One of a Kind: Adenomyoma in a Rectal Polyp

The goal of screening colonoscopies is to identify and remove premalignant polyps before they progress to invasive cancer. While most colorectal polyps can be readily resected, certain lesions—particularly those that are sessile, anatomically challenging, or involve deeper layers—may require advanced endoscopic techniques for complete resection. While adenomas and hyperplastic polyps care most common, subepithelial lesions represent a distinct subset of colorectal findings and are often difficult to characterize based on endoscopic appearance alone, frequently requiring histopathologic evaluation for definitive diagnosis and management. Although the majority of colorectal lesions arise from gastrointestinal (GI) tissue, rare entities originating from non-GI sources may also be encountered. Müllerian-derived lesions, including extrauterine adenomyosis and adenomyoma, are well described in gynecologic literature but are infrequently reported within the GI tract and are exceedingly rare in the colon. We present a case of a rectal adenomyoma presenting as a subepithelial lesion initially identified as an unresectable polyp. CASE REPORT Patient description A 60-year-old woman was referred to our advanced endoscopy clinic for endoscopic mucosal resection (EMR) of a 10 mm semisessile rectal polyp that was unresectable with hot snare during a screening colonoscopy. This patient's medical history includes uterine fibroids, anxiety/depressive disorder, and migraines. She reported symptoms consistent with irritable bowel syndrome (alternating bowel habits, bloating, and cramping with bowel movements) during a primary care appointment. She also has a remote surgical history of hysterectomy in 2003. Documentation regarding the indication and pathologic findings from the hysterectomy was unavailable because the procedure predated the electronic medical record, and no history of endometriosis was noted in the available records. Procedural timeline During the flexible sigmoidoscopy for planned EMR, endoscopic ultrasound evaluation revealed an oval, intramural (subepithelial) lesion measuring 10 × 6 mm in thickness about 20 cm from the anal verge. The lesion appeared to originate within the deep mucosa and involved the submucosa and muscularis propria. It was hypoechoic and poorly defined. Fine-needle aspirations revealed fragments of smooth muscle cells, but a conclusive diagnosis could not be obtained. Given the depth of involvement, the decision was made to attempt endoscopic full-thickness resection (EFTR) after obtaining informed consent from the patient. A second flexible sigmoidoscopy was performed, and the 10 mm polyp was successfully and completely resected with EFTR (Figure 1). In addition to the rectal lesion, screening colonoscopy identified two 4–6 mm semisessile hyperplastic polyps in the sigmoid colon. Consistent with standard screening recommendations, repeat colonoscopy was recommended in 10 years. Microscopic description Histologic examination (Figure 2) of the rectal lesion demonstrated a 1.3 cm white solid nodule composed of a cellular spindle cell proliferation with scattered benign-appearing glands. These glands were predominantly located at the periphery of the lesion, with occasional glands identified centrally. The glandular elements were morphologically distinct from the overlying colonic epithelium and were associated with a cellular stroma consistent with endometrial-type stroma (Figure 3). The lesion extended from the mucosa through the submucosa and muscularis propria into the adjacent soft tissue. No significant cytologic atypia, mitotic activity, or necrosis was identified. On immunostaining, the immunoperoxidase stains (Table 1) supported the smooth muscle nature of the nodule and failed to support a GI stromal tumor or neural differentiation. The PAX8 positivity of the associated glands supported their Müllerian/gynecologic origin with the immediately adjacent CD10-positive stroma, supporting endometrial-type stroma. Overall, 2 intradepartmental pathology reviewers concluded that the findings were most consistent with nodular adenomyosis associated with smooth muscle hypertrophy, although endometriosis associated with a leiomyoma was also considered. Table 1. Immunoperoxidase stain results and description of the polyp fragment | Immunoperoxidase stain | Result | Description | | SMA | + | | | MSA | + | | | Desmin | + | | | CD117 (c-kit) | - | | | DOG-1 | - | | | CD34 | - | | | S100 | - | | | PAX8 | + | Positive in lesional glands (unlike overlying negative colonic glands) | | CDX2 | - | Negative in lesional glands (in contrast to overlying positive colonic glands) | | CD10 | + | Positive in stroma immediately adjacent to glands (supporting endometrial-type stroma) |

We report a rare case of adenomyoma presenting as a subepithelial rectal lesion initially identified as an unresectable polyp during screening colonoscopy. Endoscopic ultrasound demonstrated a hypoechoic, poorly defined lesion involving the deep mucosa, submucosa, and muscularis propria, while fine-needle aspiration was nondiagnostic. Given the depth of involvement and indeterminate features, EFTR was pursued, achieving en bloc resection and allowing for a definitive diagnosis. Subepithelial lesions most commonly include GI stromal and neuroendocrine tumors, and their endoscopic and sonographic features are often nonspecific. In this case, the lesion closely mimicked these more common entities, highlighting the limitations of endoscopic ultrasound and fine-needle aspiration in the evaluation of deep mural lesions. Definitive diagnosis therefore required complete resection and histopathologic evaluation. The depth of involvement in this lesion precluded standard EMR, which is limited to superficial layers and carries a risk of incomplete excision in lesions involving the muscularis propria. While endoscopic submucosal dissection can achieve en bloc resection of larger lesions, it is technically challenging in the setting of deep mural involvement and poorly defined borders. EFTR was selected in this case as it permits full-thickness resection, allowing both definitive diagnosis and complete removal in a minimally invasive manner, thereby avoiding surgical intervention. Pathologic examination demonstrated an adenomyoma spanning the mucosal and deeper layers, with focal extension into adjacent soft tissue. Adenomyomas represent a focal form of adenomyosis and are an uncommon manifestation of extrauterine Müllerian pathology. The terminology surrounding these lesions can be inconsistent, as the term “adenomyoma” has been used to describe both true Müllerian-derived lesions and non-Müllerian hamartomatous processes. Recognition of this distinction is important when interpreting rare lesions within the GI tract. Rare cases of malignant transformation within extrauterine adenomyomas have been reported; however, no cytologic atypia or malignant features were identified in this case. The presence of focal extension beyond the bowel wall in this case likely reflects the growth pattern of Müllerian-type tissue rather than aggressive behavior. Several mechanisms have been proposed for its development, including persistence of embryologic Müllerian remnants, metaplastic transformation of local cells, and implantation of endometrial tissue. The metaplastic theory proposes that pluripotent mesenchymal cells differentiate into Müllerian-type glands, stroma, and smooth muscle, providing an explanation for lesions arising in ectopic locations. Alternatively, the embryologic theory suggests displacement of Müllerian remnants during organogenesis with subsequent development of ectopic Müllerian tissue. In the present case, the predominantly intramural location and remote history of hysterectomy more than 2 decades before diagnosis make recent implantation-based mechanisms less compelling and may favor an embryologic or metaplastic origin, although the exact pathogenesis remains uncertain. This case exemplifies the diagnostic challenge of atypical subepithelial colorectal lesions and demonstrates that adenomyomas can present as a rectal polypoid masses with deep mural extension. EFTR allowed for definitive diagnosis and minimally invasive resection. Awareness of rare entities such as adenomyoma expands the differential diagnosis of subepithelial colorectal lesions, particularly in patients with underlying gynecologic pathology. DISCLOSURES Author contributions: All authors contributed to the writing and revision of the manuscript. I. Radosavljevic is the article guarantor. Financial disclosure: None to report. Informed consent was obtained from the patient for publication of this case. ABBREVIATIONS: - EFTR endoscopic full-thickness resection - EMR endoscopic mucosal resection - GI gastrointestinal - MSA muscle-specific actin - SMA smooth muscle actin

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