Gastrointestinal: Endoscopic ultrasound of rectal subepithelial lesion mimicking gastrointestinal stromal tumor.

Abstract

A 60-year-old man presented with anorexia, weight loss, and constipation for 3 months. On peri-rectal examination, a bulge was palpated in the rectum. He underwent colonoscopy, which showed large subepithelial lesion (SEL) starting from anorectal junction extending proximally till rectosigmoid junction obliterating the lumen without complete obstruction (Fig. 1a). Contrast-enhanced computed tomography of the pelvis showed a large lobulated heterogeneous enhancing soft tissue lesion within the rectum (Fig. 1b). Endoscopic ultrasound (EUS) was performed, which showed a large well defined lobulated hetero-echoic lesion (70 × 63 mm) with multiple cystic spaces in peri-rectal area (Fig. 1c). Possibility of rectal malignant gastrointestinal stromal tumor (GIST) was considered looking at CT and EUS image. To confirm our diagnosis, EUS-guided fine needle aspiration (FNA) was performed. During FNA, dark brownish fluid was aspirated (Fig. 2a). Possibility of bleed within tumor was considered because of dark brownish fluid aspiration; however, black pigments were noticed on slide preparation (Fig. 2b). Histopathological examination showed pleomorphic cells with abundant brownish pigment with multinucleation suggestive of malignant melanoma (Fig. 2c). Patient was advised for surgery and referred to the surgeons. Malignant melanoma is an extremely rare and aggressive tumor of the rectum. It accounts for 0.5–4% of all anorectal malignancies and < 1% of all melanomas. It is most common site of primary gastrointestinal melanoma followed by stomach, small intestine, and colon. Patients usually present in the fifth or sixth decade, and it is more common in female patients. Anorectal melanomas usually appear as ulcerated polypoidal lesions rather than SEL. EUS is an important diagnostic tool in diagnosis and differentiation of SEL. There is lack of characterization of anorectal melanoma on EUS because of rarity of tumor. Yonal et al. described the EUS morphological features of esophageal melanoma as iso to hypoechoic solid tumor mass with variable areas of internal hyper-echogenicity and anechoic cystic spaces, similar to the findings from our case. Morphological features of some SELs are confusing; therefore, EUS FNA plays an important role in differentiation of SELs. Therapy in anorectal melanomas is dictated by the local staging of disease and also the presence of lymph node and distant metastases. Multidisciplinary approach is required depending on the stage of tumor. Surgery has been the mainstay of treatment for anorectal melanoma. Although these tumors are usually radio-resistant and chemo-resistant, combination of chemotherapy and immunotherapy remains a therapeutic option for advanced metastatic disease. Overall, 5-year survival is only 3–22%, and survival in recurrent or metastatic disease is less than 10 months. Although a rare entity, anorectal melanoma can mimic GIST and may be a differential during the evaluation of rectal SEL. Early diagnosis of tumor is of paramount importance for favorable prognosis.