The method of Iodine -131 treatment involves the oral administration of a I-131 solution or capsules, entering the bloodstream through the digestive system. It selectively targets and accumulates in residual thyroid cells and residual thyroid tumor cells. By emitting β-rays through decay, it induces swelling, degeneration, and necrosis in the targeted cells, effectively eliminating residual thyroid tissue and cancerous lesions. The objective is to reduce tumor recurrence and metastasis. The average range of β-rays within tissues is less than 1mm, with almost all energy released within residual thyroid tissue or metastatic lesions, resulting in minimal impact on surrounding normal tissues and organs.