Background: Although the WHO classification (2001) requires a great deal of morphologic, immunophenotypic, genetic, and clinical features for classifying lymphomas, it is still feasible to misdiagnose under limited resources, especially a limited panel of antibodies used for immunophenotyping. To identify pitfalls in classifying lymphomas among hematopathologist, general pathologists, and pathology residents under this situation.
Material and Method: Newly diagnosed lymphoma cases from 1 July 2002 to 30 June 2003 at Siriraj Hospital were included for two rounds of individually blinded review by a hematopathologist, two general pathologists, and three pathology residents. Final diagnoses were given by consensus. Pitfalls were determined from misdiagnosis, in each case analyzed in terms of frequency.
Results: One hundred and four lymphoma cases included 61 diffuse large B-cell lymphoma (DLBCL, 58.6%), 12 MALT lymphoma (11.5%), eight follicular lymphoma (FL, 7.7%), seven classical Hodgkin lymphoma (HL, 6.7%), four unspecified peripheral T-cell lymphoma (PTCL, 3.8%), three Burkitt lymphoma (BL, 2.9%), two subcutaneous panniculitis-like T-cell lymphoma (SPTCL, 1.9%), and seven other uncommon types (1% each). Pitfalls were low in frequency on diagnosis of DLBCL, nodular sclerosis HL, and SPTCL (8% each), but not different among the participants only in DLBCL. Pitfalls in diagnosis of MALT lymphoma, mixed cellularity HL, BL, unspecified PTCL, and FL were 60%, 50%, 33%, 29%, and 24%, respectively. However, considering hematopathologist and non-hematopathologist groups, pitfalls in the former were lower, especially in the uncommon types of lymphoma.
Conclusion: Pitfalls in classifying lymphomas are common. Interest in hematopathology reduces misdiagnosis in lymphomas other than DLBCL.

Keywords: Misdiagnosis, Pitfall, Lymphoma, Pathology, WHO Classification