Dosimetric Impact of Bladder Volume Variation in Radiotherapy for Prostate Cancer â A Pilot Study.

Background: In external beam radiotherapy of prostate cancer, the position of prostate is often affected by the bladder volume status due to their close anatomical relationship. This study aimed to evaluate the dosimetric impact of bladder volume deviation from the reference planning volume and to establish the acceptable limit of volume deviation in radiotherapy of prostate cancer. Methods: A total of 43 sets of CBCT images from prostate cancer patients treated by intensity modulated radiotherapy with full bladder were retrospectively recruited. The corresponding planning CT from these patients was retrieved from which the reference plans and 43 CBCT plans were generated respectively. The bladder volume in each plan was measured and the percentage difference of bladder volume between CBCT and planning CT (%d BV ) was calculated. The CBCT plans were stratified into 12 groups based on the magnitude of %d BV , which ranged from -75% to +75%. In each %d BV group, the dose parameters of the CBCT plans were compared with the corresponding reference plans. Results: The %d BV were ranged from -79.3% to +79.5%. The percentage differences of D 95% and V 95% of target volume between the CBCT and reference plans decreased significantly with increased %d BV . For the bladder, when the CBCT bladder volume was larger than the reference volume, increase of %d BV led to slight decrease of D mean , while when the CBCT bladder volume was smaller, the D mean increased dramatically with decreased %d BV. Conclusion: In radiotherapy of prostate cancer, increase in bladder volume from the reference planning volume led to target underdoes, while decrease in bladder volume increased the bladder dose. Keeping the treatment bladder volume within ± 20% of the reference volume can avoid unacceptable dosimetric outcome.