Clinicopathologic, immunophenotypic, and molecular cytogenetic fluorescence in situ hybridization analysis of primary and secondary cutaneous follicular lymphomas.

Although primary cutaneous follicular lymphoma (FL) is considered a distinct variant of FL in the World Health Organization classification ("cutaneous follicle center lymphoma"), its biologic relationship to nodal FL remains controversial. The clinical, morphologic, immunophenotypic, and molecular cytogenetic features of 17 patients with primary cutaneous FL were studied and compared with 16 patients with secondary cutaneous FL. The head and neck region was the most frequent site at initial skin presentation in both the primary and secondary cases. Among the primary cases, 29% of the 31 biopsies were grade 1, 48% grade 2, 13% grade 3, and 10% grade 3 with diffuse large B-cell (DLBCL) areas. Among the secondary cases, 38% of the 29 skin biopsies were grade 1, 45% grade 2, 3% grade 3, and 7% grade 3 with DLBCL areas with two not evaluable. A floral-like pattern was observed in 32% of primary FL but only 5% of secondary cases. Histologic progression was found in 21% of patients. CD10 expression was demonstrated in 90% (27 of 30) of primary cases and 96% (22 of 23) of secondary cases. Bcl-6 was expressed in all cases tested. Bcl-2 expression was detected in 57% (17 of 30) of the primary cases (100% of grade 1, 43% of grade 2, 40% of grade 3), whereas all secondary cases were bcl-2 positive (P=0.0002). The t(14;18) translocation was identified by interphase fluorescence in situ hybridization (FISH) in biopsies from 31% (4 of 13) of the patients with primary FL compared with 77% (10 of 13) of those with secondary lymphoma (P <0.05). Seven of the 17 (41%) patients with primary disease had cutaneous relapse, including 1 who also developed nodal disease. Bcl-2 positivity was seen in 4 of these 7 patients. Eight of the 16 (50%) patients with secondary FL had cutaneous relapse. Primary and secondary cutaneous FL share many clinical and phenotypic features, but primary cases may have some distinctive morphologic features, more frequently lack bcl-2 protein, and often lack the t(14;18) translocation. These findings suggest that primary cutaneous FL are distinctive and often but not always have a pathogenesis different from most of nodal and secondary cutaneous FL.