fall within the tangent fields depending on the individual patient anatomy and the amount of lung and heart you feel can be included safely within the field. iii

6 PART 1. OVERVIEW The MammoRx system provides a method of rapidly and comfortably preparing a patient for treatment of the breast or chest wall and regional nodes. The system does not include a slant board nor is a slant board recommended except in special cases for the reason illustrated in Figures 2A and 2B. A "beam-splitter" (central axis block) is used to define the inferior edge of the supraclavicular (SC) field. This forms a vertical plane which abuts with the superior edge of the tangential chest wall fields. After the SC field is set-up, the tangential fields are determined by measuring their separation and angle with a special digital caliper. These measurements, along with the estimated field width are entered into a computer that determines the set-up parameters, including the gantry angles and the necessary lateral shift of the table position from the set-up point on the medial border. The medial tangential field is set up according to the calculated parameters. A couch angle of 5 degrees is used to create a coplanar match of the superior edge of the tangential fields with the inferior edge of the SC field, and the collimator is adjusted to provide the necessary angulation along the chest wall. A bead chain is hung from the head of the simulator with its shadow projecting near the match line to define a vertical plane through the match line on the simulator films of the tangential fields. The projected shadow of the bead chain enables the construction of custom blocks to define this plane. The collimator and gantry angles of the lateral tangent are adjusted to provide a coplanar match of the deep borders of the opposed tangential fields, which is necessary to account for the opposing couch angulation of 5 degrees (10 degrees total). Day-to-day treatment is performed by setting up the SC field first and marking the match line at its inferior edge. The plane is remarked for each day's treatment. Day-to-day variation acts to wash out any dose inhomogeneity, but the daily juxtaposition of the SC and tangential field remains optimal. 1

7 The tangential fields are set up with reference to the tattooed "setup point" at the medial margin by using a lateral shift from this point to reach the treatment isocenter. Recorded gantry and collimator angles as well as couch angulation are used with custom blocks to block back the superior field margins to match the inferior edge of the SC field. During clinical trials of the MammoRx System, 80% of set-ups were considered clinically acceptable after only one set of simulator films, a major improvement over previous techniques at the testing facilities*. *Results will depend on site-specific circumstances. 2

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9 PART 2. SETUP PROCEDURES 2. 1 POSITIONING THE PATIENT FOR SETUP 1. Place the MammoRx board on the table with the pins for the arm and forearm supports snug against the table. Do not use he headrest yet. 2. Position the patient on the board with her arms by her sides and with her shoulder joint over the pivot of the arm support. 3. Straighten the patient on the table. 4. Check the straightness of the patient's position by aligning the sagittal laser or the field cross-hairs with her sternal notch and xiphoid. You may also use the fluoroscope to be sure she is lying as straight as possible. 5. Raise the patient's arm and place it into the arm support and wrist support. The MammoRx board assembly may be moved longitudinally along the table edge if necessary. 6. Adjust the arm support and shoulder to find a position that is comfortable for the patient but places her arm out of the way of the tangent fields. Make sure the upper arm is well cradled and not binding in the arm support. 7. Raise the patient's head and slip the head platform and headrest into place. Put the platform into the set of holes closest to the patient's head position. Choose a headrest that is comfortable but keeps the chin up. 8. Now invite the patient to move slightly in whatever way will make her feel straight, stable, and comfortable. 9. Make sure the patient's position is still satisfactory and the MammoRx arm and forearm support pins are snug against the table. Then record the head and arm positions from the board in the setup-instruction portion of the Breast Simulation Worksheet. A sample of this worksheet is provided in Appendix A. 10. Emphasize to the patient the importance of memorizing this position so that she can repeat it for each treatment. 4

