Keywords
anorectal melanoma - PET-CT - abdominoperineal resection
Anorectal melanomas are infrequent cancers accounting for 0.05 to 2% of all colorectal
cancers.[1] Due to lack of a specific symptom, this type of cancer can be misdiagnosed as a
benign lesion of colorectal in patients with rectal bleeding. Conventional treatment
modality is a surgery that involves abdominoperineal resection (APR) or extensive
surgical excision.[1] Although advanced staging techniques suggest a course of action in the preoperative
period, the potential of frequent relapse and lack of a standard treatment approach
currently forces the surgeon to the preference of a more aggressive operation chain.
The 18F-fludeoxyglucose (FDG) PET/CT is an up-to-date approach in determining lymph
node involvement and distant metastasis at the same time. The preferred surgical approach
and lymph nodes involved are indicative for determining disease-specific survival.
Case Report
A 72 year-old male patient presented with rectal bleeding and generalized weakness.
Colonoscopy revealed a 5-cm ulcerovegetan mass lesion on the posterior wall of the
lower part of rectum. There were diverticular openings in the sigmoid and descending
colon. Inguinal lymph nodes were not palpable on clinical examination. Hemoglobin
was 8 g/dL, carcinoembryonic antigen (CEA) was 3 ng/mL, and human immunodeficiency
virus (HIV) test was negative. The biopsy specimen underwent histopathological examination
and the diagnosis of a melanoma was established with immuno-staining HBM-45 (+), CK7
(−), CK20 (−). No lymph nodes were detected on abdominal computed tomography (CT)
for staging purposes. Positron emission tomography-CT (PET-CT) revealed an increased
hypermetabolic density in the rectal area with intraluminal protrusion and 9-mm lymph
nodes in the right internal iliac, right common iliac, aortocaval, left paraaortic
areas with minimal metabolism. There were reactive lymph nodes with hilar fat measuring
22 mm at most without and FDG uptake in the bilateral inguinal fossae ([Fig. 1]). The patient underwent APR due to lack of distant metastasis. In the rectum specimen,
a 6 × 5 cm black vegetan mass lesion was observed invading the anal sphincter and
involving a 2-cm segment above the sphincter ([Fig. 2]). The pathological examination revealed the tumor tissue in the internal sphincter.
The radial and upper resection margins were intact. Reactive hyperplasia was observed
in the lymph nodes. The S-100 and hematoxylin eosin staining showed an increased mitosis
and increased number of nucleoli. There were pleomorphic cells containing melanin
pigment ([Fig. 3]). The result was reported as stage 1 spindle-cell malignant melanoma with polypoid
appearance (Slingluff's clinical stage).[2]
Fig. 1 The F-18 FDG-PET/CT revealed primary rectal tumoral lesion, and regional lymph nodes
were FDG (−). (a) Sagital CT, (b) Sagital fusion, (c) Axial CT and (d) Axial fusion
images of the primary hipermetabolic rectal malign melanoma (e) Axial CT and (f) Axial
fusion images of milimetric FDG(−) pelvic lymph nodes.
Fig. 2 Gross findings of the resected specimen. Polypoid blackish tumor was observed at
2 cm proximal to the dentate line.
Fig. 3 The S-100 and hematoxylin eosin staining showed increased mitosis and increased number
of nucleoli. There were pleomorphic cells containing melanin pigment.
Discussion
Anorectal melanomas arise from the dentate line of which 65% are located in the anal
canal or anal wedge.[3] These lesions are located in the first 6 cm in the anal canal. The most common symptoms
are rectal bleeding, pain, altered bowel habits, and prolapse of the mass. The lesions
are often polypoid and hyperpigmented and they may sometime become ulcerated, while
30% of the lesions are amelanotic. They may present with an epithelioid, spindle cell,
lymphoma-like and pleomorphic type.[1] Anal melanomas are often associated with the involvement of inguinal lymph nodes,
distant metastasis, and synchronous and metachronous adenocarcinomas. Tumor volume
and presence of obstructive findings are indications for surgery and APR is often
preferred in such cases.[4] The local relapse is especially frequent in tumors with the volume cutoff value
of ≥ 3.5 cm. Whereas lymph node involvement is higher in those measuring > 6 cm and
their rates of sphincter preservation is lower.[5] Compared with extensive surgical excision, APR offers favorable local management;
however, it does not improve overall survival.[6]
[7] In the literature, there are several but limited data showing a contribution to
survival in long-term follow-up. Tumor biology, presence of perineural invasion, and
lymphovascular involvement are the main determinants of prognosis. In particular,
PET-CT is the most commonly used method in delineating lymphatic involvement.[8]
A wider surgical approach was preferred in the present case due to the malignant characteristic
of the tumor, lymph node involvement in the PET-CT, and its localization in the lower
rectum. The advanced age of the patient would reduce his tolerance to reexcision and
response to radiotherapy.[5] It was considered that local control would be better achieved with APR. Besides,
the detection of tumor tissue in the internal sphincter showed that a correct surgical
preference was made. No relapse was detected during the 18-month follow-up of the
patient.
Today, the preference of APR or wide local excision was investigated in numerous studies,
and no difference was detected in 5-year survival analysis, whereas local relapse
rates were found to be 15.6% for APR and 64.7% for wide excision.[2] Abdominoperineal resection provides an improved local management, whereas adjuvant
chemotherapy is not effective and the disease is often resistant to radiation therapy.[5]
The comorbidity status of patient, postoperative complications, and permanent colostomy
status caused disadvantages for APR, while the tumor volume over 5 cm, lymphovascular
invasion, presence of lymph node involvement, distance to the anal surgical wedge,
and concerns about providing safe surgical margin keeps the APR on the agenda for
the surgeon.
Conclusion
Malignant melanomas are rare aggressive tumors of the rectum. Currently, PET-CT is
the most widely adopted modality in visualizing perirectal lymph nodes and screening
for distant metastasis to evaluate the patient status for the options of curative
surgery. Today, surgical approaches are still controversial. The histopathologic characteristics
and stage of tumor, overall surveillance, and prediction of quality of life are the
factors determining the preferences of the surgeon.
