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A Comparative Study of Conventional Radiotherapy, Hypofractionation, and SBRT in Prostate Cancer Using Biological Effective Dose

M Sigler

M Sigler*, M Snyder, Wayne State University, Detroit, MI

SU-E-T-428 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Patient setup in SBRT treatments typically involves specialized immobilization devices to mitigate the greater risk of target under-dosing due to inter-fraction positional differences. However there is little evidence of their necessity for prostate treatments. Here we evaluate for prostate SBRT treatments the potential consequences of inter-fraction motion on biologically effective doses to prostate and normal tissue.

Methods: The original planning scan and daily CBCT scans for a patient treated with conventional prostate radiotherapy were used as simulated phantoms for this study. On the patient's original planning scan, a 2 Gy/fx VMAT plan was created in accordance with RTOG 0938. The plan was then transferred to each daily CBCT and recalculated to represent a daily "delivered" dose. BEDs for rectum, bladder and prostate were calculated for each day and summed to a full-course BED. The plan was then renormalized into hypofractionated (4.3Gy) and SBRT (7.25Gy) plans and recalculated on each CBCT. For the hypothetical SBRT and hypofractionation courses, each CBCT was viewed as a possible setup scan during the course of treatment. As such, all possible full-course BEDs for each tissue were found by selecting all possible combinations of 12 treatment days from 44 for the hypofractionated plan, and all possible combinations of 5 treatment days for the SBRT plan.

Results: Conventional treatment delivered a prostate dose of 159.3Gy over 44 fractions, versus an average of 175.1Gy and 182.2Gy over all possible hypofractionated and SBRT courses respectively. Conventional rectal doses were 80.8Gy, versus an average of 68.1Gy for hypofractionation and 61.8Gy for SBRT.

Conclusion: For every possible combination of fractions, SBRT rectal and bladder doses proved lower than their conventional counterparts, while prostate doses increased and remained consistent for all combinations. Intra-fraction motion notwithstanding, it appears that conventional immobilization techniques are sufficient for high dose per fraction treatments.

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