When Two Benign Ovarian Pathologies Collide: An Incidental Bilateral Collision Tumour of Mature Cystic Teratoma and Endometrioma.

Ovarian collision tumours are rare entities defined by the coexistence of two histologically distinct neoplasms, most commonly reported within the same ovary, while bilateral involvement is extremely uncommon and often under-recognised. Endometriomas are common benign ovarian lesions in women of reproductive age and may be asymptomatic or incidentally detected, whereas mature cystic teratomas are the most frequent ovarian germ cell tumours and are often discovered unintentionally during imaging or surgery. We report the case of a 27-year-old woman in whom adnexal lesions were incidentally identified, with no significant gynaecologic symptoms at presentation. Pelvic magnetic resonance imaging (MRI) showed bilateral ovarian masses, and surgical exploration revealed distinct ovarian lesions involving both ovaries. Histopathological examination confirmed a mature cystic teratoma in one ovary and an endometrioma in the contralateral ovary. This unusual and incidental bilateral presentation highlights the limitations of preoperative imaging in detecting synchronous ovarian pathologies and underscores the importance of thorough intraoperative assessment and definitive histopathological evaluation. Increased awareness of this rare entity is essential to optimise surgical management, fertility preservation and follow-up. LEARNING POINTS: Ovarian collision tumours are rare pathological entities, and presentations involving histologically distinct tumours in different ovaries are extremely uncommon.This case adds to the limited literature by illustrating an atypical and largely incidental presentation with non-specific symptoms.Such variability in clinical and radiologic presentation underscores the diagnostic complexity of these tumours and the potential for under-recognition.Although advanced imaging, particularly MRI, may provide important diagnostic clues, definitive diagnosis relies on a comprehensive histopathological assessment.Awareness of this unusual presentation is crucial, as early recognition of bilateral but distinct ovarian tumours can influence surgical planning, fertility-preserving strategies and follow-up, ultimately improving individualised patient care.