Myeloid sarcoma of the testis

Acute myeloid leukaemia (AML) can involve the testis as a site of disease recurrence or as a site of tissue involvement at the time of leukaemia diagnosis. AML may mimic primary testicular neoplasms or testicular involvement by metastatic disease processes. It is therefore relevant to utilize ancillary testing, such as immunohistochemistry, cytochemistry, flow cytometry and cytogenetics, to aid in this potentially difficult differential diagnosis. We describe a patient with primary testicular presentation of monocytic sarcoma with rapid evolution to acute monocytic leukaemia (AML-M5b), initially thought to represent a seminoma.