Estrogen Receptor-Low Breast Cancer With Sternal Metastasis Presenting as "Stiff Neck" in a Young Female.

A previously healthy 20-year-old Japanese woman was referred to the outpatient clinic for evaluation of right neck stiffness, which had persisted for two months, along with weight loss, malaise, and elevated inflammatory markers. On physical examination, spontaneous pain was noted in the right upper trapezius, along with incident pain around both clavicles and shoulders. Swelling was found around the sternal manubrium, and two palpable masses were present in the right breast. A computed tomography scan of the thorax revealed a low-density, lobulated area in the right breast, a soft-tissue mass in the sternal manubrium, multiple lymphadenopathies, and small nodules in the lungs and liver. Core needle biopsy of the breast mass confirmed estrogen receptor (ER)-low invasive ductal carcinoma, and testing for breast cancer susceptibility gene mutations was negative. Systemic chemotherapy was initiated for the treatment of metastatic triple-negative breast cancer. Breast cancer causing neck or shoulder pain is rare. The right neck stiffness likely resulted from brachial plexus compression due to bulky right axillary lymphadenopathy, leading to thoracic outlet syndrome. Lung metastasis could also cause referred pain through the vagus nerve. This case presented an atypical manifestation of "stiff neck" associated with thoracic tumors. Notably, the patient initially hesitated to share key symptoms, which may have contributed to a delayed diagnosis. We believe that the rapid progression of her lesions may have heightened her anxiety, which in turn further impaired her communication with the medical staff.