Atypical Presentation of Sarcoidosis on ¹⁸F-FDG PET/CT

DISCUSSION

¹⁸F-FDG uptake can be nonspecific and is associated with a variety of conditions, including infectious processes (pneumonia), chronic inflammatory processes (granulomatous disease), or acute inflammatory processes (postradiation changes) (1). These inflammatory conditions have in common the increased ¹⁸F-FDG uptake by neutrophils, macrophages, and lymphocytes that may mimic neoplastic ¹⁸F-FDG uptake (2). These conditions may be indistinguishable from malignancy on the basis of PET alone. The ¹⁸F-FDG PET study in this case demonstrated several discrete high-grade ¹⁸F-FDG–avid lymph nodes in the subcarinal, right paratracheal, and right suprahilar lymph nodes, with likely station 11 and 12 lymph nodes.

The appearance of several discrete, intensely avid lymph nodes is not typical of lymphoma, because cases with such high-grade activity (SUV_max > 10) typically present with lymph node conglomerations. Additionally, other clusters of lymph nodes are typically ¹⁸F-FDG–avid, such as the axillary and cervical nodes (3).

The anatomic distribution of the ¹⁸F-FDG–avid lymph nodes was consistent with the drainage pattern of a right-upper-lobe neoplasm, as the malignant cells typically deposit into the right paratracheal lymph nodes and may skip stations 10–14 (4). Nevertheless, an ¹⁸F-FDG–avid neoplasm was not identified on the imaging, suggesting another etiology.

Activated granulomatous disease (histoplasmosis or sarcoidosis) or pneumonia is often a consideration in cases with a similar appearance. Although pneumonia will also follow anatomic lymph node drainage, the activity is often higher peripherally and decreases in the central lymph drainage sites, as is the case with lung cancer (1,4). Additionally, there was no anatomic evidence of a consolidative or infiltrative process. Conversely, sarcoidosis will typically demonstrate higher activity in the central mediastinal nodes than in the hilar and lobar nodes, will affect both sides of the chest symmetrically, will not follow any specific lymph node drainage, and often does not present with a parenchymal lung lesion. Chundru et al. (1) proposed criteria based on case reviews to distinguish sarcoid from malignancy, including symmetric uptake in the hila; calcifications in the lymph nodes, liver, or spleen; and unexpected lymphatic drainage. However, the presentation of the lesions in this case did not meet any of these criteria. Therefore, the histopathologic diagnosis of sarcoidosis was unexpected.

¹⁸F-FDG PET/CT has many proven applications beyond cancer imaging; its superior anatomic localization of increased metabolic activity has been found to be more sensitive for inflammatory or infectious processes such as sarcoidosis than are other nuclear imaging techniques such as ⁶⁷Ga imaging (2). However, the presence of such metabolically active benign processes can present a challenge when one interprets PET in the setting of cancer. Despite many studies using various criteria and semiquantitative methods to predict tumor phenotype or distinguish benign from malignant findings, atypical presentations (such as sarcoid in this case) are always possibilities that should be considered. Even as the accuracy of ¹⁸F-FDG PET imaging improves, its findings should always be considered in conjunction with the clinical setting and confirmed with tissue sampling when possible.