Concurrent thyroid and abdominal diffuse large B-cell lymphoma arising in the milieu of chronic fibrosing thyroiditis.

Firm tender thyroid masses present a diagnostic challenge, with differentials including benign fibroinflammatory conditions and neoplastic processes. Imaging modalities cannot reliably distinguish between these differentials. The sampling-dependent nature of both cytopathology and core-biopsy histopathology introduces the potential for indeterminate or misleading results. Surgical biopsy can have high complication rates depending on the underlying condition. Here we report the case of a patient with chronic autoimmune thyroid disease who initially presented with fibroinflammatory thyroid disease and was subsequently diagnosed with B-cell lymphoma. This case highlights the diagnostic challenges associated with these conditions, the limitations of the sampling-dependent tissue assessment and the importance of re-evaluating the diagnosis when the clinical course is discordant with the initial biopsy findings.