RapidArc exploits customized algorithms to deliver a complete IMRT treatment in a single rotation of the linac around the patient, a procedure also known as volumetric modulated arc therapy (VMAT). Modulation is achieved by delivering successive arcs. For each arc, the beam aperture (as defined by the multileaf collimator) changes with the gantry angle to match the shape of the tumour.

"At the moment, Varian does Dynamic Arc, which means the clinician can adjust the treatment field every two degrees using the multileaf collimator in the head of the treatment machine," Neil Madle, Varian's director of investor relations, Europe, told medicalphysicsweb. "Doing this, you look at the tumour from every angle and you shape the field to the tumour. Now, we have devised a way of combining this with 3D imaging to do VMAT."

In this way, says Varian, it is possible to outstrip more time-consuming IMRT treatment methods, which require machines to make several rotations around the patient or to make repeated stops and starts to set up beam angles. According to the press release, "by varying three parameters simultaneously - the speed of rotation, the beam-shaping aperture and the dose delivery rate - doctors can create finely shaped IMRT dose distributions that more closely match the size and shape of the tumour while sparing healthy tissues."

In a conference call with medicalphysicsweb before the ASTRO meeting, Dow Wilson, president of Varian's oncology systems business, explained the commercial and clinical thinking behind RapidArc: "This is where we are with RapidArc radiation therapy. What we'd like to do is in two minutes set the patient up, know exactly where the tumour is and be ready in an image-guided fashion to deliver dose to that tumour. And then in the next two minutes, we'd like to complete the delivery of the prescription and get the patient off the table."

He continued: "Depending on where you are today, I'd say most IMRT is [delivered] in around a 10-20 minute cycle - 10 minutes on some of the simple treatments; 20 minutes, maybe even longer for some of the more complex treatments. The other thing that we're seeing [with RapidArc] is a substantial reduction in dose to critical [non-tumour] tissues. Conformality is terrific."

Dave Mellenberg, a technical leader at Varian, put it another way: "We're taking advantage of a number of unique features of our C-arm linac. It's like we had this Porsche and we were running it on city streets; now we're running it on the track at full throttle."

Varian is aiming to make RapidArc available for radiation clinics in spring 2008. The product is pending FDA 510(k) clearance and is not yet available for sale.

• See more reports from the ASTRO annual meeting at our Main Event blog.