Clinical application of 90-gene expression test in a patient with occult breast cancer: a case report and literature review.

Occult breast cancer is an exceptionally rare disease characterized by ambiguous diagnostic criteria. Currently employed diagnostic methods include breast MRI, PET-CT, and immunohistochemistry; however, challenges in achieving accurate diagnoses persist in certain cases. The utilization of the 90-gene test has been implemented to identify the origin of unknown tumors, demonstrating an overall accuracy rate of 94.4% and sensitivity ranging from 74.2% to 100%, as evidenced by clinical trials. In this particular case, the patient is presented with both occult breast cancer and thyroid cancer. Initial imaging results indicated thyroid cancer without evident breast nodules or axillary lymph node involvement. Despite treatment for thyroid cancer, the patient's pain did not alleviate until tissue specimens were subjected to a comprehensive analysis using the 90-gene expression test. The gene expression profiling revealed a similarity score of 59.5%, confirming the diagnosis of occult breast cancer as a Luminal B subtype. Following treatment with letrozole and norethindrone, significant relief was observed in the patient's pain, along with a notable reduction in the size of liver metastases and mediastinal lymph node lesions. This case highlights the immense potential of the 90-gene test as a diagnostic tool for occult breast cancer.