Purpose: This study is an integrated efficacy analysis of the five clinical trials of tositumomab and iodine-131 tositumomab in patients with relapsed or refractory low-grade, follicular, or transformed low-grade non-Hodgkin's lymphoma (NHL) that resulted in the regulatory approval of the iodine-131 tositumomab by the US Food and Drug Administration.

Patients and methods: This integrated analysis included 250 patients. Patients received a single course of iodine-131 tositumomab. Responses were assessed by an independent panel of radiologists and oncologists.

Results: Response rates in the five trials ranged from 47% to 68%; complete response rates ranged from 20% to 38%. With a median follow-up of 5.3 years, the 5-year progression-free survival was 17%. Eighty-one (32%) of 250 patients had a time to progression of > or = 1 year (termed durable response population). For the durable response population, 44% had not progressed at > or = 2.5 to > or = 9.5 years and had a median duration of response of 45.8 months. The median duration of complete response was not reached. The durable response population had many poor prognostic characteristics, including bone marrow involvement (41%), bulky disease > or = 5 cm (49%), and transformed histology (23%). Forty-three percent of the patients had been treated with more than four prior therapies and 36% had not responded to their most recent therapy.

Conclusion: The tositumomab and iodine-131 tositumomab therapeutic regimen produces high response rates in patients with relapsed or refractory low-grade, follicular, and transformed low-grade NHL, with a sizable subgroup of patients achieving long-term durable responses.