The role of imaging in modern radiotherapy methods

Imaging has become an integral part of modern radiotherapy (RT). New RT techniques rely on imaging to determine the target and the organs-at-risk, as well as verification of the correct delivery of the treatment. CT has been the required imaging modality for many years as it provides distortion-free images that contain information on tissue densities needed for dose calculation. The often superior contrast resolution offered by MRI has been utilized through image registration (fusion) with CT and, more recently, on its own in MRI-only workflows. Functional MRI, MR spectroscopy and nuclear medicine imaging (especially PET) offer additional information and, therefore, the possibility of better target delineation, especially where existence of tumor cells are not manifested by morphological changes. If functional images can provide reliable information on the spatial distribution of tumor cells within the target volume, intentional non-uniform target dose distribution ( dose painting ) also becomes an option to consider. On the treatment verification aspect, to ensure that the patient receives the planned treatment correctly (in terms of geometry and dose), image-guided radiotherapy (IGRT) has widespread use in modern RT. The imaging is performed with the patient positioned on the treatment table. Projectional electronic portal images, kilovoltage and megavoltage x-ray CT (fan-beam or cone-beam) or ultrasonography are mostly used. The availability and ease of imaging during the course of RT fractions has also led to adaptive RT, which takes into account progressive changes in target volume size and shape. This review will include the historical origins and the recent advances in the use of imaging in RT.

Keywords:

imaging , radiotherapy , IGRT