Primary Malignant Melanoma of Anorectum CT Findings: a Single Tertiary Center’s Experience

Abstract

Anorectal melanoma is a rare malignancy with very aggressive course. This case series emphasize the role of imaging, contrast-enhanced computed tomography in evaluating such cases before surgery. CT scan of four patients with pathologically proven primary anorectal malignant melanoma was included. CT findings were analyzed from the picture archiving and communication system for the site of involvement, morphological shape, presence of perirectal and anal infiltration, lymphadenopathy, liver metastasis, and occurrence of bowel obstruction for all the four patients. All the four patients presented as polypoidal mass that caused focal expansion and obscuration of rectal lumen without causing colonic obstruction. Perirectal infiltration was seen in 2/4 patients and involvement of anal canal was seen in 3 patients. Distant metastasis was seen in the liver in three out of four patients. In conclusion, primary anorectal malignant melanoma is a rare, highly aggressive tumor and should be considered in patients with a bulky intraluminal polypoid mass that does not cause colonic obstruction in the anorectal region with large-sized lymphadenopathy.

Introduction

Anorectal melanoma is a rare malignancy with an aggressive course [1]. Skin and retina are the most common location of malignant melanoma, followed by anorectum which is the third most common location among all the melanomas [2, 3]. The most common malignancy in anorectal region is adenocarcinoma or squamous cell carcinoma with incidence of anorectal melanoma only 0.5–4% of all anorectal malignancies [2, 3]. In cases where gastrointestinal tract is involved by malignant melanomas, most are blood-borne metastases, having predilection for stomach and small bowel because of their rich blood supply [4, 5]. However, the esophagus, anus, and, in exceptionally rare conditions, rectum can be involved by primary malignant mucosal melanoma [6, 7].

The most common presentation in patients with primary anorectal malignant melanoma (PARMM) is rectal bleeding, which may be mistaken for bleeding associated with hemorrhoids. In such patients, treatment strategy includes surgery, chemotherapy, and radiotherapy depending upon the staging of cancer. However, the tumor tends to show a poor response to chemotherapy and is considerably resistant to radiotherapy. As the disease is rare, there is no consensus on surgical approach; however, the choice of surgery in such patients can be in the form of wide local excision (WLE) or abdominoperineal resection (APR) or endoscopic mucosal resection (EMR) [8].

We present four rare cases of anorectal malignant melanoma (ARMM) presented to our institute with the aim to emphasize the role of contrast-enhanced computed tomography (CECT) findings in diagnosing such cases before surgery.

Case Series

Our first case is a 30-year-old male who presented to the Department of Surgical Oncology with chronic constipation, history of painful defecation, and rectal bleeding since 3 months. Digital rectal examination revealed growth 1 cm from the anal verge. Colonoscopy biopsy revealed circumferential growth 1 cm from the anal canal with feature suggestive of carcinoma rectum. CECT abdomen and pelvis done in the Department of Radiodiagnosis AIIMS Patna revealed an enhancing intraluminal polypoidal mass in the distal rectum just above the anal verge causing near-complete obliteration of the lumen with pelvic lymphadenopathy and liver metastasis (Fig. 1). The patient was taken for surgery and APR was done. The pathological specimen confirmed tumor localization at rectum and sigmoid colon with pathological type malignant melanoma.

Fig. 1.

CECT axial scan (a) and coronal reformatted image (b) showing an enhancing intraluminal mass completely filling the rectal lumen (thick arrow) just above the anal verge with perirectal lymphadenopathy (thin arrow)

Our second case is a 52-year-old male who presented with complaints of painful defecation and rectal bleeding since 5–6 months and mass coming out of the anus since 3 months. Digital rectal examination revealed growth 1 cm from the anal verge. Colonoscopy biopsy revealed growth in the anal canal and histological feature of poorly differentiated neoplasm. CECT done revealed polypoidal intraluminal mass with necrosis in the rectum extending proximally from the rectosigmoid junction to the anal verge distally with evidence of extramural extension of mass into perirectal fat planes on left side. There was loco-regional, para-aortic, and external iliac lymphadenopathy suggestive of nodal metastasis (Fig. 2). The patient was taken for surgery and APR done with 30-cm-length surgical specimen submitted along with para-aortic and pelvic lymph node dissection which revealed malignant melanoma high grade on pathology.

Fig. 2.

Primary anorectal malignant melanoma in a 52-year-old male. CECT coronal reformatted image (a) shows a bulky intraluminal polypoidal mass (thick arrow) that is focally expanding and obscuring the rectal lumen, perirectal infiltration (thin arrow), and axial CECT (b) scan shows left internal iliac lymphadenopathy (triangle)

The third case is a 55-year-old female with complaints of rectal bleeding and painful defecation since 5 months. Colonoscopy biopsy done from the growth in the anal canal and fine-needle aspiration cytology (FNAC) done from liver metastasis showed feature of malignant melanoma. On CECT, bulky, intraluminal protruding mass obscuring the bowel lumen in the rectum and the anal canal without colonic obstruction and liver metastasis was seen (Fig. 3). The patient was taken for surgery and APR was done.

Fig. 3.

