The discovery of X-rays in 1895 immediately revealed the invisible. It also revealed danger. Prolonged exposure caused skin redness, burns and tissue damage. Yet those same biological effects suggested that this new form of energy might be used therapeutically.
One of the earliest pioneers was the Viennese dermatologist Léopold Freund. In 1896, he used X-rays to treat a young girl with a large hairy birthmark, successfully removing the excessive hair. That same year in Chicago, medical student Emil Grubbe used X-rays in an attempt to treat recurrent breast cancer. These treatments were crude by modern standards, but they marked an important conceptual leap: radiation could do more than create an image.
Early machines were inconsistent, making the delivered dose difficult to control. A major improvement arrived in 1913 with William D. Coolidge’s hot-cathode X-ray tube. Unlike earlier gas-filled tubes, it produced a stable, high-intensity beam whose output could be controlled more reliably. The Coolidge tube became a foundation of radiology and early radiation treatment.
The biological cost of progress was severe. Radiation burns, tissue necrosis and cancers appeared among patients and practitioners, including some of the field’s pioneers. Their injuries made the need for dose measurement, shielding and safety protocols impossible to ignore.
Radium offered another route. Discovered by Marie and Pierre Curie in 1898, it emitted radiation continuously without an external power source. By the early twentieth century, physicians were placing small radium sources inside or near tumours. This approach—brachytherapy—could deliver a high dose locally while limiting exposure to more distant healthy tissue.
Radium was powerful but hazardous. Radiation safety was still poorly understood, and both patients and staff could be overexposed. Marie Curie’s eventual death from aplastic anaemia, likely related to prolonged radiation exposure, became part of the sobering legacy that shaped safer practice.
The 1950s brought Cobalt-60 teletherapy. Artificially produced Cobalt-60 emitted gamma rays energetic enough to treat deeper tumours. Its greater penetration and skin-sparing properties helped launch the megavoltage era, moving radiotherapy beyond the limitations of lower-energy orthovoltage machines.
At roughly the same time, the medical linear accelerator emerged. Linacs use high-frequency electromagnetic waves to accelerate electrons. The electrons can treat superficial tumours directly or strike a metal target to create high-energy X-rays for deep disease. Magnetron technology, developed for radar during the Second World War, supplied the microwaves that made this acceleration possible.
Imaging then transformed planning. CT in the 1970s and MRI in the 1980s allowed tumours and organs to be visualised in three dimensions. Radiation fields could be designed around the individual patient rather than inferred from surface landmarks and two-dimensional images.
By the 1990s and early 2000s, intensity-modulated radiation therapy shaped dose around complex tumour geometry, while image-guided radiation therapy checked anatomy during treatment. Particle therapy added another physical advantage: protons release most of their energy at a selected depth through the Bragg peak, reducing dose beyond the target—particularly valuable near critical structures and in children.
Radiotherapy evolved because clinicians and scientists repeatedly converted danger into understanding and instability into control. What began as an uncontrolled physical curiosity became a therapeutic discipline built on measurement, imaging, radiobiology and precision.