10 2. 2 SETUP FOR TANGENT AND SUPRACLAVICULAR FIELDS Setup for Supraclavicular Fields 1. The physician should use the red marking pen supplied with the MammoRx system to put preliminary marks on the patient's skin to show the intended borders of the supraclavicular field: medial, lateral, superior, and especially the position of the intended match line at the inferior border of the supraclavicular field (the match line is where the supraclavicular and tangent fields abut). Note: The ink from the red marking pen is easily removed from the skin while the ink from the green marking pen is more lasting and less easily removed. 2. Set up the supraclavicular field at a target-skin distance (TSD) of 80 or 100 centimeters, as applicable for your treatment machine. Keep in mind that the field will be treated with a half-field (beamsplitter) block, as shown in Figures 2A and Rotate the gantry to an angle of 10 to 15 degrees to move the beam exit point off the spinal cord and esophagus. 4. Recheck the TSD and check the borders. Apply BB markers where any separate calculation points are desired for the supraclavicular node area and the axilla. 5. Use fluoroscopy or radiography to inspect the borders. The medial border should be off the spinal cord and the lateral border should be approximately at the humeral head, or as otherwise clinically indicated, depending on the degree of axillary coverage required (Figure 3). 6. Take a film for the patient's permanent record and have the physician inspect the film and the field projected on the patient. 7. When the borders of the field are acceptable to the physician, mark the field on the patient's skin with the green marking pen. Use the horizontal line formed by the laser isocenter lights and/or field light as a reference and mark the extension of the match line across the midline and well into the axilla. 5

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12 8. Take any necessary measurements, such as the TSD to the supraclavicular and axillary calculation points, and record the measurements on the Breast Simulation Worksheet. 9. If a beamsplitter block alone will not be adequate for the treatment, the physician should mark the additional blocking required on the supraclavicular field (Figure 3) Setup for Tangent Fields 1. The physician should use the red marking pen supplied with the MammoRx system to put preliminary marks on the patient's skin to show the intended medial, lateral, and inferior borders of the area to be included in the tangential fields. The superior border is the previously- marked match line. 2. Rotate the gantry to vertical and position the central axis of the beam on the intended medial border (Figure 4). 3. Set the field length to match the inferior mark and extend 3 to 4 centimeters superior to the match line (this extra length will be blocked later). Use a preliminary TSD of 77 or 97, as applicable for 80- and 100- centimeter TAD machines, respectively. Note: You should be aware of the maximum working length of any wedges you are using. Do not exceed these limits. 4. Mark the central axis on the patient's skin. The central axis is referred to as the AP setup point. 5. Estimate the required field width based on breast size (a moderate-sized breast requires a field width of approximately 10 centimeters). Set the field width in accordance with your estimate. 6. As an aid to block alignment on the tangential ports, tape BB markers along the match line. Start the BB markers at the medial border of the supraclavicular field and continue them into the axilla. Space the BB markers approximately 5 centimeters apart (Figure 5). 7. Apply a BB marker at the setup point and at the intended lateral border in the axial plane of the setup point. (Figure 5). 1 Contact Diacor Inc. for information on the Portalcast Block Cutting and Casting System. 7

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14 8. Use the provided caliper to measure (in a vertical plane) the distance between the setup point and the lateral border (SEPARATION) and the angle of the chord that connects these two points (CALIPER ANGLE) (Figure 6). 9. Enter the distance measurement into the calculator as SEPARATION. Enter the estimated field width into the calculator as FIELD WIDTH. Enter the chord angle into the calculator as CALIPER ANGLE. Enter the simulator/ accelerator target-axis distance (TAD) into the calculator as MACHINE TAD. Finally, indicate whether a left or right breast is to be treated by pressing the YES or NO button of the calculator. The calculator will then compute the following setup parameters: Set up TSD Shift Medial simulator angle Lateral simulator angle TSD= SHIFT= MED SIM ANG LAT SIM ANG= 10. Center the beam on the setup point and set the TSD to the value determined by the calculator. 11. Move the field central axis over the breast to be treated (Figure 4) by shifting the table laterally the distance determined by the calculator. 12. Mark the new isocenter on the breast. This is the anterior projection of the treatment isocenter. 13. Rotate the gantry to the medial angle determined by the calculator. The edge of the field should pass through the setup point. If the edge of the field misses the setup point by more than a few millimeters, recheck the field size and all measurements to make sure they are correct. If necessary, return to step Use isocentric table rotation to swing the patient's feet 5 degrees away from the source of the beam. The basis for 5 degrees rotation is explained in Appendix B. 15. Rotate the collimator to achieve the desired beam entrance line, particularly at the upper medial corner (Figure 7 and 8). At this point, the physician should inspect the field projected on the patient. If the field is not satisfactory to the physician, recheck field size and all measurements. If necessary, return to step 8. 9

