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Breast cancer is the most commonly diagnosed malignancy among women worldwide. While survival outcomes have improved substantially over time, recurrence remains a significant concern, especially in estrogen receptor-positive (ER⁺) disease. Although most recurrences occur within five years of initial treatment, ER⁺ tumors carry a persistent risk of late relapse due to tumor dormancy. Very late recurrences occurring decades after remission are rare and poorly characterized.

We present the case of a 96-year-old female with a history of right-sided breast cancer treated with mastectomy and adjuvant radiation therapy at age 50, who presented 46 years later with a palpable chest wall mass near her prior mastectomy scar. Imaging revealed an irregular hypoechoic lesion extending into the pectoralis muscle (BIRADS-5). Core biopsy demonstrated moderately differentiated infiltrating lobular carcinoma that was strongly ER+, progesterone receptor-negative, and HER2 non-amplified. Staging studies revealed no evidence of distant metastatic disease. Given the patient's advanced age, comorbidities, and prior radiation exposure, surgical and radiation approaches were deferred. Following multidisciplinary tumor board discussion and shared decision-making, primary endocrine therapy with tamoxifen was initiated. The patient demonstrated clinical regression within three months and tolerated therapy well.

This case represents one of the longest reported intervals between initial remission and breast cancer recurrence, highlighting the prolonged biologic potential of dormant ER-positive tumor cells. Clinicians should maintain vigilance for recurrence even decades after treatment, particularly in hormone receptor-positive disease.