C Cheng*, X Zhang, B Liu, N Yue, R Parikh, Y Zhang, X Wang, Rutgers University, New Brunswick, NJ

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  C Cheng

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PO-GePV-T-375 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: Target coverage and OAR sparing deficiency may be more prone to inter-fractional anatomy change due to the fewer fraction nature of hypofractionation. This study quantifies inter-fractional anatomy variations and evaluates their dosimetric impact during prostate SBRT using image deformation technique.

Methods: Ten patients diagnosed with favorable intermediate risk prostate cancer were retrospectively included in this study. Total dose was prescribed to 36.25Gy in 5 fractions. Following guidelines from RTOG 0938, a CTV to PTV expansion of 5mm in all directions except for 4mm posteriorly were used. Patients were instructed to maintain a full bladder and empty rectum prior to CT simulation and each treatment. Localization was achieved through aligning to 3 fiducial markers in the prostate on daily CBCT. To evaluate the dosimetric impact from inter-fractional anatomy change on actual delivery, the body and OARs (bladder, rectum, penile bulb, femurs) were deformed to daily CBCT before performing forward dose calculations on each deformed CT. Inverse vector-field deformation was applied before creating a 5-fraction composite back onto the planning CT.

Results: A 4.32%±32.32%(4.58cc±81.28cc) inter-fraction volume variation was observed for bladder, and -4.06%±4.86%(-2.02cc±2.41cc) for rectum. Discrepancy was noted between fiducial marker/CTV-focused versus pelvic bone-focused rigid registrations. The translation misalignment ranged from 0.05mm-9.38mm, and rotational angles from 0.0°-3.71°. CTV coverage from the composite dose remained comparable, with D95% at 100.73%±1.29% of the prescription dose(Rx) for the original plan and 100.31%±1.48%Rx for the delivery. The dosimetric parameters of OARs, including D0.03cc, D3cc, D90%, D80%, and D50% of rectum, and D0.03cc, D90%, and D50% of bladder remained comparable to those in treatment plan.

Conclusion: In spite of the inter-fractional anatomical change (including, but not limited to, bladder filling, rectal emptying and bony anatomy change), the CTV coverage, the rectum and bladder sparing of the overall course of delivery remained similar to the planned condition.

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