Cytologic features of small cell melanoma.

Area of Science:

• Cytopathology
• Oncology
• Dermatopathology

Background:

• Small cell melanoma (SCM) is a rare, aggressive subtype of malignant melanoma (MM).
• SCM is infrequently documented in cytopathology literature, posing diagnostic challenges.
• Metastatic SCM can present at unusual sites, complicating diagnosis.

Purpose of the Study:

• To characterize the clinical and cytologic features of metastatic SCM.
• To discuss the differential diagnosis of SCM diagnosed by fine-needle aspiration (FNA).
• To highlight the utility of immunohistochemistry in SCM diagnosis.

Main Methods:

• Retrospective review of two FNA cases and one core biopsy/touch preparation of metastatic SCM.
• Analysis of clinical presentation, cytomorphologic features, and ancillary test results.
• Immunohistochemical (IHC) staining for melanocytic and neuroendocrine markers.

Main Results:

• Patients aged 69-85 years.
• Cytomorphology showed small, round blue cells with scant cytoplasm, high N:C ratios, dense chromatin, and inconspicuous nucleoli.
• Acinar arrangements and nuclear molding were observed in some cases.
• All cases were positive for SOX10 and Melan A; neuroendocrine marker expression varied.
• SCM cytomorphology differs from conventional MM, lacking typical melanoma features like melanin and macronucleoli.

Conclusions:

• Diagnosing metastatic SCM by FNA is challenging, particularly in patients with no prior melanoma history.
• Distinct cytomorphologic features, including acinar formations, differentiate SCM from conventional MM.
• Immunohistochemistry is crucial for confirming SCM diagnosis.