IsoRay Inc. (IsoRay) is a national isotope-based medical company. With its patented radioisotope technology, the company offers products including Cesium-131 seeds and other relevant treatment medicine for cancer and other malignant diseases. Its subsidiary, IsoRay Medical Inc., is the sole producer of Cesium-131 brachytherapy seeds. Brachytheraphy is when cancer-fighting medical radioisotopes are implanted in or around the affected tissue so to be as close to the cancer as possible. In 2006, its patented Cesium-131 brachytherapy seed technology was named as one of 2006's 100 most technologically significant products by R&D Magazine. The company has obtained clearance from the US Food and Drug Administration (FDA) for treatment for all solid tumor applications using its products. In 2010, the company was awarded under the IRS Qualifying Therapeutic Discovery Program (QTDP), which is a project of U.S. Department of Health and Human Services.

The company was founded in 2004 and is headquartered in Richland, Washington. The founder of IsoRay is Lance A. Bray. The company markets its products mainly in United States, besides, it also distributes the product, Proxcelan Cs-131 brachytherapy seeds, through partner UralDial LLC, in Russia. By cooperating with independent radiopharmacies, the company has enlarged its marketing channel and access to customers.