Exhibit Hall | Forum 5
Purpose: To assess daily dose for post-operative prostate cancer patients in correlation with bladder and rectum variations through use of daily iterative re-constructed cone beam CT (iCBCT).
Methods: A total of 14 prostate bed patients underwent iCBCT-guided patient setup and treated with volumetric modulated radiotherapy (VMAT) were identified (8 with rectal balloon and 6 without). Daily fractional dose was assessed for a total of 314 fractions (158 fractions in 8 and 156 fractions in 6). Bladder, rectal wall, femoral heads, and prostate bed CTV were contoured and verified on daily iCBCT for each fraction. Dose volume parameters of the contoured organs at risk (OAR) and CTV coverage were assessed for clinical impact of daily volume variation.
Results: The average difference between the maximum and minimum bladder volumes for each patient was 277.1cc. The daily bladder volumes varied from 62.4cc to 590.7cc, and ranged from 29% to 286% of the planning bladder volume for all patients. The bladder dose constraint of V65% <60% (with respect to daily bladder volume) was met in almost all fractions. CTVs (D90%, D95% and D98%) stayed consistently covered with no evident clinical correlation with absolute bladder volume daily variation, or the presence of the rectal balloon. There is statistical significance between CTV D90% vs. bladder volume and use of a rectal balloon, but no clinical significance. Patients with balloon had a smaller variation but a higher average maximum rectal wall dose (D0.03cc: 104.3% of the prescription) compared to patients without (103.3%).
Conclusion: The bladder volume variation showed no clinical impact on CTV coverage for post-operative prostate cancer patients treated with VMAT. It may not be necessary to delay treatment waiting for bladder fill when the patient is on treatment couch. The use of a rectal balloon does not provide clinically significant improvement towards inter-fractional CTV coverage.
Dose Volume Histograms, Image-guided Therapy, Target Localization