“Hadron therapy”, a word that has entered the vocabulary of medicine, indicates the therapeutic use of “hadrons”, nuclear particles known in the past only in physics research centres. Hadrons derive their name from the Greek ‘hadrós’ (‘strong’), which identifies the force with which their main constituents, the quarks, interact (coined by Soviet physicist Lev B. Okun). The hadrons used today in hadron therapy centres all over the world are protons, containing three quarks, and carbon ions—made of six protons and six neutrons and thus containing 36 constituent quarks—and very recently also helium ions
[1], composed of two protons and two neutrons, hence containing 12 quarks. The treatments with carbon ion beams are often referred to as CIRT, which stands for Carbon Ions Radiation Therapy. In particle physics, carbon ions are relatively light particles and therefore they are called light ions. The convention is different in radiotherapy where carbon ions are often referred to as heavy particles to distinguish them from protons.
Rationale and Diffusion of Hadron Therapy in the World
Charged atomic nuclei, such as protons or carbon ions, have a completely different energy deposition curve with respect to X-rays, showing the characteristic Bragg peak at the end of their path in tissues (
Figure 1)
[2].
Figure 1. Energy deposition in water by X-ray photons, protons and carbon ions. The energy of the X-rays is 21 MeV. The energy of the two particles is selected to provide the same range: 148 MeV/u for protons and 270 MeV/u for carbon ions
[2].
The energy deposited per gram of matter (i.e., the ‘dose’) is low when the hadron beam enters the body and is mainly concentrated at the end of the range. The so-called Bragg peak is rather narrow (few millimetres) and to cover the full longitudinal extension of the tumour a superposition of Bragg peaks with different heights and depths is necessary, creating a Spread Out Bragg Peak (SOBP). The use of a focused beam of millimetric transverse dimensions-displaced by scanning magnets in the plane perpendicular to the beam direction-allows the painting of slices of a tumour. The combination of longitudinal (varying the beam energy) and transversal (by magnetic scanning) displacement of the beam creates an almost ideal coverage of the tumour volume and a low radiation dose deposited outside. Carbon ions are more precise than protons because, due to the larger mass, they have a reduced longitudinal range variation (straggling) and a smaller lateral scattering thus a sharper lateral penumbra
[3].
Carbon ions have a different action on the cells of the traversed organs with respect to protons or X-rays. At the end of their range the energy deposition per unit length (the ‘Linear Energy Transfer’ or LET) is producing a larger number of irreparable double strand DNA (deoxyribonucleic acid) breaks with a much higher probability of cell killing
[4]. This action is usually expressed in terms of the relative biological effectiveness (RBE), which is defined as the ratio of the photon dose and the dose of the particle radiation leading to the same biological effect. RBE of carbon ions on the tumour can be as high as 3, thus allowing the delivery of a higher biological dose in the tumour target with respect to photons and protons, while keeping the same ‘biological’ dose in the surrounding normal tissues, or the same dose in the tumour and reduced doses and damages to the normal tissues. Moreover, the higher fraction of clustered DNA lesions produced by ions, which cannot be repaired by the usual cellular mechanisms, is processed via alternative end-joining mechanisms
[5] and opens the way to the use of smart radio-sensitizers that makes tumour cells more sensitive to ion therapy. Another quantitative advantage is that ions have a reduced Oxygen Enhancement Ratio (OER) and are hence less dependent on the availability of oxygen in the tumour tissue. This means that they are effective in the treatment of hypoxia-related radio-resistant tumours, so that they can eradicate tumours that are resistant to X-ray and proton therapy
[6][7][8][9]. The latter represent 1–3% of all patients treated with X-rays, for whom CIRT is the only effective radiation treatment.
Recent data also point out even more significant biological effects of ion therapy, including reduced angiogenesis
[10][11], reduced metastasis
[12][13] and increased immune response following exposure to light ions
[14]. This implies that ion therapy can be used to enhance the effectiveness of cancer immunotherapy.
The idea of using protons to treat tumours dates back to 1946, when the American scientist Bob Wilson
[15] understood their potential due to the physical characteristics of the deposited dose. It is important to note that, until the end of the 1980s, patients were irradiated at accelerators built for nuclear and sub-nuclear physics research and adapted to radiotherapy, with all the associated drawbacks. At the beginnings of the 1990s, the era of modern hadron therapy finally began, with centres dedicated exclusively to clinical activity. The first proton therapy facility is the Loma Linda University Medical Centre, in California
[16]. Three rooms are equipped with rotating magnetic systems (isocentric ‘gantries’) of about ten meters diameter and a mass of about 100 tons that allow—for the first time—to vary the direction of incidence of the proton beam on the patient, as usually happens in conventional radiotherapy. In Japan, in June 1994, the first patient was treated with a carbon ion beam of about 4000 MeV at the Heavy Ion Medical Accelerator Centre in Chiba
[17]. In this case, the beams are fixed, horizontal and vertical, and they serve three treatment rooms. A few years ago, the centre was upgraded with the addition of new treatment rooms, one of which equipped with a carbon ion gantry made of superconducting magnets.
The National Centre for Oncological Hadronterapy (CNAO) operates in Pave, Italy. Patient treatments started in 2011 and both protons and carbon ions are routinely delivered in three treatment rooms. Each room has a horizontal beam and room 2 has an additional vertical beamline. A fourth room, with a horizontal beamline, is fully devoted to research. Figure 2 shows the hospital facility and the synchrotron accelerator.
Figure 2. Left: in front the Centre for Oncological Hadron therapy (CNAO) hospital building, in the back the power station and the roof of the synchrotron vault. Right: view of the synchrotron and of the beam transport lines.
The growth and diffusion of hadron therapy requires many efforts and international collaborations. To this end, the European Network for Light Ion Hadron Therapy (ENLIGHT)
[18] continues to play a central role in the development and diffusion of hadron therapy and in meeting the needs of the community for the education of specialised professionals.
More recently, the four European hadron therapy facilities offering CIRT, together with 18 Partners from 14 European countries, launched a four-year collaboration project named HITRIplus (Heavy Ion Therapy Research Integration plus), that has been approved in the framework of the Horizon 2020 research and innovation programme (agreement GA N. 101008548).
HITRIplus is a multidisciplinary collaborative project, aiming to open the existing facilities to the clinical and research communities, to integrate and advance biophysics and medical research in cancer treatment with ions and, in parallel, to develop innovative technologies for the next generation of centres.
Currently, proton therapy and CIRT are expanding worldwide with 94 proton therapy centres, as reported by the Particle Therapy Group website
[19]; the synchrotrons of twelve centres perform CIRT with maximum ion energy of about 400 MeV/nucleon, which corresponds to 27 cm range in water. Proton therapy centres are mainly located in the United States (41 centres), Europe (19 centres), Japan (17 centres) and the UK (5 centres). Carbon ion centres are located in Japan (6 centres), Europe (4 centres) and China (2 centres). Four European clinical centres (CNAO, Heidelberg Ion Therapy Center—HIT, MedAustron and Marburger Ionenstrahl-Therapiezentrum—MIT), one Japanese and one Chinese centre produce both carbon ions and protons, thus they are called ‘multi-particle centres’. There are at present 32 proton centres and 6 carbon ions centres under construction. In addition, 26 new proton facilities and 2 carbon ions centres, including the first one in USA, are in the planning phase.
2.Hadron Therapy Achievements and Challenges
2.1. Pre-Clinical Radiobiology Research
In recent years, radiation biology is experiencing a shift in the research topics, from more classical cellular end points, such as DNA damage and RBE quantification, to studies of the tissues and of the microenvironment
[20]. For example, it is well recognized that low-LET ionizing radiation might promote migration and invasion of tumour cells, while the few data collected so-far with high-LET radiation studies do not lead to clear conclusions
[21]. What is known to be fundamental in the modulation of migration of tumour cells exposed to ionizing radiation is the influence of the microenvironment. Therefore, the study of the influence of radiation on the migratory and invasive capacity cannot ignore the cells that populate the tumour stroma, especially in the case of pancreatic cancers, that is characterized by abundant stroma cells including cancer-associated fibroblasts, which are known to orchestrate the crosstalk with tumour cells
[22].
For this reason, thanks to a collaboration with Polyclinic San Matteo, CNAO is evaluating the cellular effects of carbon ion irradiated pancreatic adenocarcinoma cells and mucosal melanoma cells using extracellular matrix (ECM) scaffolds obtained through organ decellularization. Compared to 2D cell cultures, these ECM-derived bio-scaffolds retain growth factors, cytokines and chemokines that facilitate cell attachment, tissue integration, remodelling and differentiation. Furthermore, these scaffolds guarantee the transport of oxygen and nutrients to the seeded cells and ensure the physiological exit of the waste metabolites produced by the cells
[23]. With this experimental approach, one can create in vitro a 3D growth microenvironment that mimics very closely the native tissue and it is, therefore, possible to evaluate more comprehensively the biological effects of radiations.
Together with direct invasion of surrounding tissues, perineural invasion is another crucial route of cancer spread, since numerous tumour cells have an innate ability to actively migrate along nerves, thanks to the signals of various molecules secreted by both tumour cells and non-tumour cells of the microenvironment
[24]. Very few studies have addressed the influence of photon radiotherapy on this type of spreading, although it is considered as a marker of poor prognosis for numerous malignant neoplasms, including head-and-neck, pancreatic, prostate, colorectal, and salivary cancers. With the aim of providing useful information for the treatment with hadron therapy of these types of tumours, at CNAO a research group has started to investigate in vitro the influence of high LET radiation on migration and invasion of salivary gland adenoid cystic carcinoma and mucosal melanoma cells after different irradiation protocols, using also Neurotrophin-3 and specific inhibitors/antagonists.
To date, the reasons for the inter-individual variability of the response to radiotherapy within the same group of tumours with the same histology/site/stage are not known, nor is the origin of the different radiosensitivity of the irradiated healthy tissues. Thus, pre-clinical carbon ion radiobiology urgently needs also studies aiming at guiding patient selection and treatment protocols to achieve optimal clinical results. In this direction, CNAO researchers have recently started an experimental project to study the radio-sensitivity and induced molecular alterations of different radiation types on cancer-derived models and on organoids of head-and-neck squamous cell carcinomas
[25]. Although local control is generally very high with carbon ion beams in most malignancies, in some cases radiotherapy must be combined with systemic therapies to control metastasis and increase survival. Nevertheless, to date very few radiobiology studies specifically investigated the potential synergistic interactions of chemotherapeutic agents/radio-sensitizer and ion irradiations. CNAO research is moving towards the evaluation of an inhibitor of apoptosis proteins as radio-sensitizer combined with protons or carbon ions, to provide the biological background supporting future clinical trials with particles radiotherapy combined with these drugs in head-and-neck cancer patients.
Finally, since the use of immunotherapy has become a critical treatment modality in many advanced cancers, one of the most asked questions in clinical radiobiology of carbon ions concerns the speculations that these irradiations may enhance tumour immunogenicity and, consequently, whether abscopal effects and the combination CIRT with immunotherapy can produce better clinical outcomes
[26].
2.2. Clinical Activities: Pathologies and Results
CNAO treated more than 3700 patients with a wide range of pathologies, whose incidence rates are indicated in Figure 3. A little more than half of them (55%) were treated with carbon ions.
Figure 10. (a) Incidence of tumour pathologies treated at CNAO; (b) Number of patients treated each year at CNAO.
After an initial clinical trial stage, it is necessary to obtain the CE certification for the CNAO medical device. The number of treated patients per year has grown year after year (Figure 10). Currently, the Centre treats almost 600 patients yearly and its operational capacity might be increased by about 20%. The Italian Ministry of Health has introduced a list of pathologies for which treatments are authorized and reimbursed by the National Health System, the so-called Essential Levels of Assistance (LEA). The admitted pathologies are the following ten: