10226- MET-20 PRIMARY UNKNOWN CANCER THAT WAS DIFFICULT TO DIAGNOSE EVEN WITH A CANCER PANEL TEST, PRESENTING AS LEPTOMENINGEAL CARCINOMATOSIS

Abstract CASE REPORT The patient was a 59-year-old male. Three years prior to the current presentation, he developed left peripheral facial nerve palsy. In October of the year before his presentation, his left facial nerve palsy rapidly progressed, and by December, MRI revealed an enhancing lesion in the left cerebellopontine angle, leading to a referral to our hospital. Gadolinium enhanced lesions were located at bilateral cerebellopontine angles, auditory nerves and trigeminal nerves. There were multiple small nodular lesions at cerebellar surface and spinal cord. In March of the current year, the patient underwent biopsy, and the pathological diagnosis was mucinous adenocarcinoma. Despite various examinations, the primary site of the cancer could not be identified. Genetic testing with FoundationOne revealed amplifications of CDK4, MDM2, and ERBB3, along with ATM mutations and STK11 deletions, but a definitive diagnosis was not available. The patient was treated with nivolumab and radiation therapy to the lesion of left cerebellopontine angle, which showed significant enhancement on imaging. While local control was achieved with radiation, the leptomeningeal carcinomatosis progressed, leading to a determination that the disease had progressed. By June of the current year, the patient experienced worsening hearing loss, tinnitus, and back pain due to spinal cord lesions, resulting in a rapid decline in ADL. Radiation therapy was administered for the spinal cord lesions, and the patient passed away about a week later. DISCUSSION AND CONCLUSION In this case, despite a pathological diagnosis of mucinous adenocarcinoma, the primary site of the tumor could not be identified. The results from FoundationOne suggested possible origins in the gastrointestinal tract or lung, but a definitive diagnosis was not reached. Molecular diagnostic methods should be developed for difficult-to-diagnose cancers of unknown primary.