Cap Polyposis Mimicking Rectal Neoplasia in an Adolescent

A 17-year-old male presented with a 2-year history of hematochezia. Initial colonoscopy (Figure 1a) at the local hospital revealed a cauliflower-like lesion in the rectum, and imaging demonstrated focal rectal wall thickening, raising suspicion of neoplasia. Based on elevated fecal calprotectin and inflammatory histology, the patient was initially diagnosed with ulcerative proctitis. However, symptoms persisted despite treatment with mesalamine and corticosteroids. (a) Colonoscopy at the local hospital. (b) Colonoscopy demonstrating nodular polypoid lesions with mucosal edema and adherent mucous exudate in our hospital. (c) Histology showing granulation tissue proliferation, distorted and branching crypts (H&E 40X magnification). (d) Follow-up colonoscopy 16 months after ESD showing a well-healed mucosa without recurrence. Repeat colonoscopy (Figure 1b) in our hospital showed multiple nodular polypoid lesions in the rectum with marked mucosal edema and adherent mucous exudate. Histopathological examination(Figure 1c) revealed granulation tissue proliferation, distorted and branching crypts, and dilated congested vessels. After multidisciplinary team discussion, a diagnosis of cap polyposis was established. The patient underwent endoscopic submucosal dissection (ESD), resulting in complete symptom resolution. Follow-up colonoscopy at 4, 9, and 16 months (Figure 1d) after ESD showed no recurrence. This case highlights that cap polyposis, although rare, can occur in adolescents and may mimic inflammatory bowel disease or colorectal neoplasia. Recognition of its characteristic endoscopic features is critical for accurate diagnosis, and endoscopic treatment is effective for localized lesions. We thank Dr. Xue He, Department of Pathology, the Second Affiliated Hospital of Nanjing Medical University, for her valuable contribution to this case. Written informed consent for publication from the patient was obtained. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.