VMAT (RapidArc) | Volumetric Modulated Arc RadioTherapy and IMRT

Also known as “RapidArc”, Volumetric Rotational Arc Therapy allows for the delivery of numerous radiation beams during a full rotation of the treatment gantry – providing the ability for the physician to conform beam-energy from any direction. As a radiation delivery technology, VMAT combines several advances in radiation accuracy, including Intensity Modulation (IMRT), Image-Guidance (IGRT), On Board Imaging (OBI), and Cone Beam CT Scanning (CBCT). This integration facilitates treatments that are not only remarkably quick (usually minutes a day only!), and also reduce radiation exposure, offering a significant advancement in patient care.

VMAT is actually the next-generation of modulated radiation and in fact is a “rotational” form of IMRT – taking precision delivery and targeting to the next level, often delivering the full dose of radiation in less than 90 seconds per fraction. This rotational “volumetric arc,” enhances both the delivery and the targeting of many forms of cancer radiation, creating a more highly shaped radiation treatment that is unlimited in its sophistication.

Using prostate cancer as an example: older technologies like IMRT, the gantry of the linear accelerator is rotated to one of several fixed beam angles, and then remains in place during that one beam delivery – but usually there are five to seven angles that the gantry must then be rotated to, with each beam delivering one-fifth or one-seventh of the total radiation for that day. Since these fields are limited to only certain angles, the ability for the radiation to avoid or minimize its exposure to normal, non-cancerous tissues is also limited. However, since VMAT is a rotational treatment, radiation can be delivered 360 degrees, literally at any and all beam angles, not just a few discrete once. Since there is no limitation to where and when the beam will be on or off, or how fast or slow it is being delivered from the linac, the physicist and physicians now have nearly unlimited choices and degrees of freedom to design an even more sophisticated and highly conformal treatment plan for the patient. The result is that less normal tissue will be exposed to the higher doses of radiation, which means fewer side effects and less normal tissue damage.

VMAT is also very fast delivery (hence the trade-name: RapidArc). It can deliver radiation in 90 seconds or less, whereas conventional IMRT or IGRT treatments take about 10 minutes to deliver radiation, which while also quite speedy, those minutes are important: Remember that the prostate moves due to the bladder and rectal volumes, and in order to make sure that a treatment can be delivered accurately, targeting technologies like IGRT and CBCT must be employed seconds before the treatment beam is turned on, so that the prostate can be in the exact and precise position. However, the longer the treatment beam is on, the more chance that some minor shifts in the prostate position can occur. Thus, VMAT provides even more targeting precision by ensuring that the treatment is started and completed in such a fast period of time. It decreases the chance that a minor shift in the prostate’s position might occur – something that becomes much more likely when the treatment beam is on for 3, 4, 5, 10 or more minutes.