Anorectal melanoma is a rare and aggressive form of cancer that originates in the mucosal lining of the anus or rectum. This malignancy develops from melanocytes, the cells responsible for producing pigment, which are present in the skin and also in mucosal tissues. Compared to skin (cutaneous) melanoma, anorectal melanoma is exceedingly uncommon, accounting for approximately 0.4% to 1.6% of all melanoma cases and about 1% of all malignant tumors found in the anorectal region. It stands as the third most frequent site for melanoma development after the skin and eyes.
Symptoms and Risk Factors
The symptoms of anorectal melanoma can be non-specific, often leading to delayed diagnosis as they may be mistaken for more common, benign conditions like hemorrhoids. Rectal bleeding is the most frequently reported symptom, affecting a significant number of patients. Other common indicators include the sensation of a mass or lump in the anorectal area, localized pain, and persistent itching. Changes in bowel habits, such as new onset diarrhea, constipation, or incontinence, along with a feeling of incomplete bowel emptying, can also occur.
The causes of anorectal melanoma are largely unknown, and it is not associated with sun exposure, unlike cutaneous melanoma. This type of melanoma affects individuals over the age of 50 more frequently and shows a slight predominance in women. Some cases have been linked to specific genetic mutations, particularly in the KIT gene, which are found in an estimated 15% to 40% of mucosal melanomas. Mutations in the BRAF gene are less common in anorectal melanoma compared to skin melanoma.
The Diagnostic Process
Diagnosing anorectal melanoma begins with a thorough physical examination, including a digital rectal exam (DRE) to manually check for any masses or abnormal areas within the anus and rectum. The groin area is also examined for any swollen lymph nodes, which could indicate spread.
Following the initial examination, specialized endoscopic procedures are performed to visualize the affected area more closely. Anoscopy involves inserting a small, lighted tool to view the anal canal, while proctoscopy or colonoscopy uses a flexible camera to examine the rectum and parts of the colon. These visual inspections help locate the lesion and assess its characteristics. Endorectal ultrasound (ERUS) provides detailed images of the rectal wall and surrounding tissues to assess tumor depth.
A biopsy is the definitive method for confirming a diagnosis of anorectal melanoma. This involves removing a small tissue sample from the suspicious area, which is then examined under a microscope by a pathologist. Immunohistochemical stains are used to identify melanoma cells and differentiate them from other types of tumors. Up to one-third of anorectal melanomas may lack pigmentation, making visual identification challenging and highlighting the necessity of biopsy for accurate diagnosis.
Once a diagnosis is confirmed, imaging tests are used to “stage” the cancer, determining its size and whether it has spread to other parts of the body. Computed tomography (CT) scans provide detailed three-dimensional images, while magnetic resonance imaging (MRI) offers a more precise view of the local extent of the disease within the pelvis. Positron emission tomography (PET) scans can detect metabolically active cancer cells throughout the body, helping identify distant metastases. Staging systems classify the cancer based on its spread, categorizing it as local, involving nearby lymph nodes, or having spread to distant sites.
Available Treatment Approaches
Surgery is considered the primary treatment for localized anorectal melanoma, aiming to remove the tumor. Two main surgical approaches are employed, depending on the tumor’s size, location, and extent. Wide local excision (WLE) involves removing the tumor along with a margin of healthy tissue around it. This approach often allows for the preservation of sphincter function, avoiding the need for a permanent colostomy and generally leading to a quicker recovery. However, WLE can be associated with higher rates of local recurrence.
Abdominoperineal resection (APR) is a more extensive surgical procedure that involves removing the anus, rectum, and a portion of the colon, which necessitates a permanent colostomy. Current evidence suggests that APR may not significantly improve overall survival compared to WLE, although it can provide better local disease control and manage symptoms such as chronic bleeding or obstruction. The choice between WLE and APR depends on the tumor’s characteristics and the potential impact on the patient’s quality of life.
Beyond surgery, systemic therapies are used, particularly if the cancer has spread beyond the primary site. Immunotherapy utilizes drugs that stimulate the body’s own immune system to recognize and destroy cancer cells. These treatments are frequently considered as first-line therapy for unresectable or metastatic anorectal melanoma. Targeted therapy involves drugs that specifically attack cancer cells by interfering with particular genetic mutations or pathways that drive their growth, such as those for BRAF or KIT mutations. These therapies are selected based on the specific molecular profile of the tumor.
Traditional chemotherapy may also be an option for anorectal melanoma, though its role is generally more limited and often less effective compared to immunotherapy or targeted therapies. Chemotherapy can be used alone or in combination with other treatments to help control symptoms. Radiation therapy may be employed after surgery to decrease the risk of local recurrence or to alleviate symptoms in advanced cases. While melanomas were historically considered resistant to radiation, recent studies indicate that radiation can be effective for local control and palliation.
Prognosis and Follow-Up Care
The prognosis for anorectal melanoma is generally poor. The stage of the cancer at diagnosis plays a significant role; tumors that have spread to lymph nodes or distant organs have a less favorable outlook than those confined to the anorectal area. Tumor size and thickness are also considered prognostic indicators, with smaller and thinner tumors associated with better outcomes. The involvement of lymph nodes and the presence of perineural invasion are recognized as negative prognostic factors.
A key reason for the challenging prognosis is the frequent delay in diagnosis, as the symptoms often mimic more common conditions, leading to the cancer being identified at an advanced stage. The outlook is generally less favorable than for cutaneous melanoma.
Following treatment, regular follow-up care is important to monitor for any signs of cancer recurrence. This involves physical examinations, including checks of the skin and regional lymph nodes. Endoscopic procedures may be performed periodically to examine the treated area. Imaging scans are also used to detect any returning cancer or spread to other parts of the body. The frequency of these follow-up appointments and tests is tailored to the individual’s initial cancer stage and risk of recurrence, with more intensive monitoring in the first five years after treatment.