Occult metastatic thyroid cancer diagnosed during breast cancer axillary sentinel node biopsy

SUMMARY: In most developed countries, breast carcinoma is the most common malignancy in women and while thyroid cancer is less common, its incidence is almost three to five times greater in women than in men. Since 1966, studies have demonstrated an association between thyroid and breast cancer and despite these studies, the mechanism/s by which they are related, remains unclear. We present a case of a 56-year-old lady who initially presented in 2014 with a screen detected left breast carcinoma but was subsequently found to have occult metastatic thyroid cancer to the axilla, diagnosed from a sentinel node biopsy from the primary breast procedure. The patient underwent a left mastectomy, left axillary dissection and total thyroidectomy followed by three courses of radioactive iodine ablation. Despite this, her thyroglobulin level continued to increase, which was secondary to a metastatic thyroid cancer parasternal metastasis. Breast and thyroid cancer presents metachronously or synchronously more often than by chance. With improving mortality in primary cancers, such as breast and differentiated thyroid cancer, it is likely that as clinicians, we will continue to encounter this association in practice. LEARNING POINTS: There has been a long-standing observation of an association between breast and thyroid cancer although the exact mechanism of this association remains unclear. Our patient presented with thyroid cancer with an incidental diagnosis from a sentinel node biopsy during her primary breast operation for breast cancer and was also found to have a parasternal distant bony metastasis. Thyroid axillary metastases are generally rare. The interesting nature in which this patient’s metastatic thyroid carcinoma behaved more like a breast carcinoma highlights a correlation between these two cancers. With improving mortality in these primary cancers, clinicians are likely to encounter this association in clinical practice. Systemic therapy for metastatic breast and thyroid cancers differ and therefore a clear diagnosis of metastasis is crucial.