IMPT – defined as the simultaneous optimization of all Bragg peaks from all incident beams – could increase dose conformality even further. The technique, pioneered by researchers at the Paul Scherrer Institute in Switzerland, is now being offered commercially by Varian Medical Systems (Palo Alto, CA).

Varian's ProBeam proton therapy system is based on a 250 MeV isochronous superconducting cyclotron. Magnets in the treatment nozzle steer the proton beam to conform to the target volume layer and the dose is painted layer by layer, starting with the deepest slice (the highest-energy beam).

In early 2009, the first patient treatment with the ProBeam system took place at the Rinecker Proton Therapy Center in Munich, Germany; and last December, IMPT treatments were introduced on the machine. Since then, the CE-marked system has received FDA clearance in the US. Varian is now working on its first US order, from San Diego-based project developer Advanced Particle Therapy.

Tami Freeman spoke to Jan Timmer, marketing manager for Varian proton therapy, to find out more about IMPT and the ProBeam.

TF: Can you explain the main benefits of IMPT over standard proton therapy?

JT: IMPT can improve the quality of the dose distributions, similar to what IMRT [intensity-modulated radiotherapy] did for photon therapy. That means coming closer to the holy grail of radiation therapy: being able to apply more dose to the cancerous tumour while sparing organs at risk in the entrance path of the beam.

With the double-scattering technique, which most commercial centres use today, a collimator shapes the lateral edges of the field and a compensator shapes the distal edge. But that means that the proximal edge of the field cannot be shaped independently, and dose to healthy tissue is increased on this side due to the compensator. With IMPT, you can irradiate voxel by voxel and really optimize the dose applied to every spot of the field.

What is the difference between pencil-beam scanning and IMPT?

IMPT is an application of pencil-beam scanning technology. For each treatment fraction you would usually use, say, two fields at different treatment angles. There are two ways in which you can apply these fields, you can optimize the individual fields to get uniform target doses and then add them. This is also referred to as single field optimization. Or you can optimize the fields simultaneously and allow inhomogeneous dose per field to get an optimized fraction. Optimizing the fields simultaneously during the therapy planning – that's the definition of IMPT.

Does Varian's ProBeam also offer scattering-based proton therapy?

With our ECLIPSE treatment planning software, Varian covers double scattering, spot scanning and IMPT. The ProBeam proton therapy delivery system offers pencil-beam scanning, and collimators can be mounted to the nozzle to simulate double scattering. Double scattering has brought proton therapy to where it is today, but it's similar to the way photon therapy was being done 15–20 years ago, in which you have to make modifications to the treatment nozzle during a treatment and for each patient. It is our expectation that the use of collimators will be reduced in the future.

IMPT doesn't require any mechanical modifications during the day to account for different patients and different fraction delivery. You can treat multiple fields without entering the room, just by making electronic changes, which makes the treatment very similar to an IMRT photon treatment today. This also means that Varian can apply the many techniques that it knows from photon therapy to the proton system.

Will the ProBeam system provide image guidance?

Yes, it offers 2D orthogonal imaging and, in addition, cone-beam CT will become available that is virtually identical to that on Varian's TrueBeam radiotherapy system. Another step on our roadmap is the implementation of gating technologies on the proton machine.

Is IMPT suitable for all clinical cases?

The majority of treatments can benefit from IMPT technology. It is especially beneficial for complex tumour shapes, such as head-and-neck tumours, tumours of the lower abdomen that have a curved shape, and tumours wrapped around the spinal cord or brain stem. IMPT can shape complex fields with a limited number of radiation angles, which keeps the treatment time as short as possible and helps to spare healthy tissue. It also provides the flexibility to apply simultaneous integrated boosts to target volumes.

IMPT doesn't give any added benefit for very small fields that require high dose rates. So, for example, if you want to treat ocular tumours, which are normally small and receive a high dose, there is no added benefit from IMPT.

Finally, can you tell me about Varian's project with Advanced Particle Therapy.

Advanced Particle Therapy is working in collaboration with Scripps Health to build the Scripps Proton Therapy Center in San Diego, CA. This will be a five-room facility containing three rotational gantries and two fixed-beam rooms. Site construction started last summer, and Varian expects to start installing equipment in early fall of this year. Clinical treatments at the new centre are planned to start in early 2013.

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