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Co-occurrence of papillary thyroid carcinoma and primary lymphoma of the thyroid in a patient with long-standing Hashimoto's thyroiditis.

BACKGROUND
Papillary thyroid carcinoma (PTC) is the most common type of thyroid cancer, whereas mucosa-associated lymphoid tissue (MALT) lymphoma of the thyroid gland is uncommon. Simultaneous occurrence of both disease entities is very rare.


PATIENT FINDINGS
A 59-year-old man with known hypothyroidism from Hashimoto's thyroiditis (HT) was seen for thyroid nodules. A thyroid ultrasound revealed a heterogeneous thyroid gland with two hypoechoic nodules, one in the right aspect of the isthmus measuring 2.0 cm×3.2 cm×1.7 cm and another one in the left lobe measuring 1.4 cm×1.3 cm×1.2 cm. A fine-needle aspiration (FNA) of the right-sided nodule revealed atypical epithelial cells and atypical lymphoid cells in a background of lymphocytic thyroiditis; FNA of the left-sided nodule showed findings of PTC. A total thyroidectomy was performed. Lymph node dissection was not performed. Pathology showed extranodal marginal zone B-cell lymphoma of MALT type with extreme plasmacytic differentiation in the right nodule and PTC in the left nodule (pT1b Nx Mx). Postoperatively, he underwent radioactive iodine ablation treatment. There was only minimal neck uptake on the post-treatment scan. Further work-up did not show any evidence of extrathyroidal lymphoma. Seven years after the surgery, the patient had no evidence of recurrence of either malignancy.


SUMMARY
PTC is the most prevalent thyroid cancer and has an excellent prognosis. Primary thyroid lymphoma is rare and accounts for <5% of all thyroid cancers. Among the primary thyroid lymphomas, MALT lymphoma tends to have a more indolent course and a better prognosis. PTC and MALT lymphoma have been associated with HT. FNA has been validated in several studies for the diagnosis of MALT lymphoma; however, distinguishing MALT lymphoma from HT remains a challenge due to their histological similarities. The treatment of MALT lymphoma remains controversial; however, surgery is generally accepted in the early-stage MALT lymphoma as was performed in the present case.


CONCLUSION
Since HT is associated with PTC and MALT lymphoma, patients with HT deserve careful surveillance for both disease entities. In our patient, the management of one malignancy did not affect the management of the other, and the prognosis did not seem to be affected.