Imagine a newly diagnosed prostate cancer patient lying on a table in a darkened room, about to undergo intensity-modulated radiation therapy (IMRT). Besides eyeing the linear accelerator anxiously and pondering the side effects radiation might induce (secondary tumors?), he's also feeling intense pressure to urinate. You made him fill his bladder to lift his reproductive organs out of the way of the beam's target area-his prostate. But he must ignore that pressure and lie perfectly still. Moving a few millimeters too much could make or break his cure.

In conventional static-field IMRT, that patient must lie prone for five, eight, 10 minutes as you maneuver the linac around him. You deliver a treatment dose; stop and reposition the machine; deliver another dose; stop and reposition the machine; deliver another dose, and so on.

Imagine achieving the same effect in just two or three minutes via the push of a button as your linac rotates around the patient in one or more arcs with the radiation continuously on. As it rotates, the system automatically varies the multileaf collimator (MLC) aperture shape, dose rate, gantry rotation speed and MLC orientation to deliver maximum doses while sparing normal tissue. That's the promise of volumetric modulated arc therapy (VMAT)-essentially, moving IMRT. "Instead of covering something from seven angles, you have an infinite number of angles," says William Bodner, MD, a radiation oncologist at Montefiore Medical Center who has used VMAT since March. "Ten to 15 years ago, we were thinking in terms of static fields and large lead blocks. Now we track these tumors as the machine rotates around, adjusting the shape of the beam. It's beyond comprehension."

What's in a name?

VMAT is a generic term but also Elekta Oncology's commercial name for its entry into this new market. Varian Medical Systems calls its system RapidArc. Siemens Healthcare's analogous product is known as IM-RealART.

At this early stage in its evolution, VMAT is used most commonly to treat prostate and head/neck cancers, according to several users. "Prostate cancer is very prevalent and fairly simple in terms of its structure, so when you introduce a new modality, it's a good place to start," says David Asche, MS, director of physics and engineeringfor RAS ROC, a multisite practice in Northern California that uses Elekta's VMAT system at two sites and a Varian VMAT system at its third site. "We believe the greatest benefit to be derived is efficiency in treatment."

Asche's first prostate cancer patient received a single-arc prostate treatment of 504 monitor units (MUs) in 1 minute, 49 seconds. A second prostate patient was transitioned from IMRT to VMAT and given a single-arc treatment of 423 MUs in 1 minute, 33 seconds. Those times compare to about 5 minutes, 30 seconds for a seven-field IMRT plan using Elekta sequencing modes, according to Asche. "Because there's a fairly high volume of prostate patients, this improves our throughput since patients are on the machine for less time," he says. "[Also] it's more tolerable for them. They have a greater capacity to retain a good position. With cancer of the pelvis, head and neck, time on table is much longer, so we think those time savings will be even greater."

Independent software

Varian and Elekta supply software for their VMAT linear accelerator systems. Not to be outdone, two independent companies have developed their own planning software. Royal Philips Electronics has introduced SmartArc, available as a module in Philips' Pinnacle³ Version 9 radiation treatment planning (RTP) software. Nucletron recently released Oncentra VMAT, VMAT-customized software that supports image-guided photon, electron and proton treatment delivery and rapid dose calculation times when creating VMAT plans for any brand of linac.

"Philips SmartArc software works for both Varian and Elekta linear accelerators that are capable of delivering VMAT treatments," says David Robinson, CMD, MBA, senior product manager for Philips. "The customer specifies the desired doses to the target volumes, and the dose limits to organs at risk, and the software designs a plan that meets these criteria."

VMAT plans are generated identically to IMRT plans, says Robinson, "but the VMAT plan is designed to be delivered by a rotational technique instead of using a number of beams set at specific gantry angles. VMAT is a dynamic delivery technique where the gantry, dose rate and MLC shapes are changing continuously throughout the delivery process."

Oncentra VMAT also generates treatment plans for Elekta and Varian linacs, says Tom Chadwick, Nucletron's marketing manager, who says his company wants to position itself as among the fastest options for independent VMAT planning.

"The word 'independent' is important," Chadwick says. "We provide an independent choice so clinicians don't have to purchase the complete system from their linac provider. We can provide a VMAT solution on an upgraded linac or a linac that has not been upgraded."

Training, QA

Besides shorter treatment times, greater conformality and increased patient throughput, VMAT offers another benefit, according to Dr. Bodner. "It's also a way to stimulate your physics staff, expose them to more sophisticated techniques," he says. "From a departmental standpoint, the interest our physicians and technical staff have taken delving into this and investigating its potential is exciting."

Like any technology, VMAT "is only as good as the people implementing and utilizing it," he adds. "The people implementing it have to be well-trained and experienced in its use."

Training his staff to deliver VMAT was "almost instantaneous," says Asche, adding that error recovery training only took about five minutes. The more involved training for the dosimetry team required a task force of dosimetrists and physicists to work out the planning process, using as a benchmark the center's own IMRT standards. "We've had this software for a while and [are] learning its intricacies," says Asche. "Our first round of clinical solutions are excellent, but they can be further refined."

Staff must be trained not only in the hardware and the planning software but in a third piece as well, quality assurance (QA) testing, "to make sure we have a high level of confidence," he says.

Future refinements

As for VMAT's future, Varian engineers are working to marry a respiratory gating system with its RapidArc technology to eradicate the perennial problem of tumor motion. Respiratory gating-tracking a patient's respiratory cycle with an infrared camera and chest/abdomen marker so radiation is delivered only when the tumor is in the treatment field-"is a feature of old IMRT," explains Meryl Ginsberg, Varian's senior public relations manager. Integrating it with RapidArc "will permit the beam to turn on and off automatically for the treatment of tumors that move with swallowing, or respiratory motions, as in lung cancers."

Elekta, meanwhile, has rolled out its vendor-neutral MOSAIQ Radiation Oncology solution to, among other things, integrate VMAT planning and delivery information technology into electronic medical records. MOSAIQ supports both Elekta VMAT and Varian's RapidArc. "When delivering a radiation therapy plan, all the information that went into designing the treatment-and how the machine will deliver it-is all part of the electronic medical record," says Steve Rubenstein, business marketing manager for Elekta. "With VMAT, you're going from static fields to dynamic arcs. Clinics must keep track of all this and MOSAIQ can seamlessly manage all the information."

Neither Dr. Bodner nor Asche believe VMAT will eventually supplant conventional IMRT.

"VMAT is an adjunct to IMRT," says Dr. Bodner. "It's a different way to treat [patients]. For more simplistic treatments-palliative treatments-VMAT is not necessary. But for centralized structures like prostate cancer, it's favorable for the decreased time."

VMAT "is not the silver bullet," agrees Asche. "We need a tool box. Cancer is many, many different things. But I do believe as the software becomes readily available, it will get better, and as delivery systems get better, it will become widespread. Adoption will follow, to an extent, the finances of departments and their abilities to upgrade. Varian and Elekta accelerators need new hardware and software, so there are costs associated beyond staff training. That will be one factor controlling the rollout on this."

Michael Gibbons is a managing editor of ADVANCE.