Minimal residual disease monitoring in early stage follicular lymphoma can predict prognosis and drive treatment with rituximab after radiotherapy.

Since 2000, we have investigated 67 consecutive patients with stage I/II follicular lymphoma (FL) for the presence of BCL2/IGH rearrangements by polymerase chain reaction (PCR), real time quantitative PCR (RQ-PCR) and digital droplet PCR (ddPCR). All patients were treated with involved-field radiotherapy (IF-RT) (24-30 Gy). From 2005, patients with minimal residual disease (MRD) after IF-RT received rituximab (R) (375 mg/m2 , 4 weekly administrations). The median follow-up is 82 months (17-196). At diagnosis, 72% of patients were BCL2/IGH+. Progression-free survival (PFS) was significantly better in patients with undetectable/low levels (<10-5 ) of circulating BCL2/IGH+ cells at diagnosis and in those who were persistently MRD- during follow-up (P = 0·0038). IF-RT induced an MRD- status in 50% of cases; 16/19 (84%) MRD+ patients after IF-RT became MRD- after R treatment. A significantly longer PFS was observed in MRD+ patients treated with R compared to untreated MRD+ patients (P = 0·049). In early stage FL, both circulating levels of BCL2/IGH+ cells at diagnosis and MRD status during follow-up bear prognostic implications. Standard IF-RT fails to induce an MRD-negative status in half of patients. Most patients become MRD- following treatment with R and this is associated with a significantly better PFS.