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 ([Formula: see text]), daily ([Formula: see text]), and delivered dose distributions ([Formula: see text]) 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. [Formula: see text] was well described by the median value of the [Formula: see text] 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 [Formula: see text] 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.