Primary extranodal non-Hodgkin’s lymphoma (NHL): the impact of alternative definitions tested in the Comprehensive Cancer Centre West population-based NHL registry

Abstract

The definition of primary extranodal non-Hodgkin's lymphoma (NHL) is a controversial issue, especially in patients where both nodal and extranodal sites are involved. The impact of different definitions of primary extranodal NHL on incidence and prognosis is explored using data from a population-based NHL registry. Using liberal criteria, 389 (34%) cases were classified as primary extranodal NHL. Overall survival (OS) rates of nodal and extranodal NHL patients defined this way were comparable; however, extranodal NHL patients had a better disease-free survival (DFS). When strict criteria were applied, 231 cases (20%) were classified as primary extranodal NHL. OS and DFS rates of extranodal NHL patients defined this way were superior to nodal NHL patients; however, the difference in OS was reversed after correction for differences in International Prognostic Index and malignancy grade. This study illustrates the selection bias that is introduced when a strict definition of primary extranodal NHL, that excludes cases with disseminated disease, is used. Patients with primary extranodal NHL were found to have a superior DFS, irrespective of which definition of primary extranodal NHL was used.