Should proton therapy be available in the UK? What are the barriers that must be overcome first?
INTRODUCTION The concept of treating cancer patients with protons was first suggested by Robert R Wilson in a paper published in 19461
. Initially, patient treatments were performed with
particle accelerators built for physics research, most notably at the Berkeley Laboratory in the USA and Uppsala in Sweden in the mid 1950s. It took another 35 years before a purpose built hospital proton facility was built with the Loma Linda University Medical Center in Loma Linda, California in 1990. Recent years have seen a growth in the number of proton radiotherapy treatment centres either opening or in the planning stage with nearly 30 centres worldwide in operation at the end of 2010.
The potential advantage of protons for radiotherapy treatment can clearly be seen from the depth dose characteristics of photons and protons using a spread-out Bragg peak (Figure 1). Proton radiotherapy treatments have the potential to reduce the integral radiation dose received by the patient, and to deliver lower doses to normal tissue proximal to the tumour compared to traditional photon radiotherapy treatments.
For proton radiotherapy a high energy accelerator capable of accelerating protons to at least 230MeV is required to provide sufficiently energetic protons to penetrate to the centre of body. The particle accelerator will either be a cyclotron or synchrotron and the choice has an effect on the performance parameters of the clinical beam. A cyclotron can produce a high current beam of one energy, the maximum required. For clinical treatments, the beam energy must be degraded to provide the range of energies required to produce a spread-out Bragg peak for treatment.
The degrading can be performed very quickly but reduces the current of the proton beam, which can be a problem when treating at lower energies. Synchrotrons generally operate at a lower current but can accelerate to different energies and so beams do not
100 - 80 - 60 - 40 - 20 - -
0
Spread-out Bragg peak
Tumor
7 2011
IMAGING & ONCOLOGY
10 MV X-rays Bragg peak (protons) 4 8 12 Depth in tissue (cm)
Figure 1. Depth dose characteristics in tissue of a single Bragg peak (pink), a spread-out Bragg Peak (red), which consists of different energy Bragg peaks added together, and a 10MV x-ray beam (dashed). Taken from Technology Insight: proton beam radiotherapy for treatment in pediatric brain tumors Torunn I Yock and Nancy J Tarbell, Nature Clinical Practice Oncology (2004) 1, 97-103 (reproduced with permission).
16
require degrading. As a result, they can produce beams with a better energy resolution but the time to switch between energies is longer. They are generally more complex machines and so more expensive and operate at a lower beam current.
The high cost of the particle accelerator for proton radiotherapy means treatment centres run multiple treatment rooms off a single accelerator. Typically, between three and five rooms can be serviced. The limiting factor is that currently only one patient at a time can be treated and it takes time to switch the proton beam between rooms. Achieving efficient throughput and avoiding patients waiting too long for the treatment beam is dependent on several
a unique opportunity for Consistent proton treatments
Relative dose (%) - - - - -
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72