Proton and Heavy Ion Therapy: An overview: January 2017
7
Particle therapy is of particular benefit in treating cancers that are difficult or dangerous to treat
with surgery, and for tumours where conventional radiotherapy would damage surrounding tissue
to an unacceptable level (e.g. optic nerve, spinal cord, central nervous system, and structures of
the head and neck). In addition, protons are becoming increasingly accepted for the treatment of
selected paediatric and young adult cancers, where the need to avoid secondary radiation-induced
tumours is also important due to the potential long life span of the patient.
4, 5
Further, there is
early
evidence that heavy ions, including carbon, may be effective in treating radio-resistant
tumours.
6-8
Since the mid-1990’s, there has been interest in the potential establishment of an Australian
particle therapy centre.
9
In 2006 and 2007, HealthPACT reviewed the available evidence regarding
proton beam therapy (PBT) for the treatment of specific cancers and concluded that further high
quality research, demonstrating improved patient outcomes in comparison to conventional
treatment, was required.
10-12
Since this time, the evidence base for both protons and heavy ions
has progressed, particularly for PBT.
Currently,
four Australia States have progressed planning towards establishing particle facilities.
HealthPACT considers it timely to review the role of particle therapy in cancer treatment to inform
broader discussion around its potential introduction in Australian and New Zealand. This paper
aims to summarise the current state of play regarding protons and carbon ions in cancer
treatment.
Particle Therapy Technology
The aims of particle therapy are similar to conventional radiotherapy, which is to induce an
ionising effect within malignant cells, and create biological effects such as DNA disruption, and cell
death.
1
However, instead of high-energy x-rays (photons) used in conventional radiotherapy,
particle therapy involves directing a beam of accelerated subatomic, electrically charged particles
to tumour targets. Different particles have been trialled, including neutrons, protons, pions, and
various ions, however the most commonly used particles are protons, followed by carbon ions.
13
The main potential benefits of particle therapy radiation (protons and carbon ions) primarily come
from the physical distribution of its radiation dose. Conventional radiotherapy, utilising photon (x-
ray) energy, deposits most of its energy near the skin surface, with a decreasing dose deposition
along its path, including healthy tissue encountered before the target tumour, the tumour itself,
and healthy tissue beyond the tumour (see Figure 1).
14
Proton and Heavy Ion Therapy: An overview: January 2017
8
Figure 1 X-rays lose energy rapidly as they travel through the body. Protons and other ions
deposit most of their energy at a specific depth depending on their energy (the Bragg Peak).
Therefore, they can deliver a high radiation dose at a tumour site, sparing surrounding healthy
tissue.
15
In contrast, particle therapy radiation has a lower entrance dose and deposits most of its radiation
energy to a small area at what is known as the Bragg Peak. The depth to which the protons or
carbon ions penetrate, and at which the Bragg Peak occurs, is dependent on the energy of the
beam, which is adjusted within the accelerator or beam transport system to correspond with the
targeted tumour.
1
As particle therapy radiation is mostly absorbed at this point, there is little or
no exit dose (i.e. irradiation of normal tissue beyond the target).
16
Therefore, particle therapy can
reduce the radiation dose to healthy
tissue around the tumour, reducing treatment side-effects. In
addition, due to the difference in energy deposition between conventional radiotherapy and
particle therapy radiation, there is an estimated reduction of approximately 60 per cent in the
integral dose (the total energy absorbed by the human body).
17
Theoretically, this would lead to a
reduction of radiation-induced secondary cancers; however there is limited clinical evidence to
date to support this assertion, in part due to the limited number of in vivo clinical trials
published.
18
As a result of both its superior depth dose distribution and reduced integral dose, particle therapy
technology is considered beneficial for paediatric and young adult patients, and in patients with
tumours located near vital organs or tissues.
19
This includes tumours with adjacent nerves whose
integrity could be compromised as a result of surgery or radiation, where a tumour cannot be
resected with appropriate margins due to proximity to critical structures, and where, with
conventional radiotherapy, there is an inability to irradiate to a curative dose without overdosing
other local organs.
20
However, some uncertainties remain in particle therapy technology planning
and delivery, including radiobiological effectiveness, planning calculations and costs, and further
research is necessary to determine optimal treatment protocols.
21
Bragg Peak