Reducing PTV Margins with Daily Adaptive Radiotherapy to the Prostatic Fossa

Abstract

<h3>Purpose/Objective(s)</h3> In modern trials, traditional planning target volume (PTV) margins for postoperative prostate radiotherapy have been large (7-10 mm) to account both for daily changes in patient positioning / motion and target deformation (from changes in adjacent bladder fill and/or rectal distension). With daily adaptive radiotherapy, these inter-fractional changes could be minimized, potentially reducing the margins required for treatment and improving adjacent normal tissue dosimetry. <h3>Materials/Methods</h3> A single-center retrospective study was conducted from 03/2021 to 11/2021. Patients receiving conventionally fractionated radiotherapy to their prostatic fossa +/- elective nodes with pre- and post-treatment CBCT imaging (CBCTpre and CBCTpost, respectively) were included. Clinical target volumes (CTV) were contoured by a single observer on all CBCTs (only the prostatic fossa CTV was analyzed in this study). CBCTpre and CBCTpost images were registered using the same shifts used on the day of treatment. Motion was calculated from the center of mass of CBCTpre to that of CBCTpost, which was used to determine predicted margins with Van Herk's formula. Adequate coverage with the proposed margins was verified by performing a symmetric expansion on all CBCTpre scans (PTVpre) and evaluating the overlap with CBCTpost. <h3>Results</h3> Twenty patients were included with 263 image sets available and with 248 image sets included after exclusion. The average time between scans was 10.7 ± 3.0 minutes. The estimated margins predicted to achieve ≥95% coverage for 90% of the population were 1.6 mm, 2.0 mm, and 2.2 mm (x, y, and z axes, respectively). After symmetrically expanding all CBCTpre's CTVs by 1mm, 2mm, 3mm, and 4mm, the percent of image sets achieving ≥95% coverage were 73.6%, 92.2%, 97.1%, and 98.8%. <h3>Conclusion</h3> Using daily adaptive radiotherapy could significantly decrease the required PTV margin expansions for postoperative prostate radiotherapy, as a symmetric expansion of 3 mm achieved excellent coverage in this cohort which compared favorably to historical data. Given these results were obtained from CBCTpre and CBCTpost scans in the non-adaptive setting, these results will need to be validated in a prospective cohort.