Positron emission tomography after response to rituximab-CHOP in primary mediastinal large B-cell lymphoma: impact on outcomes and radiotherapy strategies

End-of-treatment (EoT) PET/CT is used as a guide to omit radiotherapy (RT) patients with primary mediastinal large B-cell lymphoma (PMBCL). We present the mature and extended results of a retrospective study evaluating the prognostic significance of EoT-PET/CT after adequate response to R-CHOP. Among 231 consecutive PMLBCL patients, 182 underwent EoT-PET/CT and were evaluated according to the Deauville 5-point scale (D5PS) criteria. Freedom from progression (FFP) was measured from the time of PET/CT examination. Among 182 patients, 72 (40%) had D5PS score 1 (D5PSS-1), 33 (18%) had 2, 28 (15%) had 3, 29 (16%) had 4, and 20 (11%) had 5. The 5-year FFP was 97, 94, 92, 82, and 44% for D5PSS-1, D5PSS-2, D5PSS-3, D5PSS-4, and D5PSS-5, respectively. Among 105 patients with unequivocally negative PET/CT (D5PSS-1/D5PSS-2), 49 (47%) received RT (median dose 3420 cGy) and 56 (53%) did not with relapses in 0/49 vs. 4/56 patients (2 mediastinum and 2 isolated CNS relapses).The 5-year FFP for those who received RT or not was 100% versus 96%, when isolated CNS relapses were censored (p = 0.159). Among D5PSS-3 patients (27/28 irradiated-median dose 3600 cGy), the 5-year FFP was 92%. The 5-year FFP for D5PSS-4 and D5PSS-5 was 82 and 44%; 44/49 patients received RT (median dose 4000 and 4400 cGy for D5PSS-4 and D5PSS-5). Our study supports the omission of RT in a sizeable fraction of PET/CT-negative patients and definitely discourages salvage chemotherapy and ASCT in patients with PMLBCL who conventionally respond to R-CHOP, solely based on PET/CT positivity in the absence of documented progressive or multifocal disease. The persistence of positive PET/CT with D5PSS5 after consolidative RT should not trigger the initiation of further salvage chemotherapy in the absence of conventionally defined PD.