Key Events

• 1895 – Discovery of X-rays (Röntgen): First demonstration of invisible rays producing photographic images.

• 1898 – Discovery of Radium (Curie): Opened the field of brachytherapy.

• 1920s–30s – Radiosensitivity studies: Found that some tissues (bone marrow, gut) are more sensitive than others (muscle, nerve).

• 1930s – Oxygen effect: Oxygen was shown to enhance radiation damage, explaining why hypoxic tumors resist therapy.

• 1940s–50s – DNA identified as the main target: Experiments showed mutations and cell death track with DNA damage.

• 1970s–80s – Linear-quadratic model: Provided a mathematical way to describe dose–response and fractionation.

• 21st century – Modern radiobiology: Focus on DNA repair pathways, tumor microenvironment, radiosensitizers, immunotherapy, and particle therapy.

Definition

• Radiosensitivity: The relative susceptibility of cells or tissues to radiation damage.

• Oxygen effect: Radiation is more effective in the presence of oxygen, which “fixes” DNA damage caused by free radicals.