Abstract: This invention provides a safer and more effective treatment for non-intracavitary undesirable tissue masses, especially bone cancer and soft tissue tumors. The method involves the direct administration of a therapeutically-effective dose of a formulated radioisotope composition nearby or directly into the tissue mass. Small volumes of the composition are used. Administration of the dose for bone cancer may be done through a hole or multiple holes created in the bone using a miniature drill. Delivery of the dose directly into a tumor may be accomplished using a microsyringe or a miniature pump capable of accurately delivering microliter amounts of material.