Unicentric mixed variant Castleman disease associated with Hashimoto disease: the role of PET/CT in staging and evaluating response to the treatment

Castleman disease (CD) is a rare atypical lymphoproliferative disease, pathologically classified as hyaline vascular, plasma cell type and mixed type variant. The underlying cause of CD is unknown, however several theories including autoimmunity have been proposed. We describe a patient diagnosed with unicentric mixed variant CD and Hashimoto thyroiditis, concurrently. She was staged with fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT) and the disease was localized to the mediastinum. After 6 cycles of chemotherapy consisting of vincristine and prednisone, the mediastinal lymph nodes regressed, but did not disappear from the CT scan. However, FDG-PET/CT showed complete metabolic response. Although the role of FDG-PET/CT in staging and evaluation of treatment response is controversial, this case shows that PET/CT can be effective and even better for staging and response evaluation. This case is also unique as there no case of CD in association with Hashimoto thyroiditis has been reported previously. However, the possibility of a coincidental association must be raised, especially when the high prevalence of Hashimoto thyroiditis is considered.