MRI in non-pregnancy-related decidualisation of deep pelvic endometriosis: a case report and review of the literature

Abstract Decidual changes in deep pelvic endometriosis (DPE) are extremely rare. During pregnancy, endometriotic activity and symptoms diminish due to hormonal changes, whereas the endometrial lining undergoes decidualisation in response to progesterone. In some cases, a significant hormonal boost can promote the decidualisation of endometriotic implants, with endometriomas being the most common sites for such changes. Decidualised endometriosis at sites outside the ovaries is a rarer phenomenon, and the associated imaging features have been studied less. We present a case of non-pregnancy-related decidualisation involving multiple sites of DPE. A 33-year-old woman exposed to ovarian stimulation before in vitro fertilization (IVF) presented with chronic pelvic pain and vaginal bleeding. Emergency CT demonstrated irregular solid tissue in the pouch of Douglas and along the bowel loops, raising concern for a neoplastic process. Tumour markers were negative. Transvaginal ultrasound showed a hypoechoic, irregular, hypervascular lesion, and ultrasound-guided biopsy demonstrated decidualised stromal endometriosis. MRI confirmed a solid mass centred in the posterior cul-de-sac with intermediate T2-weighted (W) signal intensity, haemorrhagic foci on fat-suppressed T1W images, marked diffusion restriction, and avid post-contrast enhancement. Similar solid components were also present at multiple pelvic sites, adjacent to DPE implants. Short-interval MRI follow-up showed size reduction; however, due to persistent pain, fertility-sparing surgery was performed, and histology confirmed decidualisation of DPE at all sites. This rare case emphasizes that decidualisation can also occur on DPE and outside pregnancy, potentially mimicking malignant transformation, highlighting the role of MRI in aiding diagnosis and guiding proper management for these patients.