radiotherapy provides distinCt dosimetriC advantages
proton
Proton scanning systems are seen as the next generation of commercial proton technology. Scanning was first implemented at the Paul Scherrer Institute in Villigen, Switzerland. Firstly, we should consider why scanning beams are seen as being so important to proton radiotherapy. Scanning has the potential to both simplify and improve proton radiotherapy treatments. It has several advantages over passive scattering technology: • It has the potential to reduce commissioning times. • It does not require the manufacture of compensators or collimators for individual patients’ treatment beams.
• It allows the delivery of more conformal treatments. • It has the potential to increase patient throughput.
Scanning technology does not require the custom collimator or compensator because the beam can be controlled to irradiate the desired treatment positions by changing the position in the XY plane using magnets, and Z (depth) by adjusting the incident proton energy (Figure 4). Scanning reduces the cost of producing these customised devices. It also can increase throughput because less radiographer intervention is needed to insert a patient specific collimator and compensator combination for every treatment field.
Apart from avoiding patient specific compensators and collimators, scanning opens up the possibility of intensity modulated proton therapy (IMPT). The Paul Scherrer Institute has defined two main treatment modes of scanning treatments: First, single field uniform dose (SFUD), where each field delivers a homogeneous field across the volume. And second, IMPT, where the Bragg peaks from all fields are optimised such that each treatment beam provides a non-uniform dose delivery but, when combined, all treatment beams produce a uniform dose distribution. Both of these modes allow increased conformity with respect to passive scattering.
Figure 5. Conformity achievable from passive scattering, single field uniform dose and intensity modulated proton therapy (courtesy of Dr T Lomax).
Figure 5 shows an example from where a volume has been planned using passive scattering, SFUD and IMPT. In passive scattering, the modulation width of the SOBP is constant and equal to the maximum width of the target along the beam axis, which results in an under spill of dose for most volumes. Both SFUD and IMPT avoid this problem and can achieve increasing degrees of conformity.
It has also been postulated that scanning reduces the time for commissioning and planning. Passive scattering technology certainly has many treatment options that take a long time to commission and many centres do not commission all options. In theory, one would have to commission only one scanning option but, in practice, data on commissioning a commercial scanning system are scarce.
THE CURRENT SITUATION IN THE UNITED KINGDOM (UK) Proton radiotherapy is currently available to UK patients via a specialised commissioning service of the NHS,
http://www.specialisedservices.nhs.uk/service/proton-beam-
9 2011
IMAGING & ONCOLOGY
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