RESUMEN
INTRODUCTION AND IMPORTANCE: Papillary thyroid carcinoma is the most common type of malignancy in endocrine tumor. Women with breast cancer have an increased risk of developing thyroid cancer. Especially, patients with BRCA1 germline variants, which belong to the DNA DSB repair system, there may be a genetic susceptibility to thyroid cancer. CASE PRESENTATION: This study investigated a 47-year-old Vietnamese female patient with BRCA1 mutation, namely NM_007294.3 (BRCA1): c.4998insA (p. Tyr1666Terfs), who developed PTC after one year of breast cancer treatment. CLINICAL DISCUSSION: Factors that are thought to increase the risk of thyroid cancer after breast cancer treatment include treatment methods, hormonal factors, genetic susceptibility, and others. A hypothesis is breast cancer with BRCA1 mutation increases the risk of thyroid cancer. CONCLUSION: Understanding the relationship between breast and thyroid cancer helps clinicians for well management of both diseases and also contributes to the development of new preventive strategies, diagnostics and therapeutics as so as a new research direction.