Residual anorectal malignant melanoma presenting as polyp-like lesion detected during colonoscopic withdrawal observation: A case report and review of literature

Abstract

BACKGROUND

Anorectal mucosal melanoma (AMM) is a rare, aggressive malignancy with a poor prognosis. Early lesions are often small, amelanotic, and located in the endoscopic blind spots, leading to frequent missed diagnoses. This case highlights the importance of meticulous endoscopic surveillance in detecting early postoperative recurrence.

CASE SUMMARY

A 67-year-old man was followed up two months after surgical resection of perianal AMM. Routine forward-view colonoscopy revealed no abnormalities. Systematic withdrawal with rectal retroflexion identified a 0.5 cm sessile, white polyp adjacent to the surgical scar. Narrow-band imaging suggested benign features, but white-light endoscopy revealed punctate abnormal microvasculature. Biopsy and immunohistochemistry confirmed local recurrence of malignant melanoma. This case demonstrates that early, subtle recurrent lesions may mimic benign polyps and high-quality structured endoscopic examinations are required, including careful inspection of scar areas, retroflexion, and assessment of microvascular patterns, to confirm the diagnosis.

CONCLUSION

Systematic, meticulous endoscopic surveillance is critical for detecting early occult recurrence in high-risk AMM patients.

Key Words: Anorectal mucosal melanoma; Colonoscopy; Withdrawal observation; Melanoma; Case report

Core Tip: Anorectal mucosal melanoma (AMM) is rare, highly aggressive, and often presents with subtle or amelanotic lesions that are easily missed. We report a case of early local recurrence detected only through systematic colonoscopic withdrawal with rectal retroflexion and careful assessment of microvascular patterns. Despite appearing benign under narrow-band imaging, white-light endoscopy revealed suspicious lesions leading to biopsy-confirmed recurrence. This case highlights the critical role of meticulous, high-quality endoscopic surveillance in identifying early occult recurrence and underscores the need for individualized, standardized monitoring strategies in high-risk AMM patients.

INTRODUCTION

Anorectal mucosal melanoma (AMM) is a rare and highly aggressive malignancy, accounting for approximately 1% of all melanomas[1-3]. Most patients are diagnosed at an advanced stage and have a poor prognosis. Early clinical manifestations are often nonspecific, and nearly one-third of cases lack obvious pigmentation[4]. Endoscopically, AMM may resemble benign polyps, which frequently leads to delayed diagnosis.

In patients with a history of perianal malignant melanoma, small or atypical recurrent lesions may be confined to the surgical scar or adjacent mucosa and are therefore easily missed during routine colonoscopy examination. Previous case reports and clinical guidelines have suggested that systematic withdrawal techniques or rectal retroflexion should be incorporated into surveillance of high-risk patients to improve detection rates; however, clinical evidence regarding the monitoring of small, atypical lesions remains limited.

Here, we report a patient with a prior history of surgical resection for perianal malignant melanoma in whom a small, atypical lesion was detected during follow-up. To contextualize this finding, we conducted a focused narrative review of published case reports on AMM from the past five years. This report aims to supplement the existing evidence and to highlight the importance of individualized surveillance strategies in high-risk AMM patients.

CASE PRESENTATION

Chief complaints

A 67-year-old Chinese man presented to the gastroenterology outpatient clinic for follow-up two months after surgical resection of perianal malignant melanoma.

History of present illness

The patient reported no symptoms such as fever, abdominal pain, diarrhea, hematochezia, or tenesmus. He attended the gastroenterology outpatient clinic for routine surveillance and underwent painless colonoscopy.

History of past illness

Two months earlier, the patient underwent local wide excision for an “anal canal mass” at an outside hospital. Postoperative histopathology confirmed malignant melanoma, and his postoperative recovery was uneventful.

Personal and family history

The patient denied any family history of malignant tumors.

Physical examination

On physical examination, the vital signs were as follows: Body temperature, 36.4 °C; blood pressure, 123/57 mmHg; heart rate, 87 beats per min; respiratory rate, 20 breaths per min. The patient was conscious and cooperative during the examination. Cardiopulmonary auscultation revealed no obvious abnormalities. The abdomen was soft with a normal contour, without tenderness or rebound tenderness. Digital anal examination was not performed.