CECT axial pelvis image (a) shows an intraluminal polypoidal mass in the proximal rectum (thick arrow) with perirectal infiltration and perirectal lymphadenopathy. CECT axial image abdomen (b) shows multiple enhancing lesions with target appearance (thin arrow) in the liver on arterial phase suggestive of metastasis

Our fourth case is a 49-year-old male with complaints of painful defecation, weight loss, and constipation. Colonoscopy biopsy done from the growth in the anal canal showed feature of malignant melanoma. CECT done revealed bulky, intraluminal protruding mass obscuring the bowel lumen in the distal rectum with perirectal infiltration and involvement of the anal canal. There was perirectal lymphadenopathy with multiple hypodense lesions in the liver suggestive of metastasis (Fig. 4). The patient is planned for APR.

Fig. 4.

CECT axial abdomen and pelvis (a) shows an intraluminal polypoidal mass in the proximal rectum (thick arrow) obscuring the rectal lumen with multiple liver metastasis (b, thin arrow)

Material and Method

In our study, four patients from January 2019 to June 2019 with pathologically proven primary malignant melanoma of the anorectum were included. Computed tomography (CT) findings were analyzed from the picture archiving and communication system (PACS) in the Department of Radiodiagnosis AIIMS Patna. The findings analyzed in the study were site of involvement, morphological shape, presence of perirectal and anal infiltration, lymphadenopathy, liver metastasis, and occurrence of bowel obstruction on CT scan.

In the study, noncontrast images were obtained from the lower thoracic level up to the pelvic region covering the entire abdomen, following which contrast=enhanced single phase portal venous (60–70 s) was performed for all the four patients. The images were acquired on a 256-slice dual source Siemens SOMATOM Flash CT scanner. All patients were administered 1 mL/kg of intravenous contrast material at an injection rate of 4–6 mL/s with rectal contrast in three patients.

Discussion

Melanocytes are cells derived from the embryological neural crest, which give rise to melanomas. Melanocytes tend to migrate to many sites throughout the body, primarily to the skin during the course of fetal development. Therefore, cutaneous melanomas comprising more than 90% of all melanomas are by far the most common form of the disease [9]. In the remaining (< 10%) melanomas, ocular melanoma accounts for 5%, melanoma of unknown origin for 2%, and mucosal melanoma for 1% [3]. The incidence of primary malignant melanoma of the anorectal region is relatively rare accounting for 0.4–1.6% of all the melanomas [10].

There are different imaging modalities to evaluate PARMM for staging and treatment response. MRI (magnetic resonance imaging) is the most commonly used imaging modality for local staging in anorectal malignancy. However, CECT scan can be used for characterization and extension, as done in our study. The current series in our study showed that PARMM most often manifested as a bulky intraluminal polypoid mass without causing any obstruction. All the four patients in our study presented as polypoidal mass that caused focal expansion and obscuration of rectal lumen without causing colonic obstruction. The findings in our series were similar to a study by Kim et al. [11] in which 88% of the patient presented as polypoidal mass. Such an appearance of polypoid mass is similar to that of primary esophageal malignant melanoma (PEMM), in which tumor tends to grow intraluminally along the longitudinal axis of the bowel, leading to the development of a polypoid mass [12].

Perirectal infiltration was seen in 2/4 patients and involvement of anal canal was seen in 3 patients. PARMM tends to arise from normal melanocytes in the intestinal epithelium of the anal canal distal to the pectinate line and can extend proximally into the rectum [13]. However, there are reports in the literature which suggested tumors arising from the rectal mucosa without evidence of a synchronous lesion in the anal mucosa [10, 11].

All the four patients in our study had evidence of lymph node metastasis on CT scan with perirectal lymph nodes, the most common site involved. Distant metastasis was seen in the liver in three out of four patients.

The treatment of choice for ARMM remains controversial, and surgery with complete resection represents the typical treatment. The surgery in the form of WLE is recommended in initial cases, whereas patients with extensive diseases is recommended to undergo APR.

The most common differential in cases of anorectal malignancy is adenocarcinoma of the rectum. Whenever a large intraluminal mass with regional lymphadenopathy and prominent perirectal infiltration is seen, adenocarcinoma of rectum should be considered [11]. However, because of the desmoplastic reaction, it frequently narrows the bowel lumen and causes proximal bowel obstruction thus differentiating it from PARMM [14]. The other rare tumors of anorectal region is lymphoma and gastrointestinal stromal tumor (GIST). Imaging features like location, polypoidal mass, lack of colonic obstruction, bulky lymphadenopathy, and early liver metastasis may be novel features facilitating the discrimination of PARMM from rectal adenocarcinoma and other rarer tumors like lymphoma and GIST [15].

Conclusion

In conclusion, PARMM is rare malignancy with highly aggressive course and should be considered in patients with bulky intraluminal polypoid mass, large-sized lymphadenopathy, early liver metastasis, and lack of colonic obstruction in the anorectal region. Hence, early diagnosis would improve the managements of such patients resulting in improved survival rate.

Author’s Contributions

PKS and JKP designed the study. PKS, JKP, and NA acquired the data. PKS, JKP, NA, and HK analyzed and interpreted the data. PKS drafted the article and JKP and NA revised it critically for important intellectual content. All authors read and approved the final version of the manuscript, and agreed to be accountable for all aspects of the work.

Compliance with Ethical Standards

Conflict of Interest

The authors declare that they have no conflict of interest.

Informed Consent

Informed consent was obtained from all individual participants included in the study.

Ethical Approval

Not Applicable.