Osseous metaplasia has been described at many sites and in association with a large number of tumours. However, previous reports of osseous metaplasia in ovarian lesions are rare, there being only three published cases, which were associated with papillary serous carcinoma, thecoma, and endometrioma.^1–3 We report a case of osseous metaplasia hitherto undescribed in a benign ovarian cyst in a girl with a complex urogenital malformation.

The patient was a 16 year old girl, who was diagnosed with a complex cloacal anomaly at birth, requiring posterior sagittal anorectovaginourethroplasty, followed by further lower urinary tract reconstructive procedures. At the time of surgery, normal ovaries and fallopian tubes were noted. She attained menarche at 14. Two years later she complained of lower abdominal pain and dysmenorrhoea. Ultrasound examination revealed a large cystic lesion in the right side of the pelvis. No calcification was seen on pelvic imaging and tumour markers were normal. Repeated aspiration did not result in longterm symptom relief and an open resection was performed. At surgery, a large multiseptated cyst was noted occupying most of the pelvis behind the augmented bladder on the right and crossing the midline, closely adherent to the right ovary and right fallopian tube. The right ovary was partially resected along with the cyst. Postoperative recovery was uneventful. Histological examination demonstrated a complex tubal cyst with chronic salpingitis, in addition to a simple follicular ovarian cyst, in the wall of which osseous metaplasia was noted (fig 1). The entire specimen was embedded and multiple sections examined, but there were no additional features present to suggest a teratoma.

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Figure 1

 Photomicrograph of a simple follicular ovarian cyst with focal osseous metaplasia in a 16 year old girl with a cloacal anomaly. Original magnification, ×40.

The pathogenesis of osseous metaplasia is unclear, but in some circumstances appears to be an unusual reaction to tissue damage and repair. It is interesting that the previously reported case associated with an endometriotic ovarian cyst also occurred in an anatomically abnormal pelvis, in a supernumerary ovary.² Osseous metaplasia in a benign ovarian cyst appears to be an unusual incidental finding of unknown importance and aetiology, which is more common in anatomically abnormal ovarian tissue in women of reproductive age.