(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To evaluate the impact on delivered dose to clinical target volumes (CTVs) in patients with high-risk prostate cancer from daily variations in bladder and rectal filling as determined from image-guidance cone-beam computed tomography (CBCT) scans.
Methods: Patients receiving 77.4Gy to the prostate, 54Gy to the seminal vesicles (SVs) and 45Gy to the pelvic lymph nodes (PLNs) in 1.8Gy/fraction via sequential boost and receiving daily CBCTs were included in this study. Deformable image registration was used to match the bladder, rectum, SVs and prostate from the planning CT to each daily CBCT. Contour based deformable dose accumulation was used to compare planed and delivered dose for each fraction.
Results: Seven patients completing course of treatment were included, totaling 301 fractions analyzed. Five of seven patients had considerable variation in bladder and rectal filling with mean volume normalized to the planning CT of 1.06 (SD: 0.28) for rectum and 1.01 (SD: 0.57) for bladder, compared to 0.95 (SD: 0.16) and 0.98 (SD: 0.13), respectively for the two patients with little variation in structure filling. This led to significantly reduced V100% coverage of the prostate, SV and PLN CTVs (p<0.02 for all) and lower minimum dose coverage for the SV and PLN CTVs (p<0.001) when analyzing all treatment fractions. These effects did somewhat average out over the course of treatment with average V100% for the prostate CTV of 98.2% (range between patients: 89.7-100%).
Conclusion: Daily variations in bladder and rectal filling significantly impacts target coverage for particular treatment fractions and patients with consistent filling have significantly less underdosing of target CTVs. Care should be taken to manage rectal and bladder filling especially as hypofractionated treatment regimens are becoming more common.
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