Laboratory examinations

Routine blood and biochemical parameters, coagulation profiles, and levels of serum tumor markers showed no significant abnormalities.

Imaging examinations

A painless colonoscopy was performed under intravenous anesthesia. Intubation to the cecum was achieved smoothly, with fair bowel preparation. No obvious abnormalities were observed in the colonic mucosa throughout the entire colon. During withdrawal, a strictly systematic inspection protocol was followed. On initial forward-view examination, the mucosa at the prior surgical scar in the anal canal appeared only slightly coarse, without a definite protruding lesion or obvious pigmentation (Figure 1A). During careful close-up inspection of the scar-adjacent area on withdrawal, a sessile, whitish polypoid lesion measuring approximately 0.5 cm was identified at the anal verge adjacent to the surgical scar (Figure 1B). The lesion showed mild surface hyperemia, with scattered punctate pigmentation in the surrounding mucosa. After switching to narrow-band imaging, the surface glandular architecture appeared relatively uniform, corresponding to partial features of the National Institute for Health and Care Excellence (NICE) (NBI International Colorectal Endoscopic Classification) type 1 (Figure 1C). Detailed white-light examination revealed punctate microvascular structures on the lesion surface (Figure 1D). Given the lesion’s location in a high-risk area and its suspicious morphology, a biopsy was performed, and the specimen was submitted for histopathological evaluation.

Figure 1 Colonoscopy findings. A: Slightly coarse mucosa at the prior anal canal surgical scar; B: Additional view of slightly coarse mucosa at the prior anal canal surgical scar (arrow); C: Narrow-band imaging showing features consistent with National Institute for Health and Care Excellence (NICE) type 1 classification (arrow); D: Additional view of features consistent with NICE type 1 classification.

On gross examination, the specimen consisted of a single grayish-red tissue fragment measuring approximately 0.2 cm. Microscopically, the cells were densely packed and disorganized, with marked pleomorphism and significant cytologic atypia, characterized by variation in nuclear size and uneven chromatin staining. Immunohistochemical staining showed positivity for human melanoma black-45, microphthalmia-associated transcription factor, and CD63; partial positivity for S100 and CD56; and negativity for pan-cytokeratin, P63, CK5/6, chromogranin A, and synaptophysin. The Ki-67 proliferation index was approximately 55% (Figure 2).

Figure 2 Immunohistochemical findings of the biopsy specimen. A: Hematoxylin and eosin staining (× 200); B: Immunohistochemical staining for human melanoma black-45 (× 200); C: Immunohistochemical staining for microphthalmia-associated transcription factor (× 200); D: Immunohistochemical staining for CD63 (× 200).

FINAL DIAGNOSIS

The biopsy pathology supported a diagnosis of malignant melanoma. In conjunction with the patient’s clinical history, the findings were consistent with postoperative local recurrence.

TREATMENT

We recommended that the patient undergo comprehensive staging examinations followed by evaluation at our hospital’s multidisciplinary team clinic to assess the feasibility of extended radical resection and systemic adjuvant therapies, such as immunotherapy. However, the patient declined further treatment due to financial constraints.

OUTCOME AND FOLLOW-UP

The patient did not return for physical follow-up visits after declining further treatment. Telephone follow-up was conducted at 6 months and 12 months after the index colonoscopic diagnosis. The most recent telephone follow-up was performed 12 months after presentation, during which the patient reported no new symptoms and remains under ongoing observation.

