An imaging algorithm for the differential diagnosis of adrenal adenomas and metastases

The purpose of this study was to develop an algorithm using CT and chemical-shift MR imaging for the characterization of adrenal masses in patients with a primary cancer and no other evidence of metastatic disease.

Thirty-three patients with 37 adrenal masses (19 metastases, 18 adenomas), all of whom had a known primary cancer, were studied with noncontrast CT and chemical-shift MR imaging (1.5 T). Lesion size and density in Hounsfield units (H) were determined by CT. Adrenal signal intensity normalized to that of spleen was used to calculate adrenal-spleen ratio (ASR), defined as the percentage of signal remaining in the opposed-phase image relative to the in-phase image. Lesions less than or equal to 0 H were classified as benign, lesions greater than 20 H were regarded as malignant, and lesions between 0 and 20 H were regarded as indeterminate. Diagnoses were confirmed by biopsy (for 19 lesions) or by follow-up imaging (for 18 lesions). An imaging algorithm was derived by determining the relative value of CT and MR imaging for diagnosing the lesions. The reimbursement rates for CT-guided biopsy and MR imaging of the abdomen were obtained from Medicare.

All 13 lesions of 0 or less H were correctly classified as benign by CT. ASR was less than 70 in 10 of these 13. In another 13 lesions, H was greater than 20; all were malignant and all had an ASR greater than 80. Of 11 CT-indeterminate lesions, four of five adenomas had an ASR less than 70, and four of six metastases had an ASR greater than 80. Two malignant lesions had ASRs between 70 and 80 and were diagnosed by biopsy findings. One CT-indeterminate adenoma had an ASR of 84 and was diagnosed by biopsy findings. The reimbursement rate by Medicare is similar for CT-guided biopsy with pathologic interpretation and for MR imaging of the abdomen.

An algorithm was developed for diagnosis of adrenal lesions that uses the density reading on noncontrast CT as the first step, with chemical-shift MR imaging for CT-indeterminate lesions. In this algorithm, lesions of 0 H or less may be regarded as benign and further work-up is not required. Lesions with a density greater than 20 H are likely malignant and should be biopsied when the result will influence management. For CT-indeterminate lesions, we recommend chemical-shift MR imaging. An ASR threshold of 70 indicates a benign lesion, and no further workup is required in these patients. Lesions with an ASR greater than 70 should have a biopsy performed, depending on the clinical situation. The above algorithm is cost-effective and reduces the number of biopsies required without reducing the sensitivity of detecting malignant lesions.