Intensitymodulated Radiation Therapy

Intensity-modulated radiation therapy (IMRT) extends the benefits of 3D-CRT by enabling radiation oncologists to deliver non-uniform beams of radiation. This allows for more con-formal delivery of radiation to the tumor and possible dose escalation. The technique uses a computer optimization process that is based on prescribed doses to the target and constraints on normal, sensitive structures. Radiation delivery is further optimized by the use of a multileaf collimator (MLC) or complex compensators, which automatically shape the radiation beam. These two concepts - inverse treatment planning with computer optimization and computer-controlled intensity modulation of the radiation beam - form the basis of IMRT.

One of the more commonly used collimators for IMRT is the multivane intensity-modulating collimator (MIMiC), which attaches to the accessory tray of the linear accelerator (Peacock system, NOMOS Corp., Sewickley, PA). The MIMiC contains 40 small vanes, each of which can alter the intensity of the radiation beam during treatment. The system delivers the radiation dose using arc therapy and segmented fields. The beam can also be modified by using a dynamic MLC that passes across the treatment field or by superimposing a number of static fields. The patient's head can be affixed to the immobilization device either invasively or non-invasively.

Some radiation centers, including the authors' at the Cleveland Clinic, are using IMRT to treat meningiomas in the hopes of achieving excellent local control and minimizing acute and long-term side-effects. No large clinical experiences that have used IMRT to treat meningioma have been reported thus far.

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