Prostate Position Variability and Dose-Volume-Histograms in Radiotherapy for Prostate Cancer with Full and Empty Bladder

Abstract

Purpose/Objective: Evaluation of prostate position variability and dose-volume-histograms in prostate cancer radiotherapy with a full bladder (FB) compared to an empty bladder (EB). Materials/Methods: Thirty patients underwent planning CT scans in supine treatment position and a slice thickness of 5mm with a full and an empty bladder before the beginning (CT1-FB/CT2-EB), after four weeks (CT3-FB/CT4-EB) and eight weeks (CT5-FB/CT6-EB) of radiation therapy. The data was transferred to a commercially available stereotactic planning system (Brainlab®, Heimstetten, Germany). Prostate, seminal vesicles, bladder, rectum, and bowel loops were contoured. The scans were matched by alignment of the pelvic bones. Displacements of the prostate/seminal vesicles centres of mass and organ borders were compared. Treatment plans were calculated using a six-field technique for CT1 (full bladder) and CT2 (empty bladder) and dose-volume-histograms for the organs at risk (rectum, rectum wall, bladder, bladder wall, bowel loops) compared for the primary and consecutive scans. Planning target volume (PTV) was defined as the prostate with seminal vesicles plus a 10mm margin, the posterior margin reduced to 5mm. Results: The prostate+seminal vesicle volume did not change during the course of radiotherapy (61±21cm3 vs. 60±20cm3; CT1/2 vs. CT5/6; mean±standard deviation), neither the ability of the patient to fill (203±132cm3 vs. 201±105cm3; CT1 vs. CT5) or empty the bladder (75±35cm3 vs. 79±38cm3; CT2 vs. CT6). The rectal volume decreased significantly (82±38cm3 vs.67±21cm3; CT1/2 vs. CT5/6; p<0.01). Compared to the primary scan, the volume of the full bladder for each patient varied more (9±106cm3) than the volume of the empty bladder (5±47cm3). However, the prostate/seminal vesicle center of mass (see table) and organ borders position variability was the same. Rectum volume variability was similar (−17±39cm3 with FB vs. −8±36cm3 with EB). A rectal volume >100cm3 in the planning CT scan was associated with a higher probability of prostate/seminal vesicle posterior displacement in the later course of radiotherapy (30% vs. 10% posterior displacement >5mm in the following CT scans with a primary rectal volume >100cm3 vs. ≤100cm3; p<0.01). With 192±44cm3 vs. 188±43cm3 the PTV was similar for the treatment plan with a full and empty bladder. The bladder volume treated with 90% of the prescription dose was significantly larger with an empty bladder (39±14% vs. 22±10%; p<0.01). Bowel loops received a maximum dose of >90% of the prescription dose in 37% (3% with full bladder; p<0.01). Dose-volume-histograms for the rectum were the same with a full or empty bladder. Histograms for the contours of the organs and the organ walls hardly differed. Conclusions: In spite of the larger variability of the bladder filling, prostate position stability is the same with a full compared to an empty bladder. The rectum volume has a major influence on the prostate position. An increased amount of bladder volume in the high dose region and a higher dose to bowel loops result in treatment plans with an empty bladder. Tabled 1Prostate displacement with full vs. empty bladder* mean ± standard deviation * mean ± standard deviation