Uptake of (99m)Tc-MIBI by Sclerosing Pneumocytoma Raising a False Suspicion of Metastasis From Medullary Thyroid Carcinoma

Technetium-99m methoxy isobutyl isonitrile ((99m)Tc-MIBI; sestamibi) single-photon emission computed tomography (SPECT)/computed tomography (CT) performed for preoperative localization of parathyroid adenomas or for other indications can reveal incidentalomas. Interpretation of such findings can be challenging, particularly when thyroid or other endocrine tumors are also present. Preoperative staging of a 59-year-old female patient with medullary thyroid carcinoma (MTC) showing moderate hypermetabolism on (18)F-fluorodeoxyglucose positron emission tomography/CT also detected a slightly hypermetabolic pulmonary nodule (standardized uptake value normalized by body weight max = 2.0 g/mL). A sestamibi SPECT/CT performed because of concomitant primary hyperparathyroidism showed increased uptake by both the MTC and the pulmonary nodule, raising suspicion of MTC metastasis. Lung wedge resection biopsy revealed a sclerosing pneumocytoma (SPC), a rare benign pulmonary tumor not previously known to retain sestamibi. In contrast to classical knowledge that sestamibi uptake by tumors is associated with its retention by mitochondria, immunohistochemical analyses showed that the mitochondrial content of the patient’s SPC was low. This case illustrates the behavior of SPC in sestamibi scintigraphy and indicates that SPC is a potential cancer mimicker in this setting.