Practice Problems Week 4 2019

Dose Calculations – Week 4 – SAD Only A linear accelerator is calibrated to deliver 1cGy for 1 MU at 1.5cm depth, with an SSD of 100cm and a field size of 10x10cm. 1. If the output delivered at 100cm source to point distance at 5cm depth is 1.03cGy/MU. What is the output delivered at 200cm source to point distance, and a depth of 5cm? 2. What is the inverse square factor for a calculation point at 90cm SSD and a depth of 10cm? 3. If the output for a 35x35cm field at 1.5cm is 1.036cGy/MU with an SSD of 98.5cm, what is the output at 10cm depth with an SSD of 90cm [TMR(10cm)=0.809]? 4. How many MU are needed to deliver 200cGy at 5cm depth with a 10x10cm field at 95cm SSD [TMR(5cm)=0.918, Scp=1.000, Inverse Square Factor = ISF?]? 5. A patient’s treatment MU are calculated to deliver a dose of 200cGy at 100cm SAD at a depth of 10cm. The optical distance indicator (ODI) is mis-calibrated by 2cm, the resulting SSD = 88cm. Because of this discrepancy, what is the delivered dose to the 10cm depth prescription point? 6. Is the Mayneord F Factor needed when using Tissue Maximum Ratios? 7. The TMR for a 10x10cm 6MV photon field at 100cm SAD is 0.730 at 10cm depth. If the SSD were changed from 90cm to 95cm (but the depth remains the same), the expected change in the TMR would be: a. Increase 5% b. Increase 7.5% c. Decrease 5% d. Decrease 7.5% e. Zero 8. The dose rate at the patient’s midline, depth of 10cm, is found to be 300cGy/min at 100cm SAD. A protocol requires that the dose rate be no more than 100cGy/min. Therefore, the patient must be treated at what SSD? 9. For an SAD treatment with a 6MV x-ray beam, what is the MU setting to deliver 300cGy at the 100% isodose line for a 15x15cm field size at 10cm depth and 90cm SSD? [TMR(10) = 0.793, Sc=1.017, Sp=1.014, Dmax = 1cGy/MU @ 100cm SSD] 10. In regards to question #10, what is the given dose (dose at dmax)?

11. In regards to question #10, what is the dose at 20cm deep if TMR(20)=0.549?

Use the beam data tables, located on the last page of this homework assignment, to complete the remaining questions. The linear accelerator is calibrated to deliver 1cGy for 1MU at 1.5cm depth with a 10x10cm field size at 100cm SSD.

12. What is Sc, Sp, and Scp for a 10x15cm field? 13. How many MU are needed to delivery 180cGy at the 100% isodose line to a calculation point at 15cm depth for a 10x15cm field at 100cm SAD? 14. How many MU are needed to delivery 200cGy at the 95% isodose line to a calculation point at 5cm depth for a 5x12cm field at 100cm SAD? 15. How many MU are needed to delivery 200cGy at 100% isodose line to a calculation point at 7cm depth, and 3cm off axis, for a 12x12cm field at 100cm SAD? 16. What is the inverse square correction factor when your calculation point is at: a. Isocenter b. 5cm deeper than isocenter c. 5cm shallower than isocenter 17. How many MU are needed for the RLAT beam (from a RLAT/LLAT opposed pair field arrangement) if the prescription is 300cGy at the 97% isodose line is delivered to a calculation point 7.5cm deep for a 20x20cm field size, SAD of 100cm, and a 50/50 beam weighting? 18. How many MU are needed for a medial breast tangent field if the prescription is 180cGy (equally weighted between medial and lateral tangents) delivered at the calculation point, 10cm deep in a patient, 3cm off axis, at 100cm SAD, with the collimator setting at 10x15 and the blocked field size of 7.8x14 and normalized at the 100% isodose line? 19. A PA spine patient was planned to be treated at 5cm depth at 100cm SAD. What are the MU for a 5x12cm field if the prescription is 300cGy at 100% isodose line? 20. Regarding question #19, what is the dose delivered to the prescription point if the therapists set the patient up at 100cm SSD and a depth of 5cm; rather than 100cm SAD?