DISCUSSION

AMM is a rare mucosal malignancy first described by Moore in 1857[5], and is characterized by aggressive biological behavior and poor prognosis. It accounts for approximately 0.2%-3% of all melanomas and less than one quarter of mucosal melanomas[6,7]. AMM most commonly arises in the anal canal or near the dentate line[8], typically within 3 cm of the anal verge, corresponding to the anal transitional zone, where resident melanocytes provide the histological basis for tumor development. The disease predominantly affects middle-aged and elderly individuals, with a slight female predominance. Rectal bleeding is the most frequent initial symptom[9], followed by anal discomfort, altered bowel habits, or a palpable perianal mass. Because these manifestations closely resemble benign anorectal conditions, such as hemorrhoids or anal fissures, misdiagnosis at initial presentation remains common[1,10]. AMM exhibits marked morphological heterogeneity, presenting as polypoid, nodular, or ulcerative lesions. Although melanin pigment is a defining feature of melanoma, up to 20%-30% of AMMs are amelanotic[11,12], often appearing white or pink and mimicking benign hyperplastic or inflammatory lesions[13]. This atypical clinical and endoscopic appearance represents a major contributor to delayed diagnosis. Definitive diagnosis therefore relies on histopathological evaluation supported by immunohistochemical markers such as S100, human melanoma black-45, and Melan-A. Prognosis remains stage dependent, with reported 5-year survival rates declining sharply with advanced disease[2,14], underscoring the importance of early detection. To contextualize the present case, we retrieved recently published case reports of AMM, which are summarized in Table 1. While contemporary reports confirm marked heterogeneity in clinical presentation, lesions as small and as endoscopically indolent as those observed in the present case appear to be less frequently described. This observation suggests that early or recurrent AMM may occasionally manifest with particularly subtle features, increasing the risk of endoscopic oversight. A major challenge in AMM management is the low rate of early diagnosis. While cross-sectional imaging, including contrast-enhanced computed tomography and magnetic resonance imaging, is indispensable for staging and assessment of metastatic disease[15,16], its sensitivity for detecting early or superficial primary and recurrent lesions is limited. High-quality endoscopic examination therefore plays a central role throughout diagnosis, treatment, and postoperative surveillance[17]. Endoscopy allows direct assessment of lesion morphology and enables targeted biopsy; however, the anorectal region, particularly the area around the dentate line, represents a well-recognized blind spot during routine colonoscopy. Small, flat, or amelanotic lesions in this location are especially prone to being overlooked.

Table 1 Literature review of case reports on anorectal mucosal melanoma in the past 5 years.

No.	Ref.	Gender (M/F)	Age (year)	Clinical symptoms	Tumor size (cm)	Tumor location	Endoscopic features	Hyperpigmentation (Y/N)
1	Hokama et al[21], 2021	M	65	Hematochezia, anal pain	/	Rectum	Ulcerating type	N
2	Angelo et al[20], 2024	F	61	Rectal bleeding, prolapsed anal mass	4.00	Anal canal	Nodular type	N
3	De la Mora-Romero et al[22], 2024	F	50	Hematochezia, perianal pain	4.00	Anal canal	Polypoid type	Y
4	Zhong et al[23], 2025	F	51	Hematochezia	2.50	Colorectal junction	Polypoid type	/
5	Parajuli et al[24], 2025	M	71	Perianal pain, pain during defecation	6.00	Rectum	Nodular type	Y
6	Ren et al[25], 2025	F	67	Hematochezia, altered bowel habits	1.50	Near the dentate line	Nodular type	33.3% with hyperpigmentation
		F	70	Hematochezia	2.00	Near the dentate line	Cauliflower-like
		F	67	Hematochezia	3.00	Near the dentate line	Cauliflower-like
		F	53	Altered bowel habits	3.00	Near the dentate line	Cauliflower-like
		F	52	Prolapsed anal mass	2.00	Near the dentate line	Cauliflower-like
		F	69	Hematochezia	2.00	Near the dentate line	Nodular type
		F	56	Sensation of anal heaviness	3.00	Near the dentate line	Cauliflower-like
		M	50	Asymptomatic	0.50	Near the dentate line	Nodular type
		F	39	Sensation of anal heaviness	3.00	Near the dentate line	Cauliflower-like
7	Dubey et al[26], 2025	F	56	Rectal bleeding	12.40	Rectum	Ulcerating type	/
8	El Achchi et al[27], 2025	F	61	Rectal syndrome, anemia	/	Perianal	Ulcerating type	Y
		F	65	Hematochezia, rectal syndrome	6.00	Rectum	Nodular type	Y
9	Khan et al[28], 2024	F	71	Rectal bleeding, constipation	4.00	Perianal	Nodular type	/
10	Souhaib et al[29], 2024	M	73	Rectal syndrome, rectal bleeding	5.00	Perianal	Ulcerating type	Y
11	Wong et al[30], 2024	F	61	Anal pain, rectal bleeding, altered bowel habits	1.00	Perianal	Polypoid type	Y
12	Thatipalli et al[31], 2024	M	67	Altered bowel habits, pain during defecation, rectal bleeding	7.00	Rectum	Ulcerating type	Y
13	Xiong et al[32], 2024	F	58	Hematochezia	2.20	8 cm from the anal margin	Nodular type	Y
14	Amoako-Teming et al[33], 2023	M	71	Anemia, rectal bleeding	1.00	Anal canal	Ulcerating type	/
15	Elouali et al[34], 2023	M	63	Tenesmus and pain preceding evacuations, rectal bleeding	3.00	3 cm from the anal margin	Ulcerating type	Y
16	El Youssi et al[35], 2023	F	60	Altered bowel habits, pain preceding evacuations	5.00	4 cm from the anal margin	Ulcerating type	Y
17	Alazki et al[36], 2023	F	57	Lower abdominal pain	2.50	3 cm from the anal margin	Polypoid type	Y
18	Biswas et al[37], 2023	F	69	Pain preceding evacuations, rectal bleeding	/	Anorectal junction	Nodular type	/
19	Apostu et al[38], 2021	F	66	Tenesmus and pain preceding evacuations, rectal bleeding	2.80	Anorectal junction	Cauliflower-like	N
20	Nafees et al[4], 2020	M	69	Tenesmus and pain preceding evacuations	16.00	Rectum	Ulcerating type	N
21	Li et al[39], 2020	F	85	Fecal incontinence	1.00	Near the dentate line	Polypoid type	Y
22	Dai et al[40], 2020	F	48	Hematochezia, anal pain	3.00	3 cm from the anal margin	Nodular type	N
23	Lian et al[41], 2020	F	43	Asymptomatic	1.00	Anal canal	Polypoid type	Y

