What is Anorectal Melanoma?
Melanoma is the most life-threatening form of skin cancer. The malignant change originates in melanocytes, the cells which produce the pigment melanin, present in skin, hair and eyes. The vast majority of malignancies arise on the skin (cutaneous melanomas) but they can also occur elsewhere on the body.
These extracutaneous melanomas include ocular, mucosal, nail bed, conjunctival, vaginal, urogenital, orbital, esophageal and anorectal malignant melanomas. Unlike cutaneous melanomas, these tumors are not associated with risk factors such as sun damage, family history or benign nevi (moles).
Diagnosis of Primary Malignant Melanoma of the Anal Canal
Anorectal melanoma is often mistaken for benign (non-malignant) disease, since it commonly presents with nonspecific symptoms such as bleeding, similar to that seen with hemorrhoids. Hence this can cause significant delays in diagnosis. Distinguishing these lesions from other types of cancer e.g. undifferentiated carcinoma, sarcoma and lymphomas can also be challenging for the clinician. Diagnosis is generally made from clinical symptoms together with cytological and histological data (tissue biopsy). Immunostaining of biopsy specimens for the markers S100 protein, HMB45 and vimentin can aid diagnosis of anorectal melanoma.
Incidence and Outcome
Primary malignant melanoma of the anal canal is a rare tumor, accounting for less than 3 percent of all melanomas. However, an increase in the incidence of anorectal melanoma has been recently reported in the USA. Anorectal melanoma generally affects the elderly with a slightly higher incidence in women.
Due to its rare occurrence, there have not been adequate numbers of clinical trials to test different treatment options. Therefore much of the currently available evidence is based on retrospective studies. Anorectal malignant melanoma is rare but extremely aggressive and has a very poor prognosis (outcome). Average life expectancy from diagnosis around two years, with fewer than 20 percent of patients showing five year disease-free survival.
Treatment of Anorectal Melanoma
Surgical treatment of this condition is a controversial topic. Although it has not been examined in clinical trials, abdominoperineal surgical resection has traditionally been the treatment of choice. However, in recent years many clinicians have come to the conclusion that there is no clear survival benefit from such extensive surgery. This has caused a trend towards more conservative surgery, and local excision is now seen as an acceptable approach.
In many cancers, surgical removal of the primary tumor is often accompanied by removal of nearby lymph nodes, as it can improve outcome. However, in primary malignant melanoma of the anal canal, it is not known if this is beneficial. Whether there are any benefits from radiotherapy and chemotherapy after surgery, is also unclear and controversial.
Treatment and outcomes of anorectal melanoma, A.Heeney, J.Mulsow and J.Hyland, Surgeon. 2011, Vol 9, Pages 27-32.
Extracutaneous malignant melanomas. M.Hussein, Cancer Investigation, 2008, Vol 26, Pages 516-34.
Anorectal melanoma: diagnosis and treatment, A.Meguerditchian, S.Meterissian, K.Dunn, Diseases of the Colon and Rectum, 2011, Vol 54, Pages 638-44.