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

Abstract

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 plan SF ), daily (EUD ind SF ), and delivered dose distributions (EUD accum SF ) 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 accum SF was well described by the median value of the EUD ind SF 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 ind SF

Comparison of Online-Onboard Adaptive Intensity-Modulated Radiation Therapy or Volumetric-Modulated Arc Radiotherapy With Image-Guided Radiotherapy for Patients With Gynecologic Tumors in Dependence on Fractionation and the Planning Target Volume Margin

“…All accumulated dose distributions for series with PTV margins of 5 mm showed only small deviations, with %gEUDCTV values above −5%. A similar observation has been made for prostate cancer . However, with a PTV margin of 0 mm, deviations of the gEUD from the accumulated dose distribution occurred at a higher frequency of 40% of the treatment series, with the percentage deviation of gEUD values for CTV below −5%.…”

“…A similar observation has been made for prostate cancer. 17 However, with a PTV margin of 0 mm, deviations of the gEUD from the accumulated dose distribution occurred at a higher frequency of 40% of the treatment series, with the percentage deviation of gEUD values for CTV below −5%. The online ART mode is more important the smaller the PTV margin and the lower the number of delivered dose fractions.…”