M: Male; F: Female; Y: Yes; N: No.

Using meticulous endoscopic techniques is essential to overcome this limitation. Systematic withdrawal and rectal retroflexion have been shown to improve the detection of distal rectal and anal canal lesions[18,19]. By permitting direct visualization of the dentate line and upper anal canal, retroflexion provides a complementary perspective that is particularly relevant for AMM, which frequently arises in this region. Previous reports have demonstrated that failure to perform retroflexion may contribute to missed anorectal melanoma lesions[20,21]. The present case further illustrates this diagnostic challenge. The recurrent lesion detected two months postoperatively was small, polypoid, and grayish-white, with a NICE type I pattern on narrow-band imaging, closely resembling a benign hyperplastic polyp. Recognition of subtle, irregular microvascular patterns under white-light endoscopy, rather than reliance on electronic chromoendoscopy alone, prompted biopsy and diagnosis. This emphasizes that, for rare and highly deceptive lesions such as AMM, endoscopist judgment and vigilance remain as critical as standardized technical maneuvers. Finally, this case highlights the importance of close postoperative follow-up. Given the rarity of AMM, standardized surveillance guidelines are lacking, making patient education and routine endoscopic monitoring particularly important, especially after surgical resection. Heightened awareness of AMM and its diverse presentations, together with consistently high-quality endoscopic surveillance, may facilitate earlier detection of localized disease and ultimately improve clinical outcomes.

This report has several limitations. As a single-case observation, the findings cannot be extrapolated to the broader population of patients with AMM. In addition, long-term outcome assessment was constrained because the patient declined further treatment and did not undergo subsequent in-person follow-up, limiting objective evaluation of disease course. Furthermore, the accompanying literature review was narrative rather than systematic, and therefore may be subject to selection bias. Prospective, multicenter investigations are needed to refine endoscopic surveillance strategies and to establish quality benchmarks for postoperative monitoring in high-risk AMM.

CONCLUSION

This case report not only documents early recurrence of a rare malignancy but also highlights the decisive role of endoscopic quality control in the comprehensive management of gastrointestinal tumors. For diseases such as AMM, which are highly occult and associated with a poor prognosis, the point of diagnosis may, to a considerable extent, determine the ultimate therapeutic outcome. Future research should focus on developing expert consensus or quality indicators for endoscopic procedures targeting such high-risk anatomical regions. The potential of artificial intelligence-assisted diagnosis in identifying atypical lesions, thereby enabling more standardized and precise diagnostic and therapeutic pathways, should also be explored.