Intravascular large B-cell lymphoma masked by multinodular goiter of the thyroid: a case report and diagnostic pitfall.

Intravascular large B-cell lymphoma (IVLBCL) is a subtype of diffuse large B-cell lymphoma (DLBCL), which is characterized by the proliferation of neoplastic B-lymphoid cells within the lumens of small and medium-sized blood vessels, especially capillaries. It is a rare and highly aggressive malignancy with frequent multi-organ involvement, most commonly affecting the central nervous system, bone marrow, spleen, skin and adrenal glands. Nevertheless, IVLBCL arising in the thyroid gland is extremely rare. Nodular goiter is pathologically defined as thyroid nodular hyperplasia, which is a common benign thyroid lesion. Herein, we report a patient with IVLBCL complicated by bilateral nodular goiter. Notably, the IVLBCL was confined solely to the nodular goiter tissue, with no invasion of the adjacent normal thyroid parenchyma or cervical lymph nodes. Both peripheral blood circulating tumor cell detection and EBER staining were negative. Under low microscopic magnification, a typical background of nodular goiter was observed, and tumor cells were scattered in the intervals of thyroid follicles, mimicking inflammatory lesions. The lesions were almost completely obscured by the background of nodular goiter. This unique and occult growth pattern is highly prone to missed diagnosis, especially for junior pathologists. IVLBCL carries an overall poor prognosis. Early definite diagnosis and timely initiation of standardized chemotherapy can significantly prolong patient survival.