Breast Mass Revealing Hidden Lung Cancer.

Lung cancer is the second most common cancer and the leading cause of cancer-related deaths in the United States, frequently metastasizing to the brain, bones, liver, and adrenal glands. However, it is uncommon for lung malignancies to metastasize to the breast, and is generally assumed to carry a poor prognosis.  We present the case of a 62-year-old female who presented for an annual physical exam and complained of persistent neck pain without neurological deficit. A screening mammogram revealed a 0.4 cm right breast mass. A biopsy of the mass showed carcinoma with micropapillary features. Immunohistochemistry was positive for cytokeratin (CK) 7 and thyroid transcription factor 1 (TTF-1)and negative for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), indicating a primary lung origin. Next-generation sequencing (NGS) was negative for a targetable mutation. The patient was treated for metastatic lung adenocarcinoma with carboplatin, pemetrexed, and pembrolizumab for 4 cycles followed by maintenance with pemetrexed and pembrolizumab. Per response evaluation criteria in solid tumors (RECIST) the patient continues to have stable disease.  Imaging and immunohistochemistry (IHC) are critical in differentiating between primary and metastatic lesions. Additionally, using phylogenetic analysis to understand tumor evolution allows for valuable insights into how metastases develop and spread, thus assisting in personalized treatment strategies. Early and accurate diagnosis is essential for appropriate treatment planning leading to improved patient outcomes.