World J. Surg. 21, 36 – 40, 1997 WORLD Journal of SURGERY © 1997 by the Socie ´te ´ Internationale de Chirurgie Diagnostic Dilemma of Small Incidentally Discovered Adrenal Masses: Role for 131 I-6-Iodomethyl-norcholesterol Scintigraphy Richard T. Kloos, M.D., 1 Milton D. Gross, M.D., 1,2 Brahm Shapiro, M.B., Ch.B., Ph.D., 1,2 Isaac R. Francis, M.D., 3 Melvyn Korobkin, M.D., 3 Norman W. Thompson, M.D. 4 1 Division of Nuclear Medicine, Department of Internal Medicine, University of Michigan Medical Center, 1500 E. Medical Center Drive, B1G412, Box 28, Ann Arbor, Michigan 48109, U.S.A. 2 Nuclear Medicine Service (115), Department of Veterans Affairs Medical Centers, 2215 Fuller Road, Ann Arbor, Michigan 48105, U.S.A. 3 Department of Radiology, University of Michigan Medical Center, 1500 E. Medical Center Drive, B1D520, Box 30, Ann Arbor, Michigan 48109, U.S.A. 4 Department of Surgery, University of Michigan Medical Center, 2920D Taubman Center, 1500 E. Medical Center Drive, Ann Arbor, Michigan 48109-0331, U.S.A. Abstract. Incidentally discovered adrenal masses are detected in 0.35% to 5.00% of patients imaged with computed tomography (CT) for reasons other than suspected adrenal pathology. Most small adrenal masses are benign, although malignant tumors < 3 cm in diameter are well de- scribed. In the setting of normal adrenal hormonal secretion, the prefer- ential accumulation of 131 I-6-iodomethyl-norcholesterol (NP59) by ad- renocortical tissues allows the distinction of adenomas from other space-occupying or destructive lesions, with diagnostic images being obtained in 100% of lesions > 2 cm. Although some lesions < 2 cm have yielded nondiagnostic images, the frequency of this phenomenon and thus the utility of NP59 scintigraphy for the evaluation of small adrenal lesions has remained incompletely characterized. Between January 1976 and December 1994 a total of 166 patients with nonhypersecretory unilateral adrenal masses < 3 cm in maximal diameter, discovered incidentally during CT examinations of the abdomen or chest for reasons other than clinically suspected adrenal disease, were studied with NP59 scintigraphy. Nonhypersecretory masses < 1 cm, > 1 to < 2 cm, and > 2 to < 3 cm yielded diagnostic images in 52%, 89%, and 100% of patients, respectively. Lesions other than adenomas, including malignancies, > 1 to < 2 cm and > 2 to < 3 cm were present in 9% and 10% of patients, respectively. These findings emphasize the need to determine the nature of small incidentally discovered adrenal masses whose management may alter patient care and confirm the utility of NP59 scintigraphy to evaluate nonhypersecretory adrenal masses regardless of size. The likelihood of malignant incidentally discovered adrenal masses (incidentalomas) rises with increasing diameter [1]. How- ever, in the absence of known extraadrenal primary malignancy, benign adrenal masses outnumber malignant masses regardless of size [1]. Furthermore, malignant neoplasms must be small at some time in their development, and some benign masses may become large. Adrenal malignancies  3 cm are well described [1]. The large incidentally discovered adrenal mass ( 3 cm in diameter) poses little difficulty for anatomic characterization with computed tomography (CT) or magnetic resonance imaging (MRI), func- tional characterization with adrenocortical scintigraphy or positron emission tomography (PET), or tissue characterization by CT-guided adrenal biopsy [1, 2]. Smaller masses ( 3 cm in diameter) may pose greater difficulty. Yet determination of their etiology remains no less important, as subsequent critical clinical and diagnostic decisions may be based on the presence or absence of primary adrenal malignancy, metastatic disease in the adre- nal(s), or the presence of some adrenal lesions other than simple nonhypersecretory adenomas. We have previously noted that the frequency of diagnostically useful imaging patterns is reduced when lesions  2 cm are studied scintigraphically, but the regularity of this occurrence and thus the relation of lesion diameter to sensitivity of 131 I-6- iodomethyl-norcholesterol (NP59) imaging have not been docu- mented in detail. Thus when faced with the problem of a small nonhypersecretory adrenal mass, the utility of adrenal scintigra- phy has remained incompletely characterized. In this paper we report our experience with the scintigraphic characterization of small ( 3 cm in diameter) incidentally discovered euadrenal masses. Materials and Methods Between January 1976 and December 1994 a total of 166 patients with unilateral adrenal masses discovered incidentally during CT examination of the abdomen or chest performed for reasons other than clinically suspected adrenal disease were studied with NP59 scintigraphy. CT was initially performed with an EMI-5005 and later with a Picker 1200SX at the VA Medical Center, Ann Arbor, and with a GE-8800 or GE-9800 scanner at the University of Michigan Medical Center. Contiguous 5- to 10-mm sections were obtained on the newer scanners after intravenous or oral radio- graphic contrast administration. All patients referred for NP59 scintigraphy gave written in- Correspondence to: R.T. Kloos, M.D., University of Alabama at Bir- mingham, 1808 Seventh Avenue South, BDB 756, Birmingham, AL 35294-0012, U.S.A.