Once the tumor is localized by the radiation oncologist and the tumor volume defined, a three dimensional treatment plan is generated to conform a radiation beam to the tumor. During the planning process the radiation beam is sculpted to maximize the dose to the tumor while sparing the surrounding critical organs. The goal of the treatment planning process is not only to deliver an adequate dose to the tumor volume but also to minimize the dose to the surrounding normal tissues to alleviate side effects.
The planning of radiation therapy treatment has been revolutionized by the ability to delineate tumors and adjacent normal structures in three dimensions using specialized CT and/or MRI scanners and planning software. Virtual simulation, the most basic form of planning, allows more accurate placement of radiation beams than is possible using conventional X-rays, where soft-tissue structures are often difficult to assess and normal tissues difficult to protect.
An enhancement of virtual simulation is 3-dimensional conformal radiation therapy (3DCRT), in which the profile of each radiation beam is shaped to fit the profile of the target from a beam’s eye view (BEV) using a multileaf collimator (MLC) and a variable number of beams. When the treatment volume conforms to the shape of the tumor, the relative toxicity of radiation to the surrounding normal tissues is reduced, allowing a higher dose of radiation to be delivered to the tumor than conventional techniques would allow.