The dosimetric underpinning of bladder filling criteria for prostate image-guided volumetric modulated arc therapy

OBJECTIVES: Repeated CT simulation imaging is common at our institution due to failure to achieve acceptable bladder filling in patients undergoing prostate radiotherapy. There is operational value in re-assessing the validity of the bladder filling assessment criteria by comparing the quality of two plans optimized based on either an “Accepted” or “Rejected” bladder status. METHODS: Twenty prostate patients with repeated CT simulation imaging were included. For each patient, a VMAT plan created using the “Rejected” bladder was compared with the “Accepted” bladder plan. Then, delivered dose to bladder was estimated using ≥4 CBCTs to measure number of fractions with major bladderdose violation (>5% difference) for both plans. Bladder heights of fractions without major bladder dose violations were compared to those with a violation to determine a threshold height for bladder filling acceptability. RESULTS: Using the “Accepted” bladder plans for treatment resulted in 30/175 fractions with major dose violations. These 30 bladders were significantly shorter than those without major violation (mean 28 vs 43mm, p < 0.05). The “Rejected” bladder plans delivered a lower dose to the bladder by ≥5% than the ‘“Accepted” bladder plans in 59% of fractions, and the number of fractions with major dose violations was 17. CONCLUSIONS: Using a shorter bladder for plan optimization resulted in better bladder sparing during treatment and improved compliance to protocol specific bladder dose constraints. A bladder height range of 20–40 mm measured between the bladder dome and the superior aspect of the symphysis pubis is recommended for prostate radiotherapy requiring a full bladder protocol. ADVANCES IN KNOWLEDGE: Using real patient data from simulation and treatment, this study established a range of bladder height that can be measured easily in a clinical setting for assessing adequacy of bladder filling for prostate radiotherapy.