(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Patients receiving prostate cancer radiotherapy sometimes have difficulty following a full-bladder/empty-rectum protocol. The purpose of this study was to determine whether prostate patients still benefit from the full-bladder/empty-rectum protocol when online adaptive therapy on CBCT is used daily.
Methods: 10 prostate patients treated on our clinic’s Halcyon who had repeat CBCTs to ensure proper bladder/rectal filling were identified retrospectively. The first CBCT had inappropriate bladder/rectal filling, the second was acquired after the patient was given time to comply. The planned treatment was calculated on both CBCTs for each patient and was adapted and re-calculated on the first CBCT using the Varian EthosTM adaptive platform. The auto-segmented contours were manually edited in Ethos before adaptation as needed. DVH metrics for the CTV (Dmin,Dmax,Dmean), Rectum (V90%,V75%,V50%),and Bladder (V90%,V75%,V50%) were extracted for each scenario and compared using the average percent change (%?) in each and a Wilcoxon signed-rank test .
Results: As expected, re-calculating the scheduled plan on the second CBCT maintained tumor coverage while decreasing rectum and bladder dose for all metrics when compared to the scheduled plan on the first CBCT. Adapting the plan on the first CBCT increased tumor coverage significantly compared to delivering the scheduled plan on the second CBCT while maintaining Dmax: %?Dmin= 6.9%±4.6%(p<0.01), %?Dmean=1.4%±1.4%(p< 0.02), %?Dmax= 0.4%±0.8%(p<0.1). The 3 rectum metrics also significantly improved: %?V90%= 6.3%±7.0%(p<0.01), %?V75%= 7.1%±8.1%(p<0.05), and %?V50%= 10.2%±9.3%(p<0.02). However 2 of the 3 bladder metrics increased significantly: %?V90%= 3.0%±3.7%(p<0.05), %?V75%= 4.2%±4.8%(p<0.05), and % V50%= 7.3%±9.3%(p<0.1).
Conclusion: Daily adaptive therapy can improve tumor coverage and rectal sparing even when patients fail to adhere to empty-rectum protocols in any given fraction. However, a dosimetric benefit still remains for the full-bladder protocol in prostate.
Funding Support, Disclosures, and Conflict of Interest: Dr. Ray has a lab services agreement with Varian Medical Systems. Dr. Moore reports income for personal consulting and speaker's honoraria from Varian Medical Systems. This work was supported by an internal grant from the Center for Precision Radiation Medicine.
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