Rare and Aggressive Primary Amelanotic Melanoma in Anorectal Region: A Case Series

Abstract

Anorectal amelanotic melanoma is a rare and aggressive disease with high morbidity and mortality. A conclusive diagnosis of anorectal amelanotic melanoma poses a challenge to pathologists and clinicians due to the protean nature of this entity, be it in its initial clinical assessment appearing as non pigmented polypoidal growths usually mistaken for haemorrhoids or anorectal polyps. Histomorphologically, these entities get categorised in bewildering array of diagnoses like spindle cell carcinoma, Gastrointestinal Stromal Tumour (GIST), High grade sarcomas and even lymphoma. Thus Immunohistochemistry (IHC) remains a vital tool for conclusive diagnosis. The purpose of the present case series is to discuss in detail about the three patients all aged above 55 years and clinically assessed with anorectal polypoidal growths. All three cases on histopathological evaluation were diagnosed as spindle cell neoplasm with no discernible melanin pigments. The IHC performed on all three cases turned positive for S-100 and Human Melanoma Black 45 (HMB 45). This case series highlights the challenging and bewildering nature of presentation of anorectal amelanotic melanomas, by virtue of its rarity and hence justifies the need for it to be considered as a possible differential diagnosis.