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16 16. Make sure the amount of "flash" over the nipple is between 1 and 2 centimeters. If it is necessary to adjust the amount of flash, return to step 9, enter a new estimated field width in the calculator, and use new setup parameters. 17. Check the field length. The lower border should be on the intended inferior margin mark and the upper border should be at least 3-4 centimeters above the match line. If it is necessary to change the field length and it cannot be done symmetrically (keeping the same central axis position), return to step 3 and use a new field length with additional extension above the match line. 18. Hang the bead chain from the collimator head so that the shadow of the chain falls within 1 centimeter of the match line, but not overlying the match line. The chain must hang freely. The projection of the chain will show a vertical plane (parallel to the blocked edge of the supraclavicular field) during fluoroscopy and on the films (Figure 8). 19. Use fluoroscopy or radiography to verify that the BB markers along the match line are approximately parallel to the shadow of the chain. Make sure the medial and lateral entrance BB markers both lie upon the deep margin of the field, though they may be separated slightly due to the isocentric table rotation. Also check the amount of lung in the field, the depth of the upper medial corner, and any other clinically important factors. 20. When the physician is satisfied with the medial tangent, mark the field boundaries and the central axis with the green marking pen and record all setup parameters on the Breast Simulation Worksheet. 21. Take a medial tangent film. Use a straightedge to draw on the film the outline of a block that passes through the BB markers and is parallel to the chain (Figure 8). Note: The BB markers probably will not be perfectly aligned. Use the best fit. Do not use a curved line, because this block will define the edge of the beam that abuts the supraclavicular field in a coplanar manner (Figure 8). 22. Without moving the patient, rotate the gantry to the lateral angle determined by the calculator. 11

17 Note: Depending on your simulator, it may be necessary to move the patient laterally to allow sufficient clearance for the gantry. If it is necessary to move the patient, do so carefully with lateral table motion only, and return the patient to her original position after rotating the gantry. 23. Turn the collimator to the opposite (negative) of the angle used for the medial tangent (do not change the isocentric table position yet). The resultant field shows the exit line of the medial tangent. If this exit line is acceptable to the physician as the lateral entrance line, mark it with a dotted line. If the exit line is not acceptable, it is necessary to either return to step 1 and put new marks on the patient's skin or return to step 8 and take new measurements. Note: The most common correction necessary is to move the lateral entrance line either down (deeper) or up (shallower). Empirically, a +2-degree change in the chord angle moves the lateral entrance line approximately 1 centimeter deeper (and vice versa). However, it is still necessary to return to step 9. 12

18 24. Use the isocentric table rotation to swing the patient's feet to 5 degrees away from the source of the beam (10 degrees total). This will cause the projected lateral entrance line to move slightly. 25. Use a minor change in collimator angle to make the entrance line parallel with the entrance line marked in step Adjust the gantry angle until the two lines coincide (only a minor change in gantry angle should be necessary). 27. Record the new collimator and gantry angles on the Breast Simulation Worksheet. These are the final values to be used for treatment. 28. Hang the chain from the collimator head so that the shadow of the chain falls within 1 centimeter of the match line. (It may be necessary to suspend the chain from an IV pole or other temporary attachment point so that it does not rest against the simulator head and hangs freely.) 29. Use fluoroscopy or radiography to make the same checks specified in step Take a lateral tangent film and draw on the film the outline of a block as specified in step Mark the final portals on the patient's skin with the green marking pen and take AP and lateral Polaroid photographs for the patient's chart. 32. Tattoo the setup point, as shown in Figure 9. Make sure you tattoo the setup point rather than the midpoint of the medial entrance line (which is not exactly the same because of collimator rotation). The setup point may not lie exactly on the entrance line, but it is still the key point for future treatment. You may wish to tattoo other points as well (Figure 9). 33. Measure the separation from border to border and from the medial central axis to the lateral central axis, and record these measurements on the Breast Simulation Worksheet. 34. Take the patient's contour, by your usual method, for dosimetry. 13

19 35. Draw the Patient Setup Diagram (a sample is included in Appendix A) and fill in the required information. Mark tattoo placement on the diagram with an X. 36. Fabricate custom blocks. 2 2 Contact Diacor Inc. for information on the Portalcast Block Cutting and Casting System. This system is designed to allow custom blocks to be made quickly and accurately. 14

20 2.3 SETUP FOR TANGENT-ONLY FIELDS 1. The physician should use the red marking pen supplied with the MammoRx system to put preliminary marks on the patient's skin to show the intended medial, lateral, superior, and inferior borders of the tangent treatment area. 2. With the gantry vertical, put the central axis of the field on the midpoint of the intended medial entrance line. Use a preliminary TSD of 77 or 97 as applicable for 80- and 100- centimeter TAD machines, respectively. 3. Adjust the field length to match the superior and inferior marks. Note: You should be aware of the maximum working length of any wedges you are using. Do not exceed these limits. 4. Estimate the required field width based on breast size (a moderate-sized breast requires a field width of approximately 10 centimeters). Set the field width in accordance with your estimate. 5. Mark the central axis, which is referred to as the AP setup point. 6. Apply BB markers on the setup point and on the intended lateral entrance line in the axial plane of the setup point. 7. Use the caliper to measure (in a vertical plane) the distance between setup point and the lateral border (SEPARATION) and the angle of the chord that connects these two points (CALIPER ANGLE) (Figure 6). 8. Enter the distance measurement into the calculator as SEPARATION. Enter the estimated field width into the calculator as FIELD WIDTH. Enter the chord angle into the calculator as CALIPER ANGLE. Enter the simulator/accelerator target-axis distance (TAD) into the calculator as MACHINE TAD. Finally, indicate whether a left or right breast is to be treated by pressing the YES or NO button of the calculator. The calculator will then compute the following setup parameters: Set up TSD Shift Medial simulator angle Lateral simulator angle TSD= SHIFT= MED SIM ANG= LAT SIM ANG= 15

21 9. Center the beam on the setup point and set the TSD to the value determined by the calculator. 10. Move the field central axis over the breast to be treated (Figure 4) by shifting the table laterally the distance determined by the calculator. 11. Mark the new isocenter on the breast. This is the anterior projection of the treatment isocenter. 12. Rotate the gantry to the medial angle calculated by the calculator. The edge of the field should pass through the setup point. If the edge of the field misses the setup point by more than a few millimeters, recheck the field size and all measurements to make sure they are correct. If necessary, return to step Rotate the collimator to achieve the desired beam entrance line, particularly at the upper medial corner (Figures 7 and 8). At this point, the physician should inspect the field projected on the patient. If the field is not satisfactory to the physician, recheck field size and all measurements. If necessary, return to step Make sure the amount of "flash" over the nipple is between 1 and 2 centimeters. If it is necessary to adjust the amount of flash, return to step 8, enter a new estimated field width into the calculator, and use new setup parameters. 15. Check the field length. The upper and lower borders should be on the intended marks. If it is necessary to change the field length and it cannot be done symmetrically (keeping the same central axis position), return to step Use radiography to check the amount of lung in the field, the depth of the upper medial corner, the approximate coincidence of the medial and lateral entrance BB markers, and any other clinically important factors. 17. When the physician is satisfied with the medial tangent, mark the field boundaries and the central axis with the green marking pen and record all setup parameters on the Breast Simulation Worksheet. 18. Mark the central axis and take a film. 19. Without moving the patient, rotate the gantry to the lateral angle determined by the calculator. 16

22 Note: Depending on your simulator, it may be necessary to move the patient laterally to allow sufficient clearance for the gantry. If it is necessary to move the patient, do so carefully with lateral table motion only, and return the patient to her original position after rotating the gantry. 20. Turn the collimator to the opposite (negative) of the angle used for the medial tangent. 21. Check the lateral entrance line on the skin and use fluoroscopy to check field position. If both are satisfactory to the physician, mark and proceed. If unsatisfactory, return to step 1 and use new skin marks and/or measurements. Note: Empirically, a +2- degree change in the chest wall angle moves the lateral entrance line downward approximately 1 centimeter (and vice versa). However, it is still necessary to return to step Take a film and record the required data on the Breast Simulation Worksheet. 23. Mark the final portals on the patient's skin with the green marking pen and take AP and lateral Polaroid photographs for the patient's chart. 24. Tattoo as shown in Figure 10. Be sure that you tattoo the setup point, rather than the midpoint of the medial entrance line (which is not exactly the same because of collimator rotation). The setup point may not lie exactly on the entrance line, but it is still the key point for future treatment. You may wish to tattoo other points as well. 25. Measure the separation from border to border and from the medial central axis to the lateral central axis, and record these measurements on the Breast Simulation Worksheet. 26. Take the patient's contour by your usual method for dosimetry. 27. Draw the Patient Setup Diagram (a sample is included in Appendix A) and fill in the required information. Mark tattoo placement on the diagram with an X. 17

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24 PART 3. TREATMENT PROCEDURES 3.1 POSITIONING THE PATIENT FOR TREATMENT 1. Preset the headrest and arm supports at the positions established for this patient during the initial positioning steps. Make sure that the pins for the arm supports are snug against the table edge. 2. Have the patient lie down and put her head in the headrest. Then have her straighten and adjust her shoulders and hips for comfort. 3. Raise the patient's arm and place it into the arm and wrist supports. 4. Pivot the wrist support to find a position that is comfortable for the patient. Make sure the upper arm is well cradled, not binding in the support and is out of the way of the tangential fields. 5. Make sure the patient is straight on the table. 3.2 TREATING THE PATIENT USING SUPRACLAVICULAR ANDTANGENT FIELDS 1. To set up the supraclavicular field, use a TSD of 80 or 100 centimeters, as applicable for your treatment machine. 2. Rotate the gantry medially to an angle of degrees as determined during simulation. 3. Position the supraclavicular central axis and adjust the field size. 4. Put a half-field (beamsplitter) block or the patient's custom block in place. 5. Use the red marking pen to dot the match line across the field. Use either the lateral line laser or lateral table movement to continue the line well into the axilla. 19

25 Note: This line should be reapplied each day. Encourage the patient to remove the line between treatments. This will help to "feather" the match plane dose over the course of the treatment. 6. When you are satisfied with the setup, either take a film or begin the treatment, as appropriate. 7. After treating the supraclavicular field, remove the bearnsplitter, change the field size to the appropriate tangent values, rotate the gantry to vertical, and place the central axis on the tangent setup point at the proper TSD. 8. Shift the table laterally to the new isocenter. 9. Rotate the gantry to the medial tangent treatment angle. 10. Use isocentric table rotation to swing the patient's feet 5 degrees away from the collimator. 11. Rotate the collimator to the proper setting. 12. Put the patient's medial custom block in place. Make sure the blocked edge coincides with the previously marked match line. You may use up to 0.5 centimeter of longitudinal table movement to achieve the proper match line. No other movement (vertical, lateral, collimator, or gantry) should be used. If necessary, consult with the physician or your supervisor. 13. If the portal is within acceptable tolerances of the simulation tattoos, take a film or begin the treatment, as appropriate. Use a wedge or compensator, if appropriate. Note: The problem of acceptable tolerances has no easy answer. Obviously, perfection cannot be expected. Sometimes the setup for a thin, cooperative patient will not vary from the simulation more than a few millimeters over a period of weeks. Other patients offer greater challenges. Alignment along the match line must be done accurately, so a reasonable amount of longitudinal freedom is necessary. The lateral entrance line will be especially variable. It is better to be too careful rather than the opposite. 14. After treating the medial field, move the gantry to the specified lateral angle and move the collimator to the specified angle for the lateral tangent (which because of table rotation is not simply the negative of the medial collimator angle, as is the case for tangent-only treatment.) 20

26 15. Use isocentric table rotation to move the patient's feet to 5 degrees away from the collimator (a total of 10 degrees of movement). 16. Put the patient's lateral custom block in place. Make sure the blocked edge coincides with the previously marked match line. Again, a small amount of longitudinal movement is acceptable to achieve a match. 17. When the port is within acceptable tolerances and on the match line, take a film or begin treatment, as appropriate. Use a wedge or compensator, if appropriate. 3.3 TREATING THE PATIENT USING TANGENT-ONLY FIELDS 1. Set the field size and place the central axis on the setup point at the specified TSD. 2. Shift laterally the calculated amount. 3. Position the gantry and collimator at the predetermined angles for medial tangent. 4. If acceptable, either take a film or begin treatment, as appropriate. Use a wedge or compensator, if appropriate. 5. Without moving the patient, rotate to the lateral gantry angle and the negative of the medial tangent collimator angle. 6. If acceptable, take a film or begin treatment, as appropriate. If appropriate, use a wedge or compensator. 21

27 APPENDIX A WORKSHEETS The following three worksheets are included in this appendix to assist you in recording the necessary patient information. You may copy the worksheet and setup diagrams in whatever quantity you need to meet your departmental requirements, or you may use any portion of them to create your own. You may also purchase them from Diacor, Inc. in 50-page tablets. A.1 BREAST SIMULATION WORKSHEET A.2 PATIENT SETUP DIAGRAM - LEFT A.3 PATIENT SETUP DIAGRAM - RIGHT 22

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31 APPENDIX B ASSUMPTIONS B.1 SETUP ASSUMPTIONS Throughout the patient setup procedures we have made two assumptions which detract somewhat from the absolute accuracy of the geometry, but which we feel are justified for the sake of simplicity, reproducibility and avoidance of technologist error. The first assumption is that the angulation of the sternal region of the patient relative to horizontal and therefore the angulation of the collimator usually falls between 10 and 30º. Therefore, this angle was fixed in the equations at 20º(Figure B-1). Table B-1 shows the variation in the TSD, SHIFT, Medial Gantry Angle (MG) and Lateral Gantry Angle (LG) for different values of machine TAD, Separation (S), Field Width (W), Caliper Angle (A), and values of sternal angle (SA) from 0º to 30º. (These values are for a left breast). Note that significant changes in sternal angle result in relatively insignificant changes in TSD, etc. TABLE B-1 It is apparent from these calculations that there are no clinically significant changes in any of the important setup parameters within this range of sternal angles. 26

32 The second assumption is that the isocentric table angulation is fixed at 5º (we recommend you do this). Though this angle does not come into play in the equations for TSD, SHIFT, Medical Gantry Angle, and Lateral Gantry Angle, it does affect the coplanar match of the tangents with the supraclavicular field at the matchline. This angle is used to bring the source of the tangent fields into the match plane. As shown in Figure B-2, the magnitude of this angle depends upon the horizontal distance from the isocenter to the match plane, which is usually from 6 to 12 centimeters. This yields a range of angles from 3.5º to 7º for 100 cm machines and from 4.51º to 8.5º for 80 cm machines. We have chosen 5º as a reasonable approximation. However, you may wish to calculate and use an exact solution for each patient. The choice of 5º also recognizes that some tables may not have individual degree marks on the floor, whereas they will usually have 5º precision. B.2 GANTRY ANGLE CONVENTIONS The most common gantry angle conventions within radiotherapy are those of the Siemens (IEC) and Varian Corporation. The Siemens convention assumes that the machine indicates 0º when the beam is aimed at the floor and 90º when the machine aims at the left side of a supine patient. The Varian convention has 180º when the beam is aimed at the floor and 90º when the beam is aimed at the left side of a supine patient. Therefore, one can convert from the Siemens angles (S) to the Varian angles (V) through the equations (V) = (S), for 0 < (S) < 180 and (V) = (180 - (S)), for 180<(S) < 360 This conversion has been programmed into the MammoRx Breast Setup Calculator. Refer to the operating instructions provided with the MammoRx Breast Treatment System. Acknowledgments: 1. Siddon, R.L., Buck, B.A., Harris, J.R., Svensson, G.K.: Three-Field Technique for Breast Irradiation using Tangential Field Corner Blocks. Int. J. Radiation Oncology Biol. Phys., 9 : , Lichter, A.S., Fraass, B.A., Van de Geijn, J., Padikal, T.N.: A Technique for Breast Matching in Primary Breast Irradiation. Int. J. Radiation Oncology Biol. Phys. 9 : ,

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