Image guided radiation therapy

Abstract

HIGH precision radiation therapy is based on the fact that the tumor and normal surrounding structures have to be delineated in all three dimensions before the radiation oncologist plans the treatment for the patient. The three dimensional computerized treatment planning system then generates a plan in which the tumor gets a high dose and the normal tissues around it get a very low dose of radiation. The radiation is then delivered using a linear accelerator with a multi leaf or micro multi leaf collimator. This methods results in higher cure rates and minimal normal tissue complications. Based on this premise the modern radiation therapy techniques viz. stereotactic radiotherapy, stereotactic radiosurgery, 3 dimensional conformal radiation therapy (3D CRT), and intensity modulated radiation therapy (IMRT) have been developed and have become standard of care in modern radiation therapy departments around the world, including India. Such treatments are planned using CT/ MRI imaging. Till a decade ago the planning techniques were different because the planning was done based on fluoroscopy and not on CT/MRI images. This resulted in two and not three dimensional delineation of tumor and normal tissues and therefore the radiation portals were large so that there was no geographical miss of the tumor. This, however, resulted in inclusion of a larger volume of normal surrounding tissues and the radiation oncologists had to keep the total dose to a lower level in order to keep the complication rates low, but the lower tumor doses were not sufficient enough to result in higher cure rates. When the intention is to spare the normal tissues and the delineation is in three dimensions, the radiation oncologist uses a very tight margin around the tumor which helps in escalating the total tumor dose. This philosophy is utilized in all the modern treatment techniques mentioned above. It has been observed and documented over the last few years that all the organs in the body move during the radiation treatment and also over a period of 4-6 weeks over which all the radiation therapy treatments are delivered. This observation has led us to think that if the