Synchronous right colon adenocarcinoma and sigmoid endometriosis mimicking a second primary carcinoma: a case report

Intestinal endometriosis is a benign gynecological condition that frequently involves the rectosigmoid colon. Its synchronous occurrence with colorectal cancer is exceptionally rare and diagnostically challenging, particularly when mimicking a second primary malignancy. We report a case of a 48-year-old woman presenting with altered bowel habits. Colonoscopy revealed synchronous lesions: an adenocarcinoma at the hepatic flexure and a suspicious mass in the sigmoid colon. Repeated biopsies of the sigmoid lesion were inconclusive, ranging from dysplasia to chronic inflammation, raising concern for a second primary tumor. The patient underwent right hemicolectomy with partial sigmoidectomy. Intraoperative frozen section unexpectedly identified endometriosis, subsequently confirmed by definitive pathology and immunohistochemistry. At 12-month follow-up, the patient remained well with no evidence of recurrence.

Intestinal endometriosis predominantly affects the rectosigmoid colon in 3.8%–37% of women, causing chronic pelvic pain, dysmenorrhea, dyspareunia, and cyclical bowel changes that impair quality of life (–). Its clinical, endoscopic, and imaging features closely resemble colorectal cancer, yet preoperative pathological confirmation is rarely achieved because lesions reside in the submucosa and standard biopsies yield only nonspecific inflammation (, ). Consequently, definitive diagnosis is usually established postoperatively via paraffin pathology and immunohistochemistry (). Although numerous cases of intestinal endometriosis have been reported in the literature, many were misdiagnosed as colorectal cancer and underwent unnecessary surgical resection (–). However, the specific scenario of right colon cancer with synchronous sigmoid endometriosis, as presented in our case, has not been previously reported. We report such a case, highlighting the diagnostic pitfalls and the critical roles of intraoperative frozen section and multidisciplinary collaboration in guiding surgical strategy. Case presentation A 48-year-old woman was admitted to our hospital with a 6-month history of altered bowel habits, characterized by frequent (3–4 times daily), loose, and mucopurulent stools. The timeline of the main diagnostic and therapeutic events during surgical care is summarized in Table 1. She had previously undergone appendectomy, hemorrhoidectomy, and excision of breast nodules. She had no smoking or significant alcohol history, with regular menses and no dysmenorrhea. Family history was negative for hereditary or major infectious diseases. Physical examination revealed no abnormal findings on abdominal palpation, and digital rectal examination was unremarkable. Laboratory tests, including complete blood count, liver and renal function, electrolytes, coagulation profile, and tumor markers (CEA 2.24 ng/mL, CA125 32.35 U/mL, CA19-9 36.8 U/mL), were all within normal limits. The patient's hemoglobin was 123 g/L and serum albumin was 45 g/L, which were within normal ranges—likely reflecting the early stage of the tumor (as confirmed by postoperative pathology) and the absence of significant bleeding or malnutrition. Table 1 | Timeline of the case | | |---|---| | Date | Event | | 17-Dec-24 | First outpatient visit for altered bowel habits | | 20-Dec-24 | Initial colonoscopy + biopsy: ascending colon adenocarcinoma; sigmoid lesion with dysplasia | | 23-Dec-24 | Repeat biopsy: sigmoid lesion showed only chronic inflammation | | 24-Dec-24 | Preoperative CT and MRI scan | | 10-Jan-25 | Admission of patient to surgical department | | 14-Jan-25 | Laparoscopic surgery + intraoperative colonoscopy + frozen section → endometriosis diagnosed; right hemicolectomy + sigmoid resection performed | | 27-Jan-25 | Discharged, uneventful recovery | | 17-Jan-26 | 12-month follow-up: no recurrence | Timeline of the main diagnostic and therapeutic events during surgical care. Colonoscopy revealed two synchronous lesions: a laterally spreading tumor at the hepatic flexure of the ascending colon, and a semi-circumferential mass located 17–19 cm from the anal verge in the sigmoid colon, occupying approximately half of the lumen (Figure 1). Histopathological examination of biopsies from the ascending colon lesion confirmed intramucosal adenocarcinoma. Biopsies from the sigmoid lesion were inconclusive—initially showing glandular dysplasia, raising suspicion for malignancy. A repeat sigmoidoscopy with targeted biopsy was subsequently performed, which demonstrated only moderate chronic inflammation with erosion. Contrast-enhanced CT of the abdomen and pelvis demonstrated mild and heterogeneously enhancing wall thickening of the ascending colon. The distal sigmoid colon showed approximately 1.0 cm of wall thickening with ill-defined borders and homogeneous enhancement, the nature of which was indeterminate. The uterus was enlarged with an irregular contour, suggestive of adenomyosis. Pelvic contrast-enhanced MRI confirmed these findings (Figure 2). Figure 1 Figure 2 Following preoperative multidisciplinary discussion involving gastrointestinal surgery, gynecology, and oncology, the patient was diagnosed with right colon cancer with a synchronous sigmoid lesion of indeterminate nature, without evidence of distant metastasis. Laparoscopic exploration was subsequently performed. Intraoperative findings included a 2 cm × 2 cm exophytic mass at the hepatic flexure and dense adhesion of the rectosigmoid junction to the cervix, forming a firm, ill-defined mass. Intraoperative colonoscopy was employed to delineate the distal tumor margins. Frozen section analysis of the sigmoid lesion unexpectedly returned a diagnosis of endometriosis, and a gynecologist was consulted intraoperatively. The definitive procedure consisted of right hemicolectomy (en bloc resection of the terminal ileum, cecum, ascending colon, and right branch of the middle colic vessels with regional lymphadenectomy) and segmental resection of the sigmoid colon with a 5-cm proximal and distal margin, followed by primary anastomosis, as well as uterine curettage. Final pathological examination confirmed a moderately differentiated adenocarcinoma of the ascending colon (pT2N0M0, Stage I), endometriosis of the sigmoid colon, and proliferative-phase endometrium in the uterine curettage specimen. No further resection of the adnexa or uterus was performed. Immunohistochemistry supported both diagnoses: the ascending colon adenocarcinoma showed diffuse positivity for P-CK, confirming its epithelial origin; the sigmoid lesion showed strong positivity for ER (90%), PR (90%), CD10, CK7, and PAX-2, with negative staining for CDX-2, CK20, and SATB2, confirming endometrial origin (Figure 3). The patient's postoperative recovery was uneventful, and she was discharged on the 13th postoperative day. At 1-year follow-up, she remained well with no evidence of recurrence. Figure 3

Endometriosis is characterized by functioning endometrial tissue beyond the uterine cavity, affecting up to 15% of reproductive-aged women, with gastrointestinal involvement occurring in as many as 37% of cases (). Intestinal endometriosis may present with chronic cyclic abdominal pain and altered bowel habits, often mimicking irritable bowel syndrome and leading to years of misdiagnosis (). Historical clues such as dyspareunia, unexplained infertility, dysmenorrhea, known endometriosis, and perimenstrual symptom exacerbation may aid diagnosis, though physical examination is usually normal or reveals only nonspecific findings. Establishing a definitive diagnosis of intestinal endometriosis is particularly elusive, because the lesions typically spare the mucosa and reside in the deeper layers of the bowel wall (, ). Consequently, mucosal biopsies obtained via standard colonoscopy often reveal only normal mucosa or non-specific inflammatory changes, making the procedure more valuable for ruling out other pathologies than for establishing a diagnosis of intestinal endometriosis. Even in the presence of overt endoscopic abnormalities—such as stricture formation or submucosal protrusions—histopathological examination frequently shows no diagnostic features, resulting in a substantial rate of false-negative findings (). Various imaging modalities have been employed to evaluate intestinal endometriosis. Abdominal CT or MRI may demonstrate bowel wall thickening, with MRI offering superior delineation of endometriomas and multifocal lesions (). Endoscopic ultrasound (EUS) can further characterize the layers of bowel wall involvement, the length of affected segment, distance from the anal verge, and the presence of reactive lymph nodes or extracolonic disease (, ). Despite these imaging advances, a definitive diagnosis of endometriosis without laparoscopic confirmation remains difficult to establish. The diagnostic challenge posed by intestinal endometriosis is well illustrated by previously reported cases. In 2018, Masatsugu Ishii et al. reported a case series of seven patients with intestinal endometriosis who were preoperatively mistaken for colorectal cancer and underwent surgical resection (). Other reports have described rectal endometriosis misdiagnosed as adenocarcinoma or presenting with bowel obstruction (, ). Beyond the pelvis, endometriosis can also involve distant sites, further complicating the differential diagnosis—one case described a rectal cancer patient with a synchronous mediastinal mass that, after resection, proved to be endometrial carcinoma arising from mediastinal endometriosis (). This case further illustrates the ability of endometriosis to mimic metastatic disease and occasionally undergo malignant transformation. However, none of these reports described a synchronous endometriotic lesion in a patient with established right colon cancer, highlighting the distinctiveness of our presentation. Given these diagnostic pitfalls, a thorough understanding of the clinical behavior of endometriosis is essential for appropriate surgical management. Endometriosis is generally considered a benign condition, yet accumulating evidence suggests a potential link with certain malignancies. Moreover, patients with endometriosis appear to be at higher risk for several chronic diseases, and malignancy is frequently suspected in this population, particularly given that gynecologic cancers may coexist with endometriosis (). In light of these clinical concerns, timely and accurate surgical management becomes particularly important in cases of suspected intestinal involvement. Once surgical intervention is indicated for intestinal endometriosis, resection remains the mainstay of treatment. The operative objectives are early recognition, complete excision of ectopic endometrial tissue, and alleviation of symptoms. Laparoscopic approaches for colorectal conditions have evolved substantially in recent years and have proven valuable not only for therapy but also for distinguishing endometriotic lesions from colorectal malignancies (). The safety and feasibility of laparoscopic colorectal resection for endometriosis have been corroborated by multiple investigations, including a prospective randomized study conducted by Darai and colleagues (). In China, the incidence and mortality of colorectal cancer have steadily increased, with approximately 520,000 new cases and 240,000 deaths reported in 2022 (). The prevalence of synchronous colorectal cancer ranges from 1.1% to 8.1% (). In this context, the presence of a confirmed malignancy elsewhere in the colon creates a cognitive bias toward malignancy, making the consideration of benign etiologies exceedingly challenging. Given the limitations of preoperative biopsies and imaging, we turned to intraoperative tools to guide our surgical strategy. Frozen section evaluation plays a crucial role in this setting, offering real-time diagnostic insight that influences surgical decisions (). In our case, despite the indeterminate nature of the sigmoid lesion, the confirmed diagnosis of right colon adenocarcinoma without distant metastasis prompted laparoscopic exploration. Intraoperative frozen section subsequently confirmed sigmoid endometriosis, and concurrent colonoscopy precisely delineated the distal margin, ensuring negative margins. Together, these intraoperative tools transformed a planned radical resection into a more conservative, organ-sparing procedure that balanced oncologic cure with functional preservation. This team-based approach enabled us to achieve the best possible diagnostic and therapeutic outcome for the patient.

For female patients presenting with multiple colorectal lesions, the possibility of intestinal endometriosis should be carefully considered, particularly when preoperative workup yields inconclusive results. During surgery, effective communication between the operating surgeon and the histopathologist within the framework of interdisciplinary cooperation is essential to achieve the best possible patient outcomes. Statements Data availability statement The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author. Ethics statement The studies involving humans were approved by Biomedical ethics committee of Mianyang Central Hospital. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article. Author contributions KY: Conceptualization, Data curation, Writing – original draft, Writing – review & editing. PL: Writing – original draft, Writing – review & editing. XL: Supervision, Writing – original draft, Writing – review & editing. Funding The author(s) declared that financial support was not received for this work and/or its publication. Conflict of interest The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Generative AI statement The author(s) declared that generative AI was not used in the creation of this manuscript. Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us. Publisher’s note All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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case report, colorectal cancer, endometriosis, frozen section, synchronous lesions Citation Yang K, Li P and Long X (2026) Synchronous right colon adenocarcinoma and sigmoid endometriosis mimicking a second primary carcinoma: a case report. Front. Surg. 13:1923049. doi: 10.3389/fsurg.2026.1923049 Received 29 June 2026 Revised 02 August 2026 Accepted 03 August 2026 Published 13 August 2026 Volume 13 - 2026 Edited by Dimitrios Kehagias, Leeds Teaching Hospitals, United Kingdom Reviewed by Omar Hamdy, Mansoura University, Egypt Allen Omo-Ogboi, University of Pikeville Kentucky College of Osteopathic Medicine, United States Updates Copyright © 2026 Yang, Li and Long. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. *Correspondence: Xuan Long [email protected] Disclaimer All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.