Primary malignant tumours and malignant transformation of cysts in the retrorectal space: MRI diagnosis and treatment outcomes.

Background: There are no clear guidelines for the diagnosis and treatment of malignant retrorectal tumours. The purpose of this study was to increase preoperative diagnostic knowledge and to describe the outcomes of treatment for these patients. Methods: This retrospective study was conducted on patients who underwent complete retrorectal tumour resection between May 2006 and July 2018, and had confirmed post-operative pathology reports. Demographic and clinical data (including imaging, perioperative, pathological, and prognostic data) were collected and analysed. Results: Malignant lesions were identified in 15 (9 [60%], female) patients. The median age of the patients was 59 years (range, 34-72 years). Primary malignant tumours were identified in seven patients with solid tumours, in which gastrointestinal stromal tumours accounted for 71.4% (five of seven) and the remainder were chordoma or mucinous adenocarcinoma. Malignant transformation of cysts occurred in another eight patients with heterogeneous tumours, while histopathological features were present in 75% (six of eight) of patients with mucinous adenocarcinoma, and the remainder were squamous-cell carcinoma or neuroendocrine tumour (Grade 2). The malignant characteristics of the solid portions observed using magnetic resonance imaging (MRI) were as follows: the cyst wall of the tumour was irregularly thickened; the surface was convex or lobed; the solid tumour had no capsule, or the capsule was destroyed; and the surface had a gyrus-like morphology. At a median follow-up time of 52 months (range, 13-100 months), the overall recurrence-free survival rate was 40.0% and the survival rate was 46.7%. Conclusion: Some MRI features can be used to distinguish malignant retrorectal tumours from benign retrorectal tumours. The survival rate of patients with malignant retrorectal tumours is poor.