Australian Society of Cytology

Case of the Month

December 2002 - Answer and Discussion

Small Cell Melanoma

A variant of melanoma that resembles small cell carcinoma of the lung or other round blue cell tumours.
Arises in pre-exisiting congenital naevus or at mucosal sites
Metastatic small cell melanomas have been documented in the literature at varying cutaneous and visceral sites.
The list of differential diagnoses is long and generally requires an immunohistochemical profile.

The cytological features of small cell melanoma:

Most of the cytomorphological features overlap with small cell carcinoma
There is no pathognomonic cytological feature of small cell melanoma
The smears are moderately cellular
The cells are small in size and have little or no cytoplasm
Dispersed cell population but a few cohesive groups may be present
Nuclear chromatin is finely divided
Nucleoli are often inconspicous
The may be nuclear moulding and overlapping
Smearing artefact may be seen
Melanin pigment may be present

DDx Skin/Cutaneous Sites

Naevi (cellular blue naevus)
Merkel cell tumour
Basaloid squamous cell carcinoma
Non- Hodgkin's lymphoma
Metastatic small cell carcinoma
Extraskeletal PNET/EWING
Malignant adnexal neoplasm

DDx Deep Sites

Primary/metastatic melanoma
Small cell carcinoma
Undifferentiated carcinoma
Embryonal rhabdomyosarcoma
Desmoplastic round cell tumour
Lymphoma
Small cell mesothelioma
Small cell carcinoma of the ovary with hypercalcaemia
Small cell carcinoma of the ovary of pulmonary type
Granulosa cell tumour
Carcinoid tumour
Neuroblastoma

Further History

Six weeks previously the patient presented with rectal bleeding. A large tumour was identified at the anterior wall of the rectum which was initially biopsied and then resected. Histology showed a round small cell neoplasm with some intracytoplasmic brown pigment. The differential diagnosis included: poorly differentiated carcinoma, small cell carcinoma, lymphoma and melanoma. Immunohistochemistry showed S-100 positive, HMB-45 positive, E1/AE3 negative, LCA negative.

CONCLUSION

A number of variants of melanoma exist: small cell, amelanotic, epithelioid, to spindle cell and biphasic types.
It is the greatest mimic in pathology and can resemble carcinoma, lymphoma and sarcoma, both at primary and secondary sites.
Primary and metastatic small cell malignant melanoma should be considered in the differential diagnosis of cutaneous and deep seated (visceral) small round cell tumours.
Definite diagnosis rests on clinical and cyto/histomorphological correlation and further ancillary studies.