Deep rectosigmoid endometriosis: "mushroom cap" sign on T2-weighted MR imaging

The purpose of this study is to evaluate the "mushroom cap" sign on T2-weighted MR imaging in patients with submucosal tumors in the rectosigmoid colon.

From January 2001 to August 2009, 12 patients with four different diseases presenting or mimicking submucosal tumors in the rectosigmoid colon underwent colonic resection. All patients with deep endometriosis (n = 6), gastrointestinal stromal tumor (n = 4), metastasis from ovary cancer (n = 1), and carcinoid tumor (n = 1) had either an MRI of the rectum or pelvis before surgery. We evaluated the MRI findings and compared them with the macroscopic and microscopic observations in the resected specimens.

In all six cases of deep endometriosis, a characteristic "mushroom cap" shaped appearance was found on T2-weighted MR imaging. Heterogeneous low signal intensity of the hypertrophic muscularis propria, covered with high signal intensity of the mucosa and submucosa on T2-weighted MR images, looked like a "mushroom cap" with the pattern of intraluminal endophytic growth. In histological findings, deep endometriosis involved the submucosa (n = 4) or mucosa (n = 2). The "mushroom cap" sign was not present in any of the six other tumors.

The "mushroom cap" sign on T2-weighted MR imaging may be a characteristic sign for diagnosing deep rectosigmoid endometriosis.