Particle Therapy Co-operative Oncology Group (PTCOG 40) meeting, Institute Curie 2004

Clinical Radiobiology of Fast Neutron Therapy: What Was Learnt?

Neutron therapy was developed from neutron radiobiology experiments, and had identified a higher cell kill per unit dose and an accompanying reduction in oxygen dependency. But experts such as Hal Gray were sceptical about clinical applications, for good reasons. Gray knew that the increase in relative biological effectiveness (RBE) with dose fall-off could produce marked clinical limitations. After many years of research, this treatment did not produce the expected gains in tumour control relative to normal tissue toxicity, as predicted by Gray. More detailed reasons for this are discussed in this paper. Neutrons do not have Bragg peaks and so did not selectively spare many tissues from radiation exposure; the constant neutron RBE tumour prescription values did not represent the probable higher RBE values in late-reacting tissues with low α/β values; the inevitable increase in RBE as dose falls along a beam would also contribute to greater toxicity than in a similar megavoltage photon beam. Some tissues such as the central nervous system white matter had the highest RBEs partly because of the higher percentage hydrogen content in lipid-containing molecules. All the above factors contributed to disappointing clinical results found in a series of randomised controlled studies at many treatment centres, although at the time they were performed, neutron therapy was in a catch-up phase with photon-based treatments. Their findings are summarised along with their technical aspects and fractionation choices. Better understanding of fast neutron experiments and therapy has been gained through relatively simple mathematical models—using the biological effective dose concept and incorporating the RBEmax and RBEmin parameters (the limits of RBE at low and high dose, respectively—as shown in the Appendix). The RBE itself can then vary between these limits according to the dose per fraction used. These approaches provide useful insights into the problems that can occur in proton and ion beam therapy and how they may be optimised. This is because neutron ionisations in living tissues are mainly caused by recoil protons of energy proportional to the neutron energy: these are close to the proton energies that occur close to the Bragg peak region. To some extent, neutron RBE studies contain the highest RBE ranges found within proton and ion beams near Bragg peaks. In retrospect, neutrons were a useful radiobiological tool that has continued to inform the scientific and clinical community about the essential radiobiological principles of all forms of high linear energy transfer therapy. Neutron radiobiology and its implications should be taught on training courses and studied closely by clinicians, physicists, and biologists engaged in particle beam therapies.

Proton radiobiology and its clinical implications

Denser ionisation clustering and complex DNA damage in proton Bragg peaks far exceeds that seen with conventional X-rays. This results in more efficient cell sterilisation, quantified by the relative biological effectiveness (RBE). Currently, a 1.1 RBE is used to determine the clinical proton doses by dividing the usual X-rays dose by this amount. This number, derived from short-term experiments, has been criticised as being irrelevant to late normal tissue (NT) effects following radiotherapy and included many control irradiations using lower voltage X-rays (with elevated RBE values) than those used in the clinic. In principle, an increased RBE could be used for each organ at risk, by using extensions of the clinically successful linear quadratic model.

Protons undoubtedly reduce or eliminate NT radiation dose in tissues distantly located from a tumour, but the necessity to include NT margins around a tumour can result in a higher volume of NT than tumour being irradiated. Deleterious side-effects can follow if the NT RBE exceeds 1.1, including in tissue very close to these margins and which are only partially spared.

Use of a constant 1.1 RBE can ‘overdose’ NT, which may require a greater dose reduction such as 1.2 in the brain; some tumours may be ‘under-dosed’ (since they might require a lesser or no reduction in dose). More sophisticated proton experiments show that RBE values of 1.1–1.5 and higher occur in some situations. There are now mathematical models of varying degrees of complexity that can estimate the RBE from the dose, LET and the low-LET radiosensitivities. True multidisciplinary cooperation is required to implement such new ideas in proton therapy in order to improve safety and effectiveness.

A critical appraisal of the clinical utility of proton therapy in oncology

Proton therapy is an emerging technology for providing radiation therapy to cancer patients. The depth dose distribution of a proton beam makes it a preferable radiation modality as it reduces radiation to the healthy tissue outside the tumor, compared with conventional photon therapy. While theoretically beneficial, its clinical values are still being demonstrated from the increasing number of patients treated with proton therapy, from several dozen proton therapy centers around the world. High equipment and facility costs are often the major obstacle for its wider adoption. Because of the high cost and lack of definite clinical evidence of its superiority, proton therapy treatment faces criticism on its cost-effectiveness. Technological development is causing a gradual lowering of costs, and research and clinical studies are providing further evidence on its clinical utility.

Carbon Ion Radiotherapy at the Gunma University Heavy Ion Medical Center: New Facility Set-up

Carbon ion radiotherapy (C-ion RT) offers superior dose conformity in the treatment of deep-seated tumors compared with conventional X-ray therapy. In addition, carbon ion beams have a higher relative biological effectiveness compared with protons or X-ray beams. C-ion RT for the first patient at Gunma University Heavy Ion Medical Center (GHMC) was initiated in March of 2010. The major specifications of the facility were determined based on the experience of clinical treatments at the National Institute of Radiological Sciences (NIRS), with the size and cost being reduced to one-third of those at NIRS. The currently indicated sites of cancer treatment at GHMC are lung, prostate, head and neck, liver, rectum, bone and soft tissue. Between March 2010 and July 2011, a total of 177 patients were treated at GHMC although a total of 100 patients was the design specification during the period in considering the optimal machine performance. In the present article, we introduce the facility set-up of GHMC, including the facility design, treatment planning systems, and clinical preparations.

Implications of quality adjusted survival for clinical trials in radiation oncology

Clinical trials in radiotherapy sometimes compare changes in radiation dose distribution using different radiation techniques. The use of quality adjusted survival can, in special circumstances, reduce the requirement of large patient numbers in order to show a significant difference in overall outcome. The provisos are that marginal improvements in survival or tumour control endpoints and a reduction in toxicity scores are present. The converse findings would also be amenable to this approach. Random sampling methods are used to construct a patient population where the first set of conditions is met. Further work is necessary to refine the absolute indications for this technique.

The case for particle therapy

Among the most important decisions facing the British Government regarding the treatment of cancer in the National Health Service (NHS) is the purchase of charged particle therapy (CPT) centres. CPT is different from conventional radiotherapy: the dose is deposited far more selectively in Bragg Peaks by either protons or "heavy" ions, such as carbon. In this way, it is possible to "dose paint" targets, voxel by voxel, with far less dose to surrounding tissues than with X-ray techniques. At present the UK possesses a 62 MeV cyclotron proton facility at Clatterbridge (Wirral), which provides therapy for intraocular cancers such as melanoma; for deeper situated cancers in the pelvis, chest etc., much higher energies, over 200 MeV are required from a synchrotron facility. There is an impressive expansion in particle beam therapy (PBT) centres worldwide, since they offer good prospects of improved quality of life with enhanced cancer cures in situations where conventional therapy is limited due to radioresistance or by the close proximity of critical normal tissues. There is a threat to UK Oncology, since it is anticipated that several thousand British patients may require referral abroad for therapy; this would severely disrupt their multidisciplinary management and require demanding logistical support.

Modelling the expected increase in demand for particle radiotherapy: implications for the UK

The present rapid worldwide expansion of particle radiotherapy services will inevitably have an impact on clinical practice within the UK. The most recent results of developmental trials using protons and carbon ions are impressive, with high cure rates and little or no functional normal tissue changes and a very low level of serious treatment-related morbidity. The potential numbers of patients that will demand or are referred for treatment abroad are estimated, assuming different rates of change and treatment capacities with time. Even if the maximum demand were to be under 10% of all patients presently treated by radiotherapy, significant numbers (amounting to several thousand patients per year) may be advised to seek treatment abroad between 5 and 10 years from now. The gap between overall demand and the estimated numbers could be partly, although substantially, filled by the establishment of a single large UK facility. Should demand increase beyond the estimated level, for example due to improved screening of cancer, then a network of UK particle radiotherapy centres will be required.