Three Dimensional Conformal Radiation Therapy

Since computers were used for single plane treatment planning in the 1970s to 1980s, treatment planning systems have become further developed along with advances in computer hardware. These dramatic increases in computing power have allowed multiplane treatment planning to become practical. Once the planning systems became capable of handling large amounts of computed tomography (CT)-derived patient data, calculation algorithms were developed to account for the full scattering component of radiation transport in various human tissues.

The development of accurate three-dimensional dose calculations and 3-D rendering of patient anatomy provided the tool to sculpt a tumoricidal dose distribution conformed to a tumor (target) volume and to minimize dose to all other normal tissue (Figure 3.8). From these considerations, the term 3-D conformal radiation therapy (3-D CRT) gained rapid

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figure 3.8. A tumor in the pancreas was treated with three-dimensional (3-D) conformal radiation therapy (CRT). Organs spared were kidneys, liver, spinal cord, and small bowel. Five noncoplanar beams were used for treatment. Each beam was conformed to the target volume with multileaf collimator as shown above to maximize the conformity. Three-dimensional rendering can assist the planner to optimize the beam angle.

figure 3.8. A tumor in the pancreas was treated with three-dimensional (3-D) conformal radiation therapy (CRT). Organs spared were kidneys, liver, spinal cord, and small bowel. Five noncoplanar beams were used for treatment. Each beam was conformed to the target volume with multileaf collimator as shown above to maximize the conformity. Three-dimensional rendering can assist the planner to optimize the beam angle.

figure 3.9. International Commission on Radiation Units and Measurements (ICRU) reports 50 and 62 introduced the volume definitions for radiation therapy treatment planning (see References 41, 42). Outer ring, block aperture; second from outer ring, planning target volume; third ring, clinical target volume; inside ring, gross target volume.

figure 3.9. International Commission on Radiation Units and Measurements (ICRU) reports 50 and 62 introduced the volume definitions for radiation therapy treatment planning (see References 41, 42). Outer ring, block aperture; second from outer ring, planning target volume; third ring, clinical target volume; inside ring, gross target volume.

utility and acceptance40 Once dose could be highly conformed to the target volume, the margin of the target had to be more accurately determined; this has been accomplished by using multimodality imaging and carefully defining treatment and normal treatment volumes. The International Commission on Radiation Units and Measurements (ICRU) Reports 50 and 62 introduced the definition of volumes (Figure 3.9).41,42 The gross target volume (GTV) is defined as the clinically palpable volume or, more typically, as visualized by imaging. It may consist of the primary tumor, metastatic lymph-adenopathy, and local tumor extension. The clinical target volume (CTV) includes the tissue surrounding the GTV that might have microscopic malignant disease or "at-risk" regional lymph nodes. More than one CTV can be defined. The planning target volume (PTV) includes GTV and/or CTV plus a margin to account for variations in treatment delivery, including variations in treatment setup, patient motion during treatment, and organ motion. These ICRU reports also reviewed the definition of normal organs to be spared. Organs at risk (OAR) are defined as critical normal tissues, such as the spinal cord, whose radiation sensitivity may significantly influence treatment planning and/or the prescribed dose. After planning has been performed, a dose-volume histogram (DVH) can be generated from the plan (Figure 3.10). The DVH provides the information: dose versus volume of a specified organ. This information is very useful for radiobiologic studies. However, it does not provide spatial dose informa-

figure 3.10. Three-dimensional treatment planning provided a tool for analyzing dose distribution in various organs interested. This tool is very useful in the analysis of the biologic consequences of the optimized irradiation.

tion. It can be used to complement the graphic isodose displays used in treatment planning by comparing the amount of dose given to specified organ volume. High dose conformity can enable the radiation oncologist to seek dose escalation to the target volume for possibly better radiobiologic advantages and an improved therapeutic gain, as previously described in Figure 3.1.

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