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Medical radioisotopes emit alpha, beta, and gamma radiation, which differ in penetration and shielding needs. Radiation dose is described as absorbed dose in gray (Gy) and tissue-weighted effective dose in sievert (Sv). Biological injury can result from direct DNA ionization or from free radicals formed when radiation interacts with water; higher linear energy transfer is associated with denser DNA damage. Deterministic effects, such as skin erythema and cataract, have threshold doses, while stochastic effects, including cancer, are treated as probability-based. The lesson also considers radiosensitive tissues, acute and chronic effects, fetal exposure, and internal contamination management. Protection is organized around reducing time, increasing distance, and using shielding under the ALARA principle.