A possible biomedical facility at the European Organization for Nuclear Research (CERN)

Clinical radiobiology of proton therapy: modeling of RBE

PURPOSE

To better estimate relative biological effectiveness (RBE) in therapeutic proton beams by using a modeled approach, in order to improve their clinical safety and effectiveness.

INTRODUCTION

Concerns exist about the 1.1 RBE used in proton therapy, since it may lead to unintentional over- and under-dosage in patients