We present two rare cases of in situ mantle cell lymphoma ( in situ MCL ) and three cases of MCL with mantle zone growth pattern (MCL-MZGP). The patients include four males and one female, with a median age of 66 years (range, 52 to 86 years). Two present with isolated lymphadenopathy and three with multiple lymphadenopathy. At presentation, the complete blood count (CBC) and serum lactate dehydrogenase (LDH) are normal in all cases. Histologic examination reveals an in situ pattern in two cases and a mantle zone growth pattern in three cases. The staging bone marrow biopsies show minimal involvement by lymphoma in one case and no morphologic evidence of lymphoma in four cases. All cases are positive for cyclin D1, including two with typical MCL phenotype and three with CD5-negativity. Four out of five cases express kappa light chain. FISH study for t(11;14) is performed in three cases, of which one is positive and two are inconclusive. For four patients with a median follow-up of 38 months, three are in clinical remission and one has persistent disease. In conclusion, the in situ MCL is associated with incidental finding, indolent clinical course and lower tumor burden. The predominant usage of kappa light chain and frequent CD5-negativity observed in our cases are unusual. We review the clinical and laboratory features of in situ MCL cases in the literature
