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Computed Tomography and Position Emission Tomography in Radiation Oncology

US Oncology Review, 2005;1(1):1-4 DOI:


During the 1990s, the radiation oncology community made many improvements in its ability to accurately deliver high doses of radiation to complex tumor shapes. Modern treatment planning systems can calculate relatively efficiently optimized treatment plans to complex target volumes, and treatment machines can deliver these plans with high precision in short treatment times. Figure 1 shows a radiation dose distribution for the treatment of a para-nasal tumor with brain invasion. Radiation doses (shown in color wash) conform closely to the tumor volume while sparing the surrounding critical organs.

Similar dose distributions can be achieved for some other cancer sites as well. The ability to deliver radiation doses with small uncertainties (in both location and magnitude) increases the need to accurately define anatomical location and the extent of tumor volumes, and to determine biological properties of individual tumors.A better understanding of tumor extent and biology can result in improved radiation dose distributions delivered to patients, which should translate to better outcomes and/or reduced complications. Medical imaging is one of the main tools used in radiation therapy (RT) for disease detection, staging, treatment modality selection, tumor volume definition, radiotherapy treatment planning, and prognosis and follow-up. Therefore, the data contained in patient images profoundly affects patient management and delivery of radiation.

Research and development (R&D) in medical imaging offers one of the greatest avenues for improvement in radiation oncology treatments. The ultimate goal for medical imaging in RT is to allow radiation oncologists to accurately delineate and biologically characterize an individual tumor, select an appropriate course of therapy, and predict tumor response as early as possible. To biologically characterize an individual tumor means that an imaging modality does not need to image gross anatomically visible changes; rather, it must capture information about a tumor’s underlying physiology, metabolism, function, and molecular makeup. Therefore, imaging information used in RT can be classified as anatomical or biological, and the radiation oncologist relies on multiple imaging modalities (multimodality imaging) to gather this information. Four primary imaging modalities are used in RT:

• computed tomography (CT);

• magnetic resonance (MR);

• ultrasound (US); and

• nuclear medicine (primarily positron emission tomography (PET)).

CT and US provide primarily anatomical information, while PET and MR can provide biological as well as anatomical information. This article describes the current state of CT and PET use in RT. The main purpose of imaging in radiation oncology today is to gather anatomical information. The vast majority of studies are performed with CT, and CT will remain the primary imaging modality in radiation oncology for the foreseeable future. However, imaging of functional and biological tumor properties is increasing and could change many RT practices by improving disease detection, staging, therapy selection, target design, prognosis, and follow-up. Thus far, PET has been the primary imaging modality used for this purpose.