A potentially dangerous case of mistaken identity: giant asymptomatic composite phaeochromocytoma.

Summary: Small clinically silent phaeochromocytoma (PCC) can be identified in modern clinical practice as apparent adrenal incidentaloma or during screening of patients with familial tumour syndromes. Composite tumours comprising both PCC and a second tissue sharing embryological origin from the neural crest are rare, with fewer than 140 cases described in the literature. We report a 62-year-old woman with a 15 cm adrenal mass that was incidentally discovered on pulmonary imaging. A second 7 cm pelvic mass was also identified. The patient had no symptoms of catecholamine excess and normal blood pressure, even during a biopsy of the adrenal mass. Concordantly, urinary catecholamines were normal; however, urinary metanephrine and normetanephrine excretion were 23-fold and nine-fold the upper limit of normal, respectively. Surgical resection resulted in normalisation of metanephrines and normetanephrines. Histopathology showed a composite PCC/ganglioneuroma with discrete areas of both tumours within the same mass. Later resection of the pelvic mass revealed an unrelated ovarian teratoma. This case demonstrates a novel presentation of a composite PCC/ganglioneuroma and the presumptive role of catechol-O-methyltransferase in inactivating catecholamines within PCC, resulting in undetected growth of the tumour to a giant size. It highlights that metanephrines and normetanephrines are the preferred investigation for PCC. Learning points: Clinically silent PCC are increasingly common but are typically small, with relatively low levels of metanephrines. Upregulation of COMT causes intratumoural inactivation of catecholamines and may facilitate asymptomatic growth of PCC to a giant size. Measurement of metanephrine and normetanephrine levels in plasma or urine is the preferred biochemical investigation for PCC. Composite PCC are rare but have a similar clinical presentation and management to other PCC.