Ann R Coll Surg Engl 2010; 92:1 Teratomas are a type of germ-cell tumour which may contain embryological tissues from more than one germ-cell layer, namely ectoderm, mesoderm and endoderm. Teratomas usually occur in the gonads but 1-5% occur in extragonadal sites.1 Rarely, teratomas can be malignant. Here we describe a case of neuroendocrine carcinoma arising in a mature retroperitoneal teratoma. Neuroendocrine carcinomas have been described in teratomas of the gonads 2 and sacrococcygeal region. 3 To our knowledge, this is only the second reported case of neuroendocrine carcinoma arising in a mature retroperitoneal teratoma. Case historyA 48-year-old woman was admitted to our hospital with a month's history of pain in the left upper quadrant (LUQ). The pain had increased in severity over a 3-day period and was associated with one episode of bilious vomiting. She did not report any change in bowel habit or per-rectal bleeding. There was no history of dyspepsia, acid reflux symptoms, urinary symptoms, fever, weight loss or night sweats. She had no significant past medical history and was not on any regular medication. Clinical examination revealed a 10-cm, firm, immobile mass in the LUQ. Observations confirmed that she was haemodynamically stable and afebrile. Bloods showed a raised C-reactive protein level of 299 mg/l, however the Full Blood Count, Urea and Electrolytes, and Liver Function Tests were normal. The abdominal radiograph revealed a large space occupying lesion in the LUQ with displacement of bowel loops to the right. An abdominal ultrasound scan showed a large hypo-echoic mass in the left hypochondrium measuring 15.2 × 11.6 × 14.1 cm. There was a solid echogenic nodule measuring 1.3 cm within the mass with no obvious Doppler flow. A contrast-enhanced abdominal computed tomographic (CT) scan (Figs 1 and 2) revealed ascites and a large well circumscribed 14.8 × 17.6 cm mass in the LUQ. The attenuation value of the lesion was less than 25 Hounsfield Units , suggesting a cystic lesion with possibly haemorrhagic or mucinous content. Enhancing soft tissue components were noted in the cyst wall. The lesion displaced the spleen and the left kidney inferiorly and the stomach superiorly. Clear fat planes separating the cyst and the surrounding structures were seen.A multidisciplinary decision was made to proceed to laparotomy. Operative findings revealed at least 3 litres of turbid fluid in the abdominal cavity and an approximately 15-cm sized cystic mass was seen arising from the retroperitoneum, sandwiched between the left adrenal, spleen and the gastro-oesophageal junction (Fig. 3). The cyst could be easily separated from surrounding structures with no obvious communication or attachment. The cyst wall was ruptured at least in one area explaining the free fluid and perhaps her acute presentation. Cut section revealed viscous chocolate-coloured contents with a smooth cyst wall (Fig. 4). The presence of a small solid area within the cyst wall was noted.Extensive sampling of the cyst wall showed mature, cystic...