Chapter 27 • X-rays were first discovered emanating from an energized Crooke’s tube by Wilhelm Roentgen in 1895. In 1896, Henri Becquerel discovered that some naturally occurring elements emitted ionizing radiation. The radioactive elements radium and polonium were isolated and characterized by the Curies in 1898. • Within a year or two, ionizing radiation was in use worldwide for medical imaging and radiation therapy. • Several types of ionizing radiation are used to treat patients; most are of the low linear energy transfer, less biologically potent varieties. • Therapeutic x-rays (photons) and electrons are produced by linear accelerators but can also be produced by nuclear isotopes that undergo radioactive decay. These form the basis of external beam radiotherapy and brachytherapy, respectively. • Ionizing radiation interacts with matter via several processes, the most important of which for clinical radiation therapy is Compton scattering. • Megavoltage photons from linear accelerators have the desirable property of delivering their maximum dose at depth within the patient, thereby sparing the skin and, to some extent, other normal tissues. • Ionization of biomolecules from the deposition of energy by photons or particles can occur directly and indirectly. The most important cellular target for radiation is DNA, with irreparable or “misrepaired” double-stranded breaks believed to be the lesions most responsible for cell killing. • Irradiation elicits diverse cellular responses that include the sensing of DNA damage, mobilization of DNA repair proteins, repair (or attempted repair) of DNA damage, triggering of cell cycle checkpoints, and, for irreparable or mis-rejoined damage, cell death by one of several mechanisms (e.g., mitotic catastrophe, apoptosis, and senescence). • The most commonly applied model of cell survival probability is the linear quadratic (α/β) model, with the surviving fraction of irradiated cells described by the equation • DNA damage and repair were initially inferred by monitoring increases in cell survival or tissue tolerance with fractionation. These phenomena were termed sublethal and potentially lethal damage repair or recovery.
Basics of Radiation Therapy
Summary of Key Points
Introduction and Historical Perspective
Radiation Physics
The Radiobiology of Radiotherapy
. The α/β ratio is a convenient metric for describing cellular radiosensitivity and has been adapted to describe the response of irradiated tissues as a function of time, dose, and fractionation.
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Basics of Radiation Therapy
