Lymphohistiocytic anaplastic large cell lymphoma involving skin: a diagnostic challenge.

Systemic anaplastic large cell lymphoma (ALCL) involving the skin should be differentiated from primary cutaneous CD30-positive T-cell lymphoproliferative disorders. The lymphohistiocytic variant of ALCL (LH-ALCL) is rich in reactive histiocytes with relatively few neoplastic cells, which pose a diagnostic challenge. We present a case of LH-ALCL involving skin mimicking granulomatous inflammation. A 30-year-old woman presented with cervical lymphadenopathy and multiple non-tender, non-itching, erythematous papules over the neck, chest, and abdomen. Biopsy of the cervical lymph node showed LH-ALCL with null cell phenotype. Microscopically, the cutaneous lesion was located predominately around the hair follicle, with numerous reactive histiocytes and scanty medium-sized lymphoma cells expressing CD30 and anaplastic lymphoma kinase (ALK) protein. Furthermore, an ALK gene rearrangement was demonstrated by locus-specific interphase fluorescent in situ hybridization, confirming cutaneous involvement with LH-ALCL. LH-ALCL involving the skin is a rare event, and the numerous reactive histiocytes may mask scanty tumor cells. In addition to B-and T-cell markers, (dermato) pathologists must be aware of this entity in cutaneous lymphohistiocytic proliferations and perform immunostaining for CD30 and ALK to reach a correct diagnosis.