Malignant melanoma of the anorectum is a rare malignant tumor with a very poor prognosis and was first reported by Moore (1857). It is the third most common site after the skin and the eyes, with about 70-90% occurring in the dentate anal canal and the rest in the perianal skin.
I. Etiology and incidence
The occurrence of malignant melanoma of the anorectum may be related to the following factors.
1, history of benign nevi: It is reported that about 65-84% of patients with this disease have a history of benign nevi.
2.HIV infection: The incidence of malignant melanoma is significantly increased in homosexual, bisexual male population and other people infected with HIV.
3.Solar irradiation: The incidence of this disease varies by latitude. It may be that solar irradiation causes a systemic melanization effect, which increases the susceptibility to melanoma in areas not directly exposed to carcinogenic light.
II. Pathology
Malignant melanoma of the anorectum often occurs below the dentate line of the anal canal or at the anal verge, in areas covered by squamous epithelium. The anorectal junction is a complex squamous epithelium with a large number of melanocytes. Melanocytes or their parent cells originate from neural crest cells in the ectoderm and migrate to the skin, mucosal surfaces of the eyes and the nervous system during embryonic development. Later, under the action of certain factors, such as hormonal metabolic disorders, chemical irritation or high-energy radiation damage, melanin malignancy can be caused.
Nowadays, it is considered that malignant melanoma of anal canal is primary. From the point of view of embryonic development, it is impossible for such lesions to appear in the small intestine and colorectal mucosa, which originate from the endoderm, and malignant melanoma of rectum is the result of the malignant development of melanocytes in the anal canal and infiltrate along the mucosa from bottom to top, so it should be regarded as metastasis. Most of the tumors can produce melanin, a few may not, and obvious melanin exists only in 20% of the cases.
Clinical manifestations
1.Prolapse symptoms There is a dark red mass prolapsing from the anus similar to thrombosed hemorrhoid impaction. In the early stage, it is small and can be retracted by itself. Later, it gradually increases in size, about the size of a walnut, and often has to be assisted by hand to return it after defecation. The percentage of those who have masses prolapsed during stool is 14-53%.
2.Blood in the stool Because the tumor is located in the rectum and anal canal, it is easy to be rubbed by feces or traumatized, which is similar to hemorrhoid bleeding. It is mostly fresh blood, sometimes it is also mucus and blood stool, or dark brown overflow, with foul odor. Clinical consultation with bloody stool reaches 23-73%.
3, rectal and anal canal irritation symptoms Patients have similar hemorrhoid attacks, anal swelling and discomfort, change in bowel habits, often a sense of incomplete defecation, sometimes alternating diarrhea, constipation, or even obstructed defecation, anal pain is mostly because the tumor has invaded the anal sphincter muscle.
4. Local protruding masses are seen, with outer shape like mycelium, short and wide tip, or nodular, generally 3-6cm in size, sometimes cauliflower-shaped, and most of them are purple-black or brown-black.
Diagnosis
The disease is often overlooked because it is clinically rare. Because of the lack of specific symptoms, it is easy to miss and misdiagnose. The initial diagnosis rate is very low, and the literature reports that the misdiagnosis rate reaches 87%. It is often misdiagnosed as prolapsed hemorrhoids, thrombosed external hemorrhoids, bleeding necrosis of anal canal polyps or rectal adenocarcinoma. In particular, non-pigmented malignant melanoma is more likely to be misdiagnosed because of its rarity.
1. Rectal palpation and proctoscopy. Anyone who has bleeding stool, change of stool habit, or swelling in the anus should be highly alert and carefully examined. Because 70-90% of this disease is located in the dentition and anal canal, and 50-70% contains pigment, rectal finger examination and proctoscopy have an extremely important role in diagnosis. For suspicious lesions, tissue biopsy should be performed. Pathological examination requires complete and thorough excision of the tumor in order to avoid the spread of medical origin.
2.Dopa and tyrosinase reaction. Because some anal canal malignant melanoma cell pulp does not contain pigment granules, Masson-Foulana melanin silver staining or dopa staining and tyrosine reaction should be performed when examining the tumor tissue in the anal canal area to improve the diagnosis rate.
3.Electron microscopy and immunohistological staining. For non-pigmented type, when the above methods cannot be confirmed, electron microscopic ultra-thin section histological examination can be used. Melanin vesicles of 400~476.1nm round or ovoid shape, or pre-pigmented material can be seen in the tumor cell pulp. In addition, immunohistochemical staining with S100 protein with waveform protein can be used for effective differentiation.
V. Treatment
The optimal treatment plan remains controversial, and most advocate a comprehensive treatment plan based on surgical treatment supplemented with chemotherapy, radiotherapy and immunotherapy, but radiotherapy is not sensitive. The choice between combined abdominal perineal resection (APR) or wide local excision (WLE) is highly controversial. All small samples of scattered reports showed no statistical difference in the effect of the two surgical approaches, APR versus WLE, on disease survival. The recommendation for APR is that.
1, APR provides control of metastases in the mesenteric lymph nodes.
2. achieving more extensive resection of the rectum and anal canal, resulting in more complete resection of the cancer at the margins
3, to obtain a lower local recurrence rate.