Intensity Modulated Radiation Therapy

Three-dimensional CRT is used to optimize the radiation delivery to irregularly shaped volumes by manually setting individual beam angles. Intensity-modulated radiation therapy (IMRT) is a new approach to 3-D CRT. Clinical definition of complex treatment volumes often requires concave dose distributions. IMRT has been found to be most useful in this regard.43 Radiation dose delivery to a target volume by IMRT is performed by the superposition of multiple "beam-lets" from each treatment angle. Full dose uniformity is achieved by the summation of individual beams of small field size where the central axis is frequently blocked by complex multileaf collimator (MLC) patterns. The dose is figuratively painted in by discrete amounts wherever needed to maintain dose uniformity in a defined volume. Typically, in the optimization process using IMRT for radiation dose delivery, the relevant physical and geometric information of the irradiated object and radiation source serve as input data. Then, dose distribution may be calculated using the input data for volumes that have specific geometric characteristics. This dose calculation is known as a forward treatment planning process. However, an inverse treatment planning process is used to create IMRT plans. The inverse planning process essentially starts with a precise prescription of the goals of an ideal plan, and the planning system optimizes input data to meet these conditions. In addition to the availability of the physical parameters of the irradiated object (tumor volume), the relevant information about the capabilities and specifications of the available treatment machine may be inserted into the planning system. After the physician prescribes the desired doses to the different volumes including GTV, PTV, and adjacent dose-limiting normal tissues, the inverse planning system provides a delivery method to execute the prescription accurately with a specific treatment machine. The resultant computerized solutions appear as radiation intensities from the sources as a function of source location, which results in a dose function that is used in the delivery of targeted irradiation.

As shown in Figure 3.11, this head and neck treatment plan provides a sample of IMRT inverse planning process. This specific case shows a target volume for a patient with a base-of-tongue primary cancer where the GTV has been outlined and resulting in a corresponding PTV with expanded margins generated around it. The PTV has been prescribed to a uniform dose distribution of 7,200 cGy. Regional "at-risk" lymph nodes are also treated to the minimum dose of 5,400cGy. Here, there are unique features of IMRT compared to the conventional radiation therapy, in that the IMRT can deliver different doses to multiple targets simultaneously. This method may eliminate the necessity of boost plans to deliver different doses to multiple targets. The fractional doses per day to physically defined target volumes can be made to vary. The daily doses should be selected carefully in view of the radiobiologic consideration. Most clinical experience is based on the same fractional doses, although total doses specific to multiple target sites and critical structures may be different.

Critical normal tissues such as the spinal cord and right parotid can be "spared" (see Figure 3.11) to doses of 4,300 and 2,200 cGy, which should result in no significant late toxicities. Once all objectives are determined, the inverse treatment planning system is able to optimize the dose distributions that satisfy the initial objectives. Most planning systems have used an intermediate step before giving a deliverable plan with a delivery machine. This intermediate step provides users with several options: which treatment delivery machine is being selected, how many intensity levels are to be used, and what is the basic prescription of multiple beam directions (gantry angles).

Current IMRT delivery methods have been implemented by MLCs installed on conventional linear accelerators, tomotherapy, and physical beam modulators.44 Multileaf col-limators include conventional MLCs that accompany the installation on most modern linear accelerators and specifically designed MLCs which have been attached to the gantry for IMRT use. There are two methods used for delivering modulated beams using MLCs: step-and-shoot and sliding window techniques.44 The step-and-shoot method delivers dose to the target by a series of small beams. At each gantry angle, MLCs constantly form a series of segments to deliver a modulated beam. During the time of MLC movements between segments, the radiation beams are cycled to an off position automatically by the programmed accelerator software without further human interaction. However, the sliding window technique starts with all leaves closed from one side of a radiation field and opens the leaves in a dynamic pattern. The speed of each pair of leaves depends on the dose intensity desired. The radiation beams are continually on during the MLC movement.

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