Twenty-one cases were selected from 236
thyroidectomies with a diagnosis of
Hashimoto's disease for detailed clinicopathologic study on the basis of "early" changes in three cases and an unusually heavy lymphoplasmacytic infiltrate in 18 cases. These cases were studied in conjunction with ten cases of high-grade
non-Hodgkin's lymphoma of the thyroid.
Immunoglobulin light chain restriction was demonstrated in five cases of Hashimoto's
thyroiditis and the diagnosis was accordingly changed to
low-grade lymphoma. All ten
high-grade lymphoma cases were of B phenotype and light chain restriction could be demonstrated in eight of them. The study revealed close homology between the lymphoplasmacytic infiltrate in Hashimoto's
thyroiditis and normal mucosa-associated lymphoid tissue (MALT). Like
lymphomas of mucosal sites, thyroid
lymphoma appears to be derived from the parafollicular ("centrocyte-like") B cells. The high-grade thyroid
lymphomas appear to be derived from low-grade
tumors. There were close histologic, immunohistologic, and clinical similarities between low- and high-grade non-Hodgkin's
lymphomas of the thyroid and those appearing in mucosal sites. This study confirms the close association between Hashimoto's
thyroiditis and
B cell lymphoma of the thyroid gland and suggests that this
tumor belongs to the group of non-Hodgkin's
lymphomas derived from MALT.