Clinical Practice Evolvement for Post-Operative Prostate Cancer Radiotherapy—Part 1: Consistent Organs at Risk Management with Advanced Image Guidance

Abstract

Purpose: Post-operative prostate cancer patients are treated with full bladder instruction and the use of an endorectal balloon (ERB). We reassessed the efficacy of this practice based on daily image guidance and dose delivery using high-quality iterative reconstructed cone-beam CT (iCBCT). Methods: Fractional dose delivery was calculated on daily iCBCT for 314 fractions from 14 post-operative prostate patients (8 with and 6 without ERB) treated with volumetric modulated radiotherapy (VMAT). All patients were positioned using novel iCBCT during image guidance. The bladder, rectal wall, femoral heads, and prostate bed clinical tumor volume (CTV) were contoured and verified on daily iCBCT. The dose-volume parameters of the contoured organs at risk (OAR) and CTV coverage were assessed for the clinical impact of daily bladder volume variations and the use of ERB. Minimum bladder volume was studied, and a straightforward bladder instruction was explored for easy clinical adoption. Results: A “minimum bladder” contour, the overlap between the original bladder contour and a 15 mm anterior and superior expansion from prostate bed PTV, was confirmed to be effective in identifying cases that might fail a bladder constraint of V65% <60%. The average difference between the maximum and minimum bladder volumes for each patient was 277.1 mL. The daily bladder volumes varied from 62.4 to 590.7 mL and ranged from 29 to 286% of the corresponding planning bladder volume. The bladder constraint of V65% <60% was met in almost all fractions (98%). CTVs (D90%, D95%, and D98%) remained well-covered regardless of the absolute bladder volume daily variation or the presence of the endorectal balloon. Patients with an endorectal balloon showed smaller variation but a higher average maximum rectal wall dose (D0.03mL: 104.3% of the prescription) compared to patients without (103.3%). Conclusions: A “minimum bladder” contour was determined that can be easily generated and followed to ensure sufficient bladder sparing. Further analysis and validation are needed to confirm the utility of the minimal bladder contour. Accurate dose delivery can be achieved for prostate bed target coverage and OAR sparing with or without the use of ERB.

Target coverage and organs at risk dose in hypofractionated salvage radiotherapy after prostatectomy

“…While we did not use any rectal preparation protocol, we observed a relatively small rectal volume change (median of −10%), compared to studies utilizing strict rectal preparations [16] , [24] , [27] , [28] . For instance, a study on 18 patients undergoing ultra-hypofractionated SRT (5 fractions) instructed patients to use an enema the night before and the morning of each fraction.…”

“…Moreover, change in bladder volume may not only affect target coverage, but our results also showed that bladder volume change can lead to increased bladder dose, subsequently raising the risk of toxicity [10] , [18] , [19] , [20] , [21] , [22] . Several studies have explored alternative bladder preparation techniques and ultrasound scanning before treatment, all concluded that the current methods offer limited reproducibility of bladder filling [6] , [16] , [17] , [22] , [23] , [24] . As such, a universally effective bladder filling protocol remains elusive.…”

“…A prospective study on the use of antiflatulent medication in 78 patients, found no difference in rectal size nor toxicity [28] . Furthermore, a study on 314 CBCT from 14 patients showed that using endorectal balloon (N = 8) resulted in higher rectal wall dose compared to patients treated without endorectal balloon (N = 6) [24] . Another study reported stable rectal volumes during moderately hypofractionated (20 fractions) radiotherapy to the prostate without any rectal preparation [29] .…”

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