Context. In patients with a history of extraadrenal tumor, incidental discovery of an adrenal mass necessitates to exclude the possibility of metastatic malignancy. Detection of the malignant tissue is a difficult challenge when metastasis occurs in an adrenal adenoma, forming a collision/composite tumor.

Objective, Design and Setting. We report two patients with adrenal collision/composite tumors referred to two French University Hospitals.

Patients and Results. Two patients with histories of mammary and sigmoid carcinomas, respectively, presented with adrenal mass discovered eight and three years after surgical removal of the primary tumor. In the two cases, computerized tomographic scan showed that the adrenal tumor contained two components with low and high attenuation values, respectively. Uptake of iodocholesterol by the adrenal tumor in case one and elevated plasma ACTH-stimulated 17OH progesterone values in case two strongly argued for the diagnosis of primary adrenocortical tumors. Enlargement of the adrenal mass during follow-up in case one and association of the adrenal lesion with a hepatic mass in case two led to adrenalectomy. In both cases, histological examination of the tumor demonstrated the presence of metastatic carcinoma tissue in an adrenocortical adenoma allowing to classify the neoplasia as a collision/composite tumor.

Conclusion. These observations show that collision/composite tumors of the adrenal gland formed by carcinoma metastasis in benign adenomas are a pitfall of iodocholesterol scintigraphy and/or plasma steroid assays to exclude the diagnosis of adrenal metastasis. Conversely, CT scan is a useful tool for the distinction between the benign and malignant tissues in adrenal collision/composite tumors.