Definitive radiotherapy for localized follicular lymphoma staged by 18F-FDG PET-CT: a collaborative study by ILROG

Radiotherapy (RT) can be curative in patients with localized follicular lymphoma (FL), with historical series showing a 10-year disease-free survival of 40-50%. As 18F-FDG PET-CT upstages 10-60% of patients compared to CT, we sought to evaluate outcomes in patients staged by PET-CT, to determine if more accurate staging leads to better patient selection and results. We conducted a multicenter retrospective study. Inclusion criteria were: RT alone for untreated stage I-II FL (grade 1-3A) with dose equivalent ≥24 Gy, staged by PET-CT, age ≥18 years, and follow up ≥3 months. Endpoints were freedom from progression (FFP), local control, and overall survival (OS). FFP and OS were estimated with Kaplan-Meier, and uni- and multivariable analyses of prognostic factors performed with Cox Regression. 512 patients treated from 2000-2017 at 16 centres were eligible for analysis. Median age was 58 years (range 20-90). 410 patients (80.1%) had stage I disease. Median RT dose was 30 Gy (24-52). Median follow up was 52 months (3.2-174.6). 5y-FFP and OS were 68.9% and 95.7%. For stage I, 5y-FFP was 74.1%, vs 49.1% for stage II (p<0.0001). 8 patients relapsed infield (1.6%).4 had marginal recurrences (0.8%) resulting in local control rate of 97.6%. On multivariable analysis, stage II (HR=2.11, 95%CI=1.44-3.10) and BCL2 expression (HR =1.62, 95%CI 1.07-2.47) were significantly associated with less favorable FFP. Outcome after RT in PET-CT staged patients appears to be better than in earlier series, particularly in stage I disease, suggesting that the curative potential of RT for truly localized FL has been underestimated.