Chronic myelomonocytic leukemia (CMML) is a rare hematopoietic stem cell neoplasm. Indeterminate dendritic cell neoplasm (IDCN) is an extraordinarily rare histiocytosis that may manifest secondarily to CMML. A 75-year-old man with a 2-year history of CMML presented for multiple cutaneous lesions on his head and neck. Biopsy results yielded a dense diffuse infiltrate of large pleomorphic cells, which were positive for CD1a, S100, and CD56 with weak positivity for CD43 and CD68. Given his history of CMML, the patient was diagnosed with IDCN. This may indicate a progression of his CMML or transformation to acute leukemia; therefore, a systemic workup was recommended. IDCN may manifest secondary to a wide number of hematopoietic malignancies, with CMML being a rare occurrence. Recorded responses to phototherapy are reassuring, whereas systemic therapy may be appropriate for widespread cases. Remaining vigilant for cutaneous changes in patients with CMML will help prevent misdiagnosis and encourage prompt initiation of appropriate treatment.
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