An uncommon site for papillary thyroid carcinoma metastasis: a case of breast involvement.

[BACKGROUND] Papillary thyroid carcinoma (PTC) is a slow-growing neoplasm with an overall favorable prognosis, frequently disseminated via lymphatic channels in the cervical region. The occurrence of thyroid carcinoma metastasizing to the breast is infrequent, with the mechanism of dissemination remaining unclear. A 63-year-old female presented with a painful, progressive mass on the right breast upper outer quadrant for 3 months with bloody discharge from the nipple and cervical lymphadenopathy level 2-5 by axillary lymphadenopathy and shortness of breath. On examination, 8 cm ×6 cm, a well-defined lump was palpable on the right breast, with soft to firm consistency. The swelling was fixed to the underlying tissues. The patient had undergone a total thyroidectomy for papillary carcinoma 10 years back and another surgery for pre-sternal thyroid swelling 3 years back which was also papillary carcinoma. Fine needle aspiration cytology (FNAC) was inconclusive and a core cut biopsy from the breast was taken which was suggestive of papillary thyroid carcinoma. Henceforth a Positron emission tomography (PET) scan was done that showed increased fluorodeoxyglucose (FDG) uptake by the lesion, cervical and axillary lymph nodes. The patient was advised for radioactive iodine ablation and palliation.

[DISCUSSION] Papillary thyroid cancer (PTC) is the most common thyroid malignancy, often spreading via lymphatics. Regional metastasis to the neck is frequent, though metastasis outside the deep cervical chain is rare. Distant metastases occur in 1% of PTC patients mainly in the lungs and bones. The precise mechanisms enabling the spread of thyroid carcinoma to the breast remain insufficiently understood. A small subset of medullary thyroid carcinomas has been observed to display metastasis to the breast, which majorly disseminates by hematogenous route. Ours is a rare case of PTC showcasing metastasis to the breast. Proposed mechanisms encompass dissemination via intraoperative seeding and lymphatic routes.

[CONCLUSION] Metastasis of papillary thyroid carcinoma to the breast is very rare in the current body of literature; however, a small number of cases of medullary thyroid carcinomas in the breast have been identified, predominantly disseminated via the hematogenous route. Therefore, the identification of a mass in the breast may warrant consideration as a metastatic lesion in the setting of pre-existing thyroid carcinoma. Radioactive iodine ablation (RAI) and radiotherapy might be recommended for palliation.