Rosai-Dorfman disease or sinus histiocytosis with lymphadenopathy is a rare benign histiocytic proliferative disorder of unknown etiology first described in 1969. It typically affects older females and most common presentation is with massive lymphadenopathy and nonspecific systemic symptoms; therefore, it is often confused with lymphoproliferative disorders [1, 2]. We present the case of a 69-year-old woman with nasal obstruction as only complaint. Laboratory tests showed normal leukocyte count with elevated ANC (absolute neutrophil count), normal RBC count with normal MCV (mean corpuscular volume) and MCH (mean corpuscular hemoglobin), elevated ESR (erythrocyte sedimentation rate), and normal IgG, IgA, and IgM values. Evaluation revealed a nasopharyngeal mass, which was biopsied and reported emperipolesis with positive CD68 and S-100; typical and differential findings of this disease [1, 2]. 18F FDG PET/CT was performed to determine the extent and involvement of the disease. Considering the presence of few symptoms and no significant laboratory abnormality, treating physicians decided to start a regimen of corticosteroids (prednisolone) for a period of 4 months, after which a follow-up with 18F FDG PET/CT will be performed.

The World Health Organization classification divides thymomas according to morphology, epithelial component, and cell atypia. They are grouped into 3 large subgroups: low-risk thymomas (types A, AB, and B1), high-risk thymomas (types B2 and B3), and thymic carcinomas. Tumor subtype represents an independent prognostic factor, which determines therapeutic decision. All thymomas show some degree of 18 F-FDG uptake, which tends to increase with the grade of malignancy; this is related to glucose transporter 1 (GLUT1) expression. This review collects all types of thymomas with illustrative images and provides a guide to get familiar with histological characteristics of the lesions and have them in mind because, even imaging findings can overlap among subtypes, certain characteristics can be combined to make an accurate diagnosis based on 18 F-FDG PET-CT findings.