Intensity Modulated Radiation Therapy (IMRT) is an advanced form of three-dimensional conformal radiotherapy. It uses sophisticated software and hardware to vary the shape and intensity of radiation delivered to different parts of the treatment area. It is one of the most precise forms of external beam radiation therapy available.
Evergreen was one of the first hospitals to implement IMRT and to use PET/CT fusion scanning for cancer treatment planning. Evergreen Radiation Oncology has offered the therapy since 2002. Currently, Evergreen uses IMRT to treat select patients with prostate cancer, head and neck cancer, some brain tumors and selected lung cancers. Breast cancer patients are treated with a combination of 3D treatment planning and IMRT treatment delivery.
Regular 3D conformal radiation therapy and IMRT differ in how the pattern and volume of radiation delivered to the tumor is determined. In conventional 3D conformal radiation therapy, radiation oncologists input delivery patterns into the computer. In IMRT, the physician designates specific doses of radiation (constraints) that the tumor and normal surrounding tissues should receive. The physics/dosimetry team then uses a sophisticated computer program to develop an individualized plan to meet the constraints. This process is called inverse treatment planning. Treatment with IMRT is slightly longer than with 3D conformal radiation therapy, but generally the side effects are less.
IMRT uses the same medical linear accelerator that delivers radiation beams in conventional 3D conformal radiation therapy. A unique feature involves dynamic multi-leaf collimators (DMLCs), computer-controlled devices that use up to 120 movable “leaves” to conform the radiation beam to the shape of the tumor from any angle, while protecting normal adjacent tissues as much as possible.
DMLCs allow the dose of radiation to vary within a single beam – in other words, to deliver higher radiation in some areas and lower radiation in others. Earlier technology could also shape radiation beams but could deliver them only at a single, constant dose. The ability to vary the radiation dose with DMLCs is accomplished by “sliding windows” of radiation beams across the target cancerous area.
To more easily picture how DMLCs work, imagine a shower head with many nozzles, with the water representing radiation. Standard radiation techniques only allow a constant flow of water to be delivered through all nozzles. But with DMLCs, individual nozzles may be turned off and on, or set to deliver water at different intensities. In radiation therapy, the net effect is that radiation doses can be “wrapped” around tumors, or “painted” within tumors, far more precisely than was previously possible.
Treatment process and side effects for IMRT are similar to those for 3DCRT.