Abstract # 1068 In Vivo Verification of Dose Delivery to Prostate Cancer by Utilizing PET/CT Images Taken after Proton Therapy

Presenter: Lin, Liyong

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The PET‐determined proton beam path varied from the planned gantry and couch angles with a standard deviation of 1.7° and 1.3°, respectively. This difference is attributable to target position variation and set‐up accuracy. The PET‐determined treatment isocenter was 0.2 ± 0.4 cm posterior to, and 0.5 ± 0.4 cm superior to, the posttreatment CT isocenter, revealing that gradual bladder filling moves the prostate in an inferior‐anterior direction. Using the ischioanal and perivesical fat as surrogates for the in‐vivo position of rectal and bladder walls, the recalculated V74 were found to be 9.5% (9 ± 8.1 cc) and 14.6% (27.9 ± 14.7 cc), respectively, for all 36 cases. This is a variation of 8% to 284% of the planned volumes for rectal wall and 74% to 132% for bladder wall. Correspondingly, the V39 of rectal and bladder walls were 24.5% (23.3 ± 13.1 cc) and 28.4% (54.4 ± 20.4 cc) overall, respectively, reflecting a variation of 65% to 147% of the planned volumes for rectal wall and 49% to 183% for bladder wall.