Lecture 5-13: Fractionated Radiation & Dose-Rate Effect PDF

• • • • • Summary of Fractionated Radiation and the Dose-Rate Effect Potentially Lethal Damage Repair (PLD): • Definition: Radiation damage modification post-irradiation is known as PLD. • Conditions: PLD repair occurs if cells are prevented from dividing for 6 hours or more after irradiation....

• • • • • Summary of Fractionated Radiation and the Dose-Rate Effect Potentially Lethal Damage Repair (PLD): • Definition: Radiation damage modification post-irradiation is known as PLD. • Conditions: PLD repair occurs if cells are prevented from dividing for 6 hours or more after irradiation. • Demonstration: In vitro, cells are kept in saline or plateau phase for 6 hours; in vivo, delayed removal and assay show PLD repair. • Significance: Significant for x-rays, absent after neutron irradiation. • Controversy: Suggested in resistant tumors (e.g., melanoma), still debated. Sublethal Damage Repair (SLD): • Definition: Increase in survival if radiation dose is split into two fractions separated in time. • Timing: Half-time of SLD repair in mammalian cells is about 1 hour, potentially longer in late-responding normal tissues. • Occurrence: Evident in tumors, normal tissues in vivo, and cultured cells in vitro. • Mechanism: Reflects repair of DNA breaks before forming lethal chromosomal aberrations. • Radiation Types: Significant for x-rays, nearly nonexistent for neutrons. Dose-Rate Effect: • Effect: Reduction in dose rate (e.g., from 1 Gy/minute to 0.3 Gy/hour) leads to reduced cell killing due to SLD repair during prolonged exposure. • Survival Curve: Shallower slope (increased D0) and disappearance of the shoulder with reduced dose rates. • Inverse Effect: Some cell lines show an inverse dose-rate effect, where reducing the dose rate increases cell killing, due to cell accumulation in G2 phase. Brachytherapy Techniques: • • Definition: Implanting radioactive sources into or close to tumors. Types: • • • Intracavitary Brachytherapy: Sources placed in body cavities near tumors (e.g., uterine cervix). 1 • LDR: Lasts 1 to 4 days, being replaced by HDR (3 to 12 dose fractions). 3 Interstitial Brachytherapy: Implanting sources directly into tumors and adjacent tissues (7 to 10 days). Permanent Implants: Use radionuclides with short half-lives (e.g., iodine-125) or novel sources. Radiation Protection: Low-energy photon emissions simplify radiation protection issues.