Basic Radiotherapy Physics And Biology Apr 2026

: Cells are most sensitive to radiation during certain phases of the cell cycle (like mitosis). Fractionating treatment (giving it in small daily doses) allows surviving cancer cells to move into these sensitive phases.

: Normal cells are often better at repairing sublethal DNA damage than cancer cells.

: Modern techniques like IMRT (Intensity Modulated Radiotherapy) and Proton Therapy allow physicists to conform the dose strictly to the tumor, sparing nearby healthy organs like the salivary glands or the liver. 2. The Biology: The "5 R’s" of Radiotherapy Basic Radiotherapy Physics and Biology

: At the energies used in treatment, photons (X-rays) typically interact via Compton scattering , where they bounce off electrons and transfer energy to them. Direct vs. Indirect Ionization : Direct : The radiation beam itself breaks DNA strands.

: Between treatments, both normal and tumor cells begin to grow again. The trick is to ensure the "kill rate" exceeds the "growth rate". : Cells are most sensitive to radiation during

: Oxygenated cells are about three times more sensitive to radiation than hypoxic (oxygen-poor) ones. As a tumor shrinks, previously hidden hypoxic areas get better blood flow and become easier to kill.

Radiotherapy primarily uses high-energy , such as X-rays, gamma rays, protons, and electrons. Unlike visible light, this radiation carries enough energy to knock electrons out of atoms, a process called ionization . Direct vs

: The radiation hits water molecules in the cell, creating reactive oxygen species (free radicals) that then attack the DNA.