Imaging in Radiation Therapy Planning

Management of cancer is fundamentally based on surgery, chemotherapy and radiation therapy. In 1896 radiation was first used for the treatment of cancer which is just one year after discovery of X-ray by Wilhem Roentgen. In current days radiation treatment can be broadly classified as external beam radiation therapy (teletherapy), brachytherapy and targeted radionuclide treatment. External beam radiation therapy (EBRT) is the most commonly used modality for curative or palliative options for management of solid tumors. Basic principle of EBRT is the delivery of high doses of radiation (photons or electrons or protons) to the tumor with minimal possible radiation induced injury to the neighboring and intervening structures. In early years of radiotherapy the magnitude of side effects was enormously high and primarily was due to limited cross sectional details required to spare or keep the vital structures away from radiation field. However, thanks to Hounsfield who in 1972 invented computerized tomography (CT) which provided a better insight to tumor geometry, its relation with surrounding structures and also made it possible to calculate the radiation dose in 3 dimensions (3D). In current clinical practice, CT based radiation planning is the standard of care and this is due to its ability to calculate the tumor dose despite of limited tissue contrast. This low tissue contrast is the major drawback of CT which poses difficulty for delineating the gross tumor volume (GTV). But robust development in linear accelerators and CT scanners has paved the path of more sophisticated and target specific 3D-conformal radiotherapy, image guided radiation therapy (IGRT), intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMART) which are the main cuisines of radiation oncology suite.