Quality assurance of contouring and dosimetric consistency in the GETUG-AFU 31 salvage prostate stereotactic reirradiation trial.

[BACKGROUND AND PURPOSE] Salvage stereotactic body radiotherapy (SBRT) represents a promising approach for managing intraprostatic recurrence following radiotherapy. The multicenter phase I/II GETUG-AFU 31 trial evaluated 6 × 6 Gy SBRT, with delineation and dosimetric dummy runs performed to ensure consistent target and organ-at-risk (OAR) delineation and treatment plan quality across centers. We report the outcomes of this pre-accrual quality reassurance process.

[MATERIALS AND METHODS] Eighteen centers received CT, PET, and multiparametric prostate MRI (mpMRI) datasets of recurrent lesions and contoured the gross target volume (GTV), clinical target volume (CTV), prostate, bladder/rectum (whole and wall), penile bulb, and urethra + 3 mm. The Dice similarity coefficient (DSC) and average Hausdorff distance (HD avg) were calculated against an expert reference. Each center subsequently generated a 6 × 6-Gy plan on another anonymized dataset. Dose-volume indices were compared with the protocol limits. Major deviations were subsequently revised.

[RESULTS] Of the 18 centers, 5 (28 %) required resubmission. Variability was high for the GTV (median DSC: 0.63 [interquartile range {IQR}: 0.45-0.70; HD avg: >1.8 mm), CTV: (0.79 [0.71-0.82]; HDavg: >1.8 mm), and urethra: (0.10 [0.05-0.26]; HD avg: >3.6 mm). Larger structures showed higher agreement: bladder, 0.93; rectum, 0.85; prostate, 0.85. All revised plans satisfied constraints: CTV D > 38.1 ± 1.4 Gy; rectal-wall V averaged 0.33 ± 0.23 cc (≤1 cc), bladder-wall V averaged 0.88 ± 0.32 cc (≤2 cc); urethral D was 37.0 ± 1.0 Gy (≤39 Gy).

[CONCLUSIONS] The reproducibility of the target volume and urethral delineation is essential in the context of salvage radiotherapy, for which formal guidelines are anticipated.