Physics and Clinical Aspects of Brachytherapy

Brachytherapy was the original intensity‐modulated radiotherapy, providing the ability to deliver high doses to custom‐shaped targets while preserving normal, neighboring structures. Brachytherapy has many manifestations, for example permanent implants and treatments using high‐dose rate and remote afterloaders. The treatments may be executed by placement of needles or catheters into a tumor or placement of the sources in body cavities near the tumor. The choice of sources, approaches and techniques should be dictated by the patient’s presentation. The rate of treatment delivery affects both the biological results of the radiation and the physical facets of therapy. Biologically, low‐dose rate treatments tend to be relatively gentler to normal tissues but high‐dose rate approaches provide more stable and precise dose distributions. Most commonly, dose distribution calculations follow the protocol of Task Group 43 of the American Association of Physicists in Medicine, computing the doses or dose rates in space from each source separately and adding them together to obtain the composite. At the time of writing, commercial treatment planning systems have just begun to incorporate computational algorithms that can include the effects of tissue composition and density. Brachytherapy has moved from simple localization using planar images from radiographs that identify source positions and skeletal anatomy to volume imaging that also shows soft tissues and allows identification of target and normal tissue structures. Physiological and molecular imaging can further enhance the ability to shape target volumes, just as with external‐beam treatments. Treatment planning entails decisions on the approach to the brachytherapy. The classical systems, such as the Paris or Manchester systems, provide guidance for source or needle distributions that results in more controlled dose distributions from the modern optimization routines. These systems, along with other measures of implant quality, also serve as benchmarks for quality‐assurance evaluations of an application.