Application of PET (PET-CT) in Radiation Therapy Planning

Abstract

The purpose of radiation therapy planning is to maximize dose delivery to the target while simultaneously decreasing radiation dose to the surrounding normal tissues. In the era of image-guided radiation therapy (IGRT), the greatest challenge is target delineation. Over the last two decades, technological advances in radiographic imaging, biochemistry, and molecular biology have played an increasing role in radiation treatment planning, delivery, and evaluation of response. In earlier times, fluoroscopy was the basis of radiation treatment planning. In the late 1980s, computed tomography (CT) became the basis for modern radiation treatment planning and delivery. Also multimodality anatomic imaging was found to be the solution to augment delineation of tumors and surrounding structures on CT-based treatment planning. Although these imaging modalities provide the customary anatomic details necessary for radiation treatment planning, they have limitations, including difficulty with identification of tumor extension, and distinction from scar tissues. To overcome these limitations, PET and, more recently, PET-CT have been innovative regarding the extent of disease appraisal, target delineation in the treatment planning, and assessment of therapy response. The use of multi-modality imaging fusion and the introduction of more sensitive and specific PET-CT tracers may further assist target definition. Novel markers of tumor hypoxia or proliferation have the potential to modify the delineation of target volumes, allowing for “dose painting” in selected subvolumes. Furthermore, the potential to predict early outcome or even detect early recurrence of tumor may allow for the tailoring of intervention in cancer patients. The implementation of three-dimensional radiotherapy and IMRT requires adequate selection and delineation of target volumes on the basis of anatomic or molecular imaging modalities, appropriate dose prescription and (dose) specification with regard to dose volume constraints, and quality control for both the clinical and the physical aspects of the entire procedure.