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Intensity modulated radiation therapy (IMRT) involves the use of radiation beams in which the intensity of parts of the beam are varied during the patient irradiation procedure. The intensity can be varied by the use of beam modifying filters, or the shapes of a multileaf collimator (MLC) are changed during the irradiation.  The result is a highly conformal dose distribution which covers the target volume with a relatively tight margin. The net effect is that less normal tissue gets irradiated allowing for the possibility of increasing the tumour dose and thereby increasing the probability of tumour control.

Important Principles

IMRT requires the use of a treatment planning system that has inverse planning capabilities. Because of the very high number of beam shapes that may be used for IMRT delivery with an MLC, it is impractical to perform manual optimization. By giving specific dose-volume constraints for the target volume and critical tissues, the inverse planning system automatically determines the best dose delivery configuration. In addition, because of the complexity of the treatment techniques, individualized dose verification is required for each patient. This can be done by recalculating the patient dose delivery technique for a phantom of known composition and shape and performing measurements in that phantom for that specific patient dose delivery technique. It can also be done by using verification software that is totally independent of the software used by the treatment planning system.

Introduction to References

Chapter 15 of the IAEA Radiation Oncology Physics Handbook gives a brief summary of the use of MLC and IMRT. The report by the AAPM IMRT subcommittee gives guidance to the medical physicist in developing and implementing a viable and safe IMRT program.