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The discovery of X-rays by Roentgen in 1895 and of radium by the Curies in 1898 revolutionized medicine at the turn of the 20th century. Roentgen’s first paper on X-rays illustrated the power of diagnostic imaging with a remarkably detailed radiographic image of Frau Roentgen’s hand. As researchers around the world built vacuum tubes and acquired radioactive sources for their studies, it rapidly became apparent that these invisible radiations could produce dangerous, and even lethal, injuries.13 Erythema, chronic dermatitis, ulceration, loss of hair, and eye injuries were soon reported in patients who received large doses of radiation during prolonged fluoroscopy procedures. Even greater injuries were reported among the physicians, technicians, and scientists who performed diagnostic procedures or laboratory studies using unshielded X-ray-generating equipment and highly radioactive sources. The development of these radiation injuries suggested that radiation might be useful in the treatment of cancer; indeed, patients with cancer were treated with radiation therapy as early as 1896.13

Radiation was found to inhibit the growth of tumors, but this benefit came with the cost of injury to normal tissues within the irradiated areas. Because of the very low energies of the early X-ray and gamma-ray sources, radiotherapy in its early days was limited to using poorly penetrating radiations, which delivered much higher doses of radiation to skin than to even very superficial tumors. As a result, severe early radiation reactions in the skin limited the doses of radiation that could be delivered to tumors. Studies of these skin reactions led to the development of the concept of normal tissue tolerance and an appreciation of the benefits of “fractionated” radiotherapy, using multiple treatments with small doses of radiation.2 The relative sensitivity of the lung to injury from radiation became apparent early in the development of radiation oncology. The clinical syndromes of dyspnea, cough, fever, and radiographic infiltrates occurring weeks to months after irradiation of the thorax were dramatic enough to be described as early as 1922.4

The field of radiation oncology has matured immeasurably over the last century and has incorporated significant advances from fields as diverse as theoretical and applied physics, radiation biology, pathology, cell biology, and immunology.1,2,57 The importance of advances in physics and engineering to the maturation of radiation oncology is especially notable.2,7 These advances have led to the development of modern linear accelerators capable of delivering very high-energy, deeply penetrating radiations, which can be used to deliver high radiation doses with great precision to tumors deep within the body. Precise systems for radiation dose measurement, or dosimetry, rapid computers, and precise algorithms for the rapid computerized three-dimensional planning of individualized radiotherapy treatments based on computed tomography (CT) scans and magnetic resonance imaging (MRI) studies have been developed. These advances have changed the dose-limiting toxicities of radiation therapy from painful early reactions in the skin to ...

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