Intensity-Modulated Radiotherapy
Intensity-modulated radiotherapy (IMRT) is a radiation delivery technique that allows “dose sculpting” to improve target coverage while sparing normal tissues. In an IMRT plan, the number of photons delivered (or “fluence”) varies within a field. Such var
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Bridget F. Koontz, Devon Godfrey, and W. Robert Lee
Intensity-modulated radiotherapy (IMRT) is a technique for delivering external beam radiation. IMRT is defined as a radiation treatment in which the number of photons delivered (or “fluence”) varies within a field (Fig. 63.1). Such variability can be used to avoid normal structures with lower radiation tolerance than the cancer and increases the ability to provide focal treatment. Its use has become widespread in a variety of malignancies that require high radiation doses, avoidance of critical normal structures, or both. For prostate cancer, the cancer control benefit of dose escalation, which also leads to greater rectal toxicities, has driven a widespread adoption of IMRT for definitive external beam radiotherapy.
History Development of IMRT Radiation therapy has undergone an evolution in treatment field design over the past 25 years. Initial treatment of prostate cancer used nonconformal (“2D”) techniques using skeletal anatomy to guide the design of treatment fields. Multiple fields with different angles were used to create a homogenous high-dose region internally while reducing peripheral dose. Beam shaping was performed by customized solid blocking (Fig. 63.2). With computed tomography (CT), the tumor location and shape, as well as that of other nearby organs, could be individualized to each patient (“3D conformal” treatment). However, the dose delivered remained constant within each field, with multiple field plans still generating a homogeneous treatment volume.
B.F. Koontz, M.D. (*) • D. Godfrey, Ph.D. W.R. Lee, M.D., M.S., MEd Department of Radiation Oncology, Duke University Medical Center, DUMC Box 3085, Durham, NC 27710, USA e-mail: [email protected] A. Tewari (ed.), Prostate Cancer: A Comprehensive Perspective, DOI 10.1007/978-1-4471-2864-9_63, © Springer-Verlag London 2013
Intensity-modulated radiotherapy (IMRT) was initially a mathematical solution to the problem of developing a heterogeneous treatment plan. In 1982, physicists in Stockholm published a mathematical method to generate an annulus of uniform dose with minimal central dose using a rotating modulated beam [1]. Techniques to accomplish delivery of IMRT were developed throughout the early 1990s with continued technical and dosimetric advances extending into early 2000s [2, 3]. Beam shaping evolved from the premade solid blocks to multileaf collimators (MLCs), small motorized “leaves” with variable positioning in the head of the linear accelerator (Fig. 63.2). Widespread availability of MLCs in linear accelerators allowed changes to how treatment plans were developed since numerous MLC shapes at a single gantry angle could be used to vary the number of photons delivered to differing areas of a single field [4]. While MLCs are not required for IMRT delivery, they are perhaps the most widely used technique for delivering an IMRT plan. IMRT depends on high-quality imaging and computerized planning to individualize treatment fields to ensure adequate coverage of the target and a
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