Primary cutaneous melanoma 90% Primary non cutaneous melanoma 10% Melanomas originate from melanocytes which are derived from embryological neural crest cells Primary cutaneous melanoma 90% Primary non cutaneous melanoma 10% Uveal & retinal Rectal Prostatic Urethral Sinonasal Metastatic

PRIMARY ANORECTAL MELANOMA Arise from dentate line, the transition zone & squamous zone On cross sectional imaging bulky, intraluminal, poypoidal or fungating masses in the distal rectum, focally expanding & obscuring the lumen No significant obstruction Perirectal infiltration peri rectal enlarged lymph nodes

MRI MRI local staging as extraluminal extent is better assessed MRI is helpful to detect melanotic component which is high on T1w due to free radicals of paramagnetic substances Other rectal lesions do not have melanin hence are low on T1 Exceptions occur as 10 to 30 % melanomas are amelanotic Melanomas are hypervascular & they may bleed . Acute bleed is also on T1w and they show avid enhancement on post contrast images

COMPUTED TOMOGRAPHY Contrast enhanced CT scan analyze gross extent of primary disease, not only extension into perirectal space but invasion of adjacent organs, involvement of regional and distant lymphnodes and distant mets

At the time of diagnosis 26 -38 % patients show metastasis Mets go to lymphnodes (mesenteric, inguinal, hypogastric, paraortic regions) Liver Lung Skin Brain

AMERICAN JOINT COMMISSION ON CANCER STAGING SYSTEM FOR ANORECTAL MELANOMA Stage IA localized 0.75 mm depth Stage IB localized 0.76-1.5 mm Stage IIA localized 1.5-4 mm Stage IIB localized > 4 mm Stage III regional nodes any depth Stage IV distant mets any depth

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