Spatial and dosimetric evaluation of residual distortions of prostate and seminal vesicle bed after image‐guided definitive and postoperative radiotherapy of prostate cancer with endorectal balloon

Purpose To quantify daily residual deviations from the planned geometry after image‐guided prostate radiotherapy with endorectal balloon and to evaluate their effect on the delivered dose distribution. Methods Daily kV‐CBCT imaging was used for online setup‐correction in six degrees of freedom (6‐dof) for 24 patients receiving definitive (12 RT def patients) or postoperative (12 RT postop patients) radiotherapy with endorectal balloon (overall 739 CBCTs). Residual deviations were evaluated using several spatial and dosimetric variables, including: (a) posterior Hausdorff distance HD post (=maximum distance between planned and daily CTV contour), (b) point P worst with largest HD post over all fractions, (c) equivalent uniform dose using a cell survival model (EUD SF ) and the generalized EUD concept (gEUD a with parameter a = −7 and a = −20). EUD values were determined for planned ( EUD SF plan ), daily ( EUD SF ind ), and delivered dose distributions ( EUD SF accum ) for plans with 6 mm (=clinical plans) and 2 mm CTV‐to‐PTV margin. Time series analyses of interfractional spatial and dosimetric deviations were conducted. Results Large HD post values ≥ 12.5 mm (≥15 mm) were observed in 20/739 (5/739) fractions distributed across 7 (3) patients. Points P worst were predominantly located at the posterior CTV boundary in the seminal vesicle region (16/24 patients, 6/7 patients with HD post  ≥ 12.5 mm). Time series analyses revealed a stationary white noise characteristic of HD post and relative dose at P worst . The EUD SF difference between planned and accumulated dose distributions was < 5.4% for all 6‐mm plans. Evaluating 2‐mm plans, EUD SF deteriorated by < 10% (<5%) in 75% (58.5%) of the patients. EUD SF accum was well described by the median value of the EUD SF ind distribution. PTV margin calculation at P worst yielded 8.8 mm. Conclusions Accumulated dose distributions in prostate radiotherapy with endorectal balloon are forgiving of considerable residual distortions after 6‐dof patient setup if they are observed in a minority of fractions and the median value of EUD SF ind determined per fraction stays within 95% of prescribed dose. Common PTV margin calculations are overly conservative because after online correction of translational and rotational errors only residual deformations need to be included. These results provide guidelines regarding online navigation, margin optimization, and treatment adaptation strategies.