Is MRI more accurate than CT in estimating the real size of adrenal tumours?

Background: The size of adrenal tumour plays an important role in the indications for surgical excision of non-functioning adrenal tumours and in selecting the best surgical approach. Computed tomography (CT) has been reported to underestimate the real size of adrenal lesions. The accuracy of magnetic resonance imaging (MRI) in predicting the true tumour size has not been previously investigated. The present retrospective study investigates the accuracy of MRI and CT in the pre-operative determination of true adrenal tumour size. Methods: The medical records of 65 patients who underwent adrenalectomy for an adrenal mass were reviewed. The size of adrenal tumours as determined by pre-operative MRI and/or CT was compared with the «true» histopathological size. The impact of histological diagnosis on size estimation was also investigated. Results: The median age at diagnosis was 42 years (range 1–82 years) and more patients were female (60%). Five patients had bilateral adrenalectomy, thus giving rise to 70 adrenal specimens. The histopathological size of adrenal tumours ranged from 0.9 to 26 cm with a mean of 5.96 cm and a median of 4.70 cm. For tumours larger than 3 cm, MRI significantly underestimated the real tumour size by 20% (P<0.001). CT also underestimated the size of such tumours by 18.1% (P<0.003). Adrenal phaeochromocytomas were consistently underestimated by both modalities. Conclusions: MRI and CT significantly underestimated the true size of adrenal tumours larger than 3 cm by 20% and 18%, respectively. Surgeons and endocrinologists should interpret the pre-operative size of adrenal lesions with caution.