Session Item

The aim of this study was to validate the clinical introduction of a planning target volume (PTV) margin reduction from 5 to 3 mm in MRI-guided online adaptive radiotherapy for prostate cancer. Safety was assessed by quantifying clinical target volume (CTV) coverage of the prostate gland taking intrafraction motion into account.

Twelve patients with prostate cancer were treated between August and October 2022 in our department with 5 fractions on a 1.5 T MR-linac (Unity, Elekta AB, Stockholm Sweden) using a 3 mm PTV margin for the prostate gland (total dose 36.25 Gy) and 5 mm PTV margin for the seminal vesicles (total dose 30 Gy).

For each fraction, patients were treated according to an adapt-to-shape (ATS) protocol, i.e. recontouring followed by a plan optimization. After the plan was ready, a position verification scan was made to verify whether the prostate had moved 2 mm or more. If this was the case, an additional adapt-to-position (ATP) plan adaptation was performed. During each fraction at least four 3D T2 weighted MRI images were acquired; the first for plan adaptation (pre), a second for position verification (PV) immediately before irradiation, and subsequent scans to analyse intrafraction motion during and after irradiation. In the analysis, for each fraction the daily adapted CTV is rigidly moved from the online plan image to all subsequent MR scans.  

The applicability of a PTV margin was quantified by calculating the overlap of the PTV from the pre or PV scan depending on whether ATS or ATS/ATP were used with the CTV on the during treatment MRI. In line with the van Herk margin recipe, the margin was considered acceptable if at least 98% of the CTV was overlapping with the PTV for 90% of patients.

In total 60 fractions of 12 patients were analysed. The range of CTV overlap in all fractions with the prostate PTV was 93-100% (Figure 1) and the applied margin yielded more than 98% averaged CTV coverage in 90% of patients. Patients 2 and 5 had an averaged CTV overlap of 97.5% and 98.2%, respectively. For the other 10 patients the CTV overlap was higher than 99%.

Figure 1: CTV overlap within PTV in percentage per fraction (in blue) and averaged per patient (in yellow). Fractions with an additional ATP are marked with dotted bars.

The applied 3 mm PTV margin for the prostate gland had at least a 97.5% averaged CTV overlap with the PTV in all patients. An additional ATP adaptation was applied in 32% of all fractions. By changing the margin the PTV volume was on average reduced by 25% (20 cc).

Based on intrafraction motion as observed in this patient cohort the 3 mm PTV margin expansion remained large enough to compensate for the observed intrafraction variations if combined with an additional ATP correction if a shift of the prostate exceeding 2 mm is observed in the PV scan. This indicates that the PTV margin reduction from 5 to 3 mm with online adaptive MR-guided radiotherapy on a MR-linac is safe to implement in the clinic.