Thyroid Lymphoma: A Rare Cause of Thyroid Malignancy

Background: Primary Thyroid Lymphoma is rare accounting for 2-5% of all thyroid malignancies and less than 2% of extranodal lymphomas. Data has suggested annual incidence to be 2 per 1 million. (1) Clinical Case: A 72-year-old female with coronary artery disease, hyperlipidemia and hypothyroidism was diagnosed with extranodal follicular cell lymphoma of the breast. FDG-PET showed evidence of diffuse avidity within her thyroid gland, with SUV of 13.8. She denied any drastic change in the size of her neck or any compressive symptoms. Thyroid ultrasound performed months before her diagnosis of lymphoma, showed a two-fold increase, most notable to the right lobe compared to prior imaging; her gland remained otherwise unchanged with heterogenous echotexture. Fine needle aspiration and flow cytometry were obtained on thyroid tissue that showed follicular cell lymphoma. She was started on single-therapy Rituxan regimen with oncology. Several months later appearance and examination of her neck had improved with decrease in size of thyroid gland. Repeat FDG-PET scans, with the most recent being 11 months after starting Rituxan therapy, showed reduction in the avidity within the thyroid gland and decreasing size; with SUV of 7.8. Repeat thyroid ultrasound continued to show heterogeneous echotexture with overall decrease in volume in gland size by nearly 50% within the right lobe and approximately 30% within the left lobe. Conclusion: Thyroid lymphoma should be suspected in a rapidly enlarging thyroid gland with or without pain or compressive symptoms. However, we propose that it should also be considered in the setting of FDG-PET avidity in thyroid tissue in setting of clinical suspicion. Diffuse FDG-PET scan avidity in the thyroid gland does not automatically equate to malignancy as prospective and retrospective information has shown that diffuse uptake can be seen incidentally, in cases of diffuse goiter, Graves disease or chronic lymphocytic thyroiditis and that the uptake has been seen to range between 5.6-16.8 SUV. (2) The American Thyroid Association 2015 thyroid cancer guidelines propose that if diffuse uptake is seen within the thyroid gland on PET imaging that thyroid ultrasound imaging and functional testing be undertaken. If imaging shows findings suggestive of only chronic lymphocytic thyroiditis (diffuse heterogeneous gland) that no further action is required. In this patient a different approach was taken with investigations pursued in the setting of enlarging thyroid gland on imaging and known extranodal lymphoma allowing for ultimate diagnosis. References: 1. Stein, S., et al. “Primary Thyroid Lymphoma: A Clinical Review.” The Journal of Clinical Endocrinology & Metabolism, vol. 98, no. 8, 2013, pp. 3131-3138 2. Karantanis, D., et al. “Clinical Significance of Diffusely Increased 18F-FDG Uptake in the Thyroid Gland.” Journal of Nuclear Medicine, vol. 48, no. 6, 2007, pp. 896-901