Incidence of transformation to aggressive lymphoma in limited-stage follicular lymphoma treated with radiotherapy

The established treatment of limited-stage follicular lymphoma is radiotherapy (RT). There is an inherent risk of transformation of follicular lymphoma to aggressive lymphoma; however, the frequency and impact on the outcome are unknown in limited-stage patients. We identified 237 patients with limited-stage follicular lymphoma treated with curative intent RT. Cases were reviewed to determine the frequency of transformation and subsequent survival. With a median follow-up of 7.4 years, the 10-year risk of transformation was 18.5%. With a median follow-up after transformation of 4.7 years, the 3-year post-transformation progression-free survival (PFS) and overall survival (OS) were 42% and 44%, respectively. The addition of rituximab improved the 3-year post-transformation PFS and OS compared with combination chemotherapy alone (78% versus 15%, P < 0.00001) and (87% versus 38.5%, P < 0.00001), respectively. In multivariate analysis, only rituximab was associated with OS [HR 0.07 (95% CI 0.015-0.312, P = 0.001)] and PFS [HR 0.19 (95% CI 0.55-0.626, P = 0.007)] following transformation. There is a moderate risk of transformation in limited-stage follicular lymphoma treated with curative intent RT, and it substantially impacts outcome in these patients. Treatment with rituximab at the time of transformation appears to improve survival in this otherwise poor-risk population.