Biochemical and radiological relationships in patients with pheochromocytoma: lessons from a case control study

Summary Background An elevation of fractionated plasma or urinary metanephrine ( MN ) or nor‐metanephrine ( NMN ), collectively called metanephrines ( MN and NMN ), >4‐fold above the upper limit of normal ( ULN ) is usually considered to be diagnostic for pheochromocytoma ( PHEO ). There are a greater number of false positive results when the elevations are more modest. Aim To identify biochemical and radiological features in PHEO s with modest elevations (<4‐fold above ULN ) of metanephrines. Methodology We retrospectively reviewed the charts of 112 patients with PHEO (10% extra‐adrenal) and 208 patients with a non‐ PHEO adrenal mass operated from 1997–2011, who had metanephrines measured pre‐operatively. We divided PHEO into group 1 ( n  = 90) with metanephrines ≥4‐fold ULN and group 2 ( n  = 22) with metanephrines <4‐fold ULN . The non‐ PHEO group was designated as group 3. Results The median (range) tumour size in group 1 and group 2 was 4·8 cm (1·7–22) and 3·0 cm (1·7–5) respectively ( P  < 0·001). All patients with PHEO in group 2 had a tumour <5 cm in size. The MN fraction was elevated in about 65% of groups 1 and 2; only 2 (1%) patients in group 3 had an elevated urinary MN fraction, and none were associated with an elevated plasma MN fraction. All PHEO s had a pre‐contrast attenuation ≥17 Hounsfield Units ( HU ). Conclusions Modest elevations (<4‐fold ULN ) of the NMN fraction in an adrenal mass >5 cm are almost always falsely positive. Elevations in plasma and urinary MN fraction are less likely to be false positive. The CT pre‐contrast attenuation of PHEO s is >10 HU .