IMRT has been advancing for about ten years now, in part due to the assistance of image guided radiation therapy (IGRT), which ensures that the patient remains in the exact position they were in during simulation for the entire course of actual treatment, which may be "fractionated" over several days or weeks. IGRT has been in development for three or four years. Proper positioning obtained by IGRT is sparing more healthy tissues and increasing the therapeutic value of treatment more than ever before. IGRT is made possible with onboard image-guidance systems that are integrated into the linear accelerator - either 2-D portal imaging or 3-D cone-beam CT systems. Now there are even "4-D" imaging systems built in that can mimic fluoroscopic imaging. This kind of IGRT helps account for patient respiration and registers the motion of lung cancer tumors and liver tumors, which move with the diaphragm.

Varian is the market-leading manufacturer of linear accelerators. Corey Zankowski, Varian's senior director of oncology product management, says that modern IMRT is making it possible to move therapy into the realm of stereotactic radiosurgery (SRS) for some forms of cancer.


SRS and stereotactic body radiation therapy (SBRT) have revolutionized the way ionizing radiation is used to treat and cure cancer. Rather than delivering radiation over the course of several sessions, stereotactic procedures ensure highly accurate beam positioning in three-dimensional space and involve higher electron and photon energies and much higher absorbed radiation dose, which is measured in grays.

Today's linear accelerators offer spectrums of photon and electron beam strengths, measured in megavoltage units, or MV. Different cancers call for different beam strengths depending upon the tumor type, location and stage of disease.




"We treat some disease sites with photons and others with electrons," says Brent Evans, linac product manager for Siemens' Oncology Care Systems. "When you deliver electron radiation and it hits the skin, it scatters. Electrons don't penetrate deep into the skin, but it's great for treating skin cancer and melanomas. We also use electrons with some TBI - total body irradiation, which is sometimes used before chemo to keep the white blood cells low or before a bone marrow procedure."

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