Cumulative dose analysis following elective pelvis re-irradiation at junction with primary prostate radiotherapy from the OLIGOPELVIS GETUG P07 trial.

OBJECTIVE

Re-irradiation for pelvic lymph node recurrences at junction with primary prostate radiotherapy remains challenging due to the risk of adverse events from excessive cumulative doses. Clear dosimetric constraints for pelvic OAR are currently lacking in this situation.

METHODS

The OLIGOPELVIS GETUG P07 prospective Phase 2 trial evaluated elective nodal radiotherapy combined with androgen deprivation therapy in patients with oligorecurrent pelvic lymph nodes. We focused on patients with prior prostate radiotherapy. Biologically equivalent (EQD2) dose accumulation was performed using rigid and deformable image registration (RIR and DIR) in RayStation v12.A. Normal tissue complication probabilities (NTCPs) were estimated. To assess robustness, image registration and dose summation were repeated for 10 randomly selected patients using an alternative software (MIM 7.3.5).

RESULTS

Between 2014 and 2016, 33 patients with prior prostate radiotherapy were included. The median interval between courses was 52 months. Long-term grade ≥ 2 genito-urinary toxicity occurred in 12%, with no grade ≥ 2 gastro-intestinal toxicity. DIR versus RIR improved contour alignment, notably for bladder and rectum, with Dice similarity coefficients increasing from 0.3 to 0.8. Cumulative dose analysis revealed a broad range of doses, particularly in the bladder: median Dmean 53 Gy (IQR, 44-61 Gy), yet late toxicity showed no correlation with cumulative dose levels as in NTCP or time intervals. Cross-software comparison showed moderate absolute differences for DIR. A web-based tool, CASPER-RT (https://www.casper-rt.com/en/), was released to help clinicians compare individual patient doses to the OLIGOPELVIS GETUG P07 cohort.

CONCLUSIONS

Pelvic nodal re-irradiation at junction with prior prostate irradiation is feasible with acceptable long-term tolerance.