[Clinicopathological features and prognosis of anorectal melanoma: A report of 68 cases].

Objective: To investigate the clinicopathological characteristics of anorectal mucosal melanoma (ARMM), and to evaluate the prognostic factors.

Methods: A total of 68 primary ARMM surgical specimens from 2010 to 2018 were retrospectively studied. Slides were reviewed to evaluate pathological features. Slingluff staging method was used for staging.

Results: (1) Clinical features: The median age at diagnosis in this group was 61.5 years, with a male-to-female ratio 1 ∶1.62. The most common complaint was blooding (49 cases). For anatomic site, anorectum was the prevalent (66.2%), followed by rectum (20.6%). At the time of diagnosis, 28 cases were stage Ⅰ (localized stage, 41.2%), 25 cases were stage Ⅱ (regional lymph node metastasis, 36.8%), and 15 cases were stage Ⅲ (distant metastasis, 22.1%). Five patients underwent wide local excision, the rest abdominoperineal resection, and 48 patients received adjuvant therapy after surgery. (2) Pathological features: Grossly 88.2% of the tumors were exophytic polypoid masses, with the median tumor size 3.5 cm and the median tumor thickness 1.25 cm. Depth of invasion below lamina muscularis mucosae ranged from 0-5.00 cm (median 1.00 cm). The deepest site of tumor invasion reached muscular layer in 27 cases, and perirectal tissue in 16 cases. Melanin pigmentation was absent or not obvious in 67.6% of the cases. The predominant cytology was epithelioid (45 cases, 66.2%). The rate for ulceration, necrosis, lymphovascular invasion, and perineural invasion was 89.7%, 35.3%, 55.9%, and 30.9%, respectively. The median mitotic count was 18/mm. The positive rate of S100, HMB-45 and Melan-A were 92.0%, 92.6% and 98.0%, respectively. The median of Ki-67 was 50%. The incidences of mutations within , and genes were 17.0% (9 cases), 3.8% (2 cases) and 9.4% (5 cases), respectively. (3) Prognosis: Survival data were available in 66 patients, with a median follow-up of 17 months and a median survival time of 17.4 months. The 1-year, 2-year and 5-year overall survival rate was 76.8%, 36.8% and 17.2%, respectively. The rate of lymphatic metastasis at diagnosis was 56.3%. Forty-nine patients (84.5%) suffered from distant metastasis, and the most frequent metastatic site was liver. Univariate analysis revealed that tumor size (>3.5 cm), depth of invasion below lamina muscularis mucosae (>1.0 cm), necrosis, lymphovascular invasion, gene mutation, lack of adjuvant therapy after surgery, deep site of tumor invasion, and high stage at diagnosis were all poor prognostic factors for overall survival. Multivariate model showed that lymphovascular invasion and gene mutation were independent risk factors for lower overall survival, and high stage at diagnosis showed borderline negative correlation with overall survival.

Conclusion: The overall prognosis of ARMM is poor, and lymphovascular invasion and gene mutation are independent factors of poor prognosis. Slingluff staging suggests prognosis effectively, and detailed assessment of pathological features, clear staging and genetic testing should be carried out when possible. Depth of invasion below lamina muscularis mucosae of the tumor might be a better prognostic indicator than tumor thickness.