Intensity Modulated Radiation Therapy (IMRT)
What is IMRT?
Intensity-modulated radiation therapy (IMRT) is an advanced form of three-dimensional conformal radiotherapy (3DCRT). 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.
PAMF was the first outpatient facility in the Bay Area to implement IMRT. The Radiation Oncology Department has offered the therapy since June 2001, using Varian Medical Systems’ SmartBeam IMRT equipment. Currently, PAMF uses IMRT to treat select patients with prostate cancer, head and neck cancer, some brain tumors, peripheral soft-tissue extremity tumors and selected lung cancers.
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How does IMRT work?
Like conventional 3DCRT, IMRT links CT scans to treatment planning software that allows the cancerous area to be visualized in three dimensions. However, regular 3DCRT and IMRT differ in how the pattern and volume of radiation delivered to the tumor is determined. In conventional 3DCRT, clinicians 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 team then uses a sophisticated computer program to develop an individualized plan to meet the constraints. This process is termed “inverse treatment planning.” Treatment with IMRT is slightly longer that with 3DCRT, but generally produces fewer side effects.
IMRT uses the same medical linear accelerators that deliver x-ray beams in conventional 3DCRT.
As a unique feature, it also 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 tissue 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.
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