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Pelvic lymph node (PLN) relapses of prostate cancer is a frequent scenario for which salvage pelvic radiotherapy (PLN-RT) is commonly proposed, even for patients with prior prostate radiotherapy (P-RT). Regarding the OAR in the pelvis, the management of the radiation field junction with prior P-RT remains an issue, since the dose accumulation may exceed the value commonly recommended for one radiation course. Our objective is to report on the dose accumulation for the pelvic OAR and the organ-related tolerance based on the prospective OLIGOPELVIS GETUG-P07 trial.

OLIGOPELVIS GETUG-P07 trial was a prospective multicenter phase 2 trial, whose efficacy and tolerance (CTCAE v4 and EORTC questionnaires) were already proven. The prescribed dose was 54 Gy/1.8 Gy fractions with up to 66 Gy/2.2 Gy fractions to the pathologic PLN. We focused on the patients with prior P-RT history. Based on international recommendations, following pelvic organs (bladder, rectum, sigmoid, ureter, lumbosacral plexus) were centrally delineated on both P-RT and PLN-RT CT scans. Rigid and deformable registrations were performed in order to enable physical dose summation. The quality of the registration was assessed by the calculation of Dice Similarity Coefficient (DSC). 

Thirty-three patients (30 with prior salvage prostatic bed and 3 with prior prostate conservative radiotherapy), were included. The median P-RT dose was 66 Gy and 76 Gy in 2 Gy fractions for prostatic bed and prostate radiotherapy, respectively. The median time interval with PLN-RT was 50 months. With rigid registration, median DSC were 0.3, 0.3, 0.2, 0.2 and 0.9 for bladder wall, rectal wall, sigmoid wall, ureter and lumbosacral plexus, respectively ; with deformable registration: 0.8, 0.8, 0.5, 0.8 and 0.9, respectively. Median DVH values within deformable registration were : bladder wall V65=43.5%, V50=66.5%, rectal wall V70=5.6% , V60=30%, V50=51%, sigmoid wall V40=43%, ureter D0.5cc=69 Gy, lumbosacral plexus D0.5cc=60 Gy. Compared to QUANTEC recommendations, this corresponded to median relative spread of: bladder wall V65= -13%, V50= +33%, rectal wall V70= -72%, V60= -14%, V50= +2%, sigmoid wall V40= +7.5%, lumbosacral plexus D0.5cc= +20%. With median follow-up of 62.5 months after PLN-RT, 5 patients (15%) had grade ≥2 toxicity, among them 2 had grade 3 urinary incontinence or obstruction. Neither ureter stenosis nor lumbosacral plexopathy were noted. No significant correlation was found between grade ≥2 toxicities and specific dose accumulation. Five patients received additionnal pelvic bone or nodal SBRT after PLN-RT, one of them developped grade 2 sigmoid stenosis and the other grade 2 sacral fracture.

Cumulative DVH at low dose (≤50Gy) exceeded the commonly recommended values for the pelvic OAR but without notable late toxicity, while standard limits were respected at higher dose. Our data aim to provide a prospectively-validated support for the dosimetric process of salvage PLN-RT following prior P-RT.