Background: Primary thyroid lymphoma (PTL) is a rare thyroid tumor that accounts for only 5% of thyroid malignancies, but is associated with higher mortality than differentiated thyroid cancer. The major risk factor for thyroid lymphoma is the presence of Hashimotos thyroiditis (HT) with an estimated 60-fold increased risk. The lymphocytic infiltrate in HT appears to develop into lymphoma in a minority of patients. The differential diagnosis between Hashimotos thyroiditis and thyroid lymphoma can be difficult.
Case report: We report a 34-year-old man who was referred to the endocrinologist for a large, rapidly growing thyroid nodule. The patient had not noticed the nodule until 2 months earlier. He had no known thyroid disease. Blood analysis showed normal thyroid hormone levels with positive antithyroid autoantibodies (TSH 1.97 mUI/L; free T4 14 pmol/L; aTg 437 mUI/L; aTPO 283 mUI/L; and TRAb 1.84 mUI/L). Ultrasound examination (US) showed a large solitary hypoechogenic thyroid nodule of 53 mm in the right lobe, EU-TIRADS V. Scintigraphy confirmed the large nodule in the right lobe with low iodine uptake. Fine needle aspiration cytology was performed and showed the prevalence of lymphoid population with abundant presence of CD20+ lymphocytes by immuno-histochemistry (IHC). The diagnosis of lymphoproliferative disease could not be ruled out. To obtain a definite diagnosis, an anatomopathological examination with flow cytometry was necessary. The patient underwent a right hemithyroidectomy. The postoperative anatomopathological examination showed a lymphocytic infiltrate with the formation of germinal centers, with CD20+ and CD10+ lymphocytes, and expression for both chain kappa and lambda by plasma cells. There was no t(14; 18) chromosome translocation on BCL2 FISH, making the diagnosis of lymphoma less likely. Following the results of the pathological examination and IHC, the diagnosis of a thyroid nodule in the context of Hashimotos thyroiditis was made. Given the diagnostic complexity, a 18F-FDG PET-CT scan was also performed which showed no signs of malignancy or extrathyroidal disease.
Discussion: the reported case describes a difficult diagnosis between Hashimotos thyroiditis and thyroid lymphoma in a patient presenting with a single large thyroid nodule. There is a correlation between the presence of Hashimotos thyroiditis and the occurrence of thyroid lymphoma (25-75 % of patients with PTL also have a diagnosis of Hashimoto). The association between HT and PTL seems to originate from the development and alteration of intrathyroidal lymphoid tissue in HT. Cytological examination (FNAC) with IHC is often sufficient for definitive diagnosis, but in rare cases, histology is necessary for a definite diagnosis (as in our patient). The two pathologies share similarities on FNAC with IHC. The cellular infiltrate of PTL is characterized by CD20+ and CD19+ lymph nodes and the presence of K/L monoclonality (usually between 3-4), whereas the cellular infiltrate of the nodular form of Hashimotos disease lymph node-like pattern is characterized by a CD19+ prevalence and also by a variable K/Lambda monoclonality (usually between 0. 5 to 3.0). The diagnostic difficulty in our patient was due to the presentation with a large and rapidly growing nodule, with FNAC showing a large CD20+ lymphocyte population, for which a definite diagnosis could only be made with histology and other diagnostic tests such as PET-CT to exclude extra-thyroid involvement.
Conclusions: We present the case of a 34-year-old man with a large, rapidly growing thyroid nodule. Faced with a large and fast-growing nodule, one immediately thinks of a possible malignancy such as PTL or anaplastic thyroid cancer, but a similar clinical presentation can also be observed in the presence of Hashimotos thyroiditis. In these cases, only a biopsy can confirm or deny the diagnosis of malignancy.
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