Introduction

Leiomyomas are benign mesenchymal tumors arising from smooth muscle cells and are most frequently identified in the female genital tract, particularly the uterus. (1) In the gastrointestinal tract, these tumors are uncommon and more often located in the esophagus and stomach. Colonic leiomyomas are rare accounting for less than 3% of all gastrointestinal leiomyomas. They often discovered incidentally, making them diagnostically challenging. (2)

Accurate differentiation between leiomyomas and other mesenchymal tumors, particularly gastrointestinal stromal tumors (GISTs), is essential due to significant differences in management and prognosis. (3) While GISTs may require adjuvant targeted therapy, leiomyomas are benign and typically managed with surgical excision alone. (4) Histopathology supplemented by immunohistochemistry (IHC) plays a pivotal role in this distinction.(5) GISTs typically express CD117 and DOG1, whereas leiomyomas are characterized by positivity for smooth muscle markers like desmin.(6)

Furthermore, colonoscopy, while a valuable diagnostic modality for intraluminal pathology, may fail to detect extraluminal or submucosal masses, underscoring the need for comprehensive imaging and pathological correlation.(7) Large exophytic leiomyomas of the ascending colon are exceedingly rare, with most reported lesions being small and intraluminal. The present case is unique due to its size, extraluminal growth, and initial misdiagnosis as GIST.

Case Report

A man in his late forties presented with a two-week history of right upper abdominal pain. There were no associated dyspeptic symptoms or history of nausea, vomiting, constipation, diarrhea, hematemesis, or melena. Clinical examination was unremarkable except for mild tenderness in the right hypochondrium. No palpable mass or organomegaly was noted.

Ultrasonography of the abdomen, performed as part of the evaluation for suspected cholelithiasis, revealed soft tissue lesion in right iliac fossa of 67 x 50 mm, in contact with a bowel loop, likely a gastrointestinal stromal tumor (GIST) and Cholelithiasis

To further evaluate the lesion, the patient then underwent an optical colonoscopy which showed a normal mucosal study with no evidence of intraluminal lesions.

As the mass couldn’t be picked up on colonoscopy, he underwent a CECT of the abdomen and pelvis (Figures 1 and 2) which revealed, an irregular soft tissue attenuating, heterogeneously enhancing lesion in the right paracolic gutter, measuring 8.5 x 5.1 x 4.8 cm, located adjacent to the caecum and ascending colon suggestive of neoplastic etiology likely a gastrointestinal stromal tumor (GIST) and cholelithiasis

After necessary pre anesthetic evaluation, the patient was initially taken up for diagnostic laparoscopy. Intraoperatively, an exophytic mass measuring approximately 8 × 5 × 5 cm was identified arising from the ascending colon, without invasion into adjacent structures (Figure 3). A laparoscopic cholecystectomy was performed, followed by conversion to a laparotomy. Given the large size, uncertain preoperative diagnosis, and inability to exclude malignancy intraoperatively, an oncological resection (right hemicolectomy) was performed in accordance with standard surgical principles along with an ileotransverse anastomosis. A 24 French abdominal drain was placed, and the resected specimens were sent for histopathological examination.

Figure 1: CECT Abdomen and pelvis (Axial view) with red arrows depicting the lesion
Figure 2: CECT Abdomen and pelvis (Coronal view) with orange arrows depicting the lesion
	Figure 3: Diagnostic laparoscopy image of the lesion
Figure 4: Intra operative image of the lesion in the ascending colon. The black arrow depicting the lesion and the blue arrow showing the appendix	Figure 5: Resected right hemicolectomy specimen with the black arrow depicting the lesion

Outcome And Follow Up

The postoperative period was uneventful. The patient was managed according to Enhanced Recovery After Surgery (ERAS) protocols, with early mobilization and initiation of oral intake on postoperative day 1. He was discharged once stable after removal of the abdominal drain on postoperative day 3. There were no post-operative complications.

Histopathological examination revealed spindle cells arranged in short fascicles with elongated nuclei and a mitotic rate of <5 per 10 high-power fields, suggestive of a moderate-risk GIST. Based on this, the patient was counselled regarding the potential need for adjuvant chemotherapy, pending immunohistochemical (IHC) confirmation.

However, on IHC, tumor cells were positive for vimentin, desmin, and H-caldesmon, and negative for CD34, CD117, DOG1, and S-100 proteins. The Ki-67 proliferation index was 10%. These findings were consistent with a diagnosis of leiomyoma. The patient was informed that no further oncological treatment was necessary to his delight.

At one-week and one-month follow-up, the patient remained asymptomatic with no postoperative complications or gastrointestinal symptoms. Bowel habits were regular, and he was able to resume daily activities and return to work within 15 days of surgery.

Discussion

Colonic leiomyomas are rare benign tumors that can closely mimic GISTs clinically, radiologically, and even histologically. While leiomyomas have been reported throughout the colon, they most frequently arise in the sigmoid and transverse segments.(2,8) Their occurrence in the ascending colon however is rare. Dharmavaram et al. reported a rectosigmoid leiomyoma diagnosed after endoscopic polypectomy and immunohistochemistry (IHC).(2) Similarly, Caliskan et al. described a 8 mm sigmoid polyp histologically confirmed as a leiomyoma.(9) Lee et al. presented an ascending colon leiomyoma removed via colonoscopy, which, like most reported cases, presented as a small, sessile, intraluminal mass.(10) In contrast, exophytic colonic leiomyomas, as in our case, are exceptionally rare and often pose diagnostic challenges.

Immunohistochemistry (IHC) remains the cornerstone of differentiation between leiomyomas and GISTs. GISTs typically express CD117, DOG1, and CD34, whereas leiomyomas are characteristically positive for smooth muscle actin, desmin, and H-caldesmon, and negative for CD 117 and DOG1. (3) This similarity may result in initial misdiagnosis and overtreatment, especially since GISTs may require adjuvant therapy with tyrosine kinase inhibitors. (6) In this case, initial imaging and histopathology were suggestive of a GIST, but IHC confirmed a leiomyoma, sparing the patient unnecessary chemotherapy.

Additionally, this case illustrates the limitation of colonoscopy in detecting extraluminal or submucosal lesions, as the tumor was not visible during optical colonoscopy. This supports the value of incorporating cross-sectional imaging such as contrast-enhanced CT or CT colonography into the diagnostic workflow. Comprehensive pathological evaluation, including IHC, is critical to guide appropriate management and avoid overtreatment.

Conclusion

Leiomyoma of the ascending colon is a rare entity that can mimic GIST and pose significant diagnostic challenges. This case emphasizes the importance of immunohistochemistry in establishing a definitive diagnosis and preventing unnecessary treatment. It also highlights the limitations of colonoscopy in detecting extraluminal lesions and the importance of comprehensive diagnostic evaluation.

Informed Consent

Written informed consent was obtained from the patient for publication of this case report and accompanying images. Institutional ethical approval was not required for a single case report as per institutional policy.

References

1. Wang X, Ren J, Chen G, Xu S. Giant abdominal leiomyoma of male: a case report and literature review. Int J Clin Exp Pathol. 2015;8(7):8532–5.
2. Silva C, Mendes J, Gaspar A, Leal F, Almeida R. Colonic leiomyoma: a case report of an uncommon endoscopic finding. Cureus. 2023;15(12):e50879
3. Çığ Taşkın O, Armutlu A, Adsay V, Aslan F, Kapran Y. Clinicopathologic and immunohistochemical characteristics of upper gastrointestinal leiomyomas harbouring interstitial cells of Cajal: a potential mimicker of gastrointestinal stromal tumour. Ann Diagn Pathol. 2020;45: p: 151476.
4. Miettinen M, Lasota J. Histopathology of gastrointestinal stromal tumor. J Surg Oncol. 2011;104(8):865–73.
5. Miettinen M, Wang ZF, Lasota J. DOG1 antibody in the differential diagnosis of gastrointestinal stromal tumors: a study of 1840 cases. Am J Surg Pathol. 2009;33(9):1401–8.
6. Dematteo RP, Heinrich MC, El-Rifai WM, Demetri GD. Clinical management of gastrointestinal stromal tumors: before and after STI-571. Hum Pathol. 2002;33(5):466–477.
7. Pickhardt PJ, Hassan C, Halligan S, Marmo R. Colorectal cancer: CT colonography and colonoscopy for detection--systematic review and meta-analysis. Radiology. 2011 May;259(2):393-405.
8. Sandrasegaran K, Rajesh A, Rydberg J, Rushing DA, Akisik FM, Henley JD. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features. AJR Am J Roentgenol 2005;184(3):803–11.
9. Pham N, Nguyen EY, Jacob N. An Extremely Rare Case of Colonic Leiomyoma Disguised as Pedunculated Colonic Polyp. J Gastrointestin Liver Dis. 2024 Dec 28;33(4):453.
10. Lee J, Elshafey S, Soumekh A. Leiomyoma of the ascending colon: an uncommon lesion in an uncommon location. Am J Gastroenterol. 2024;119(10S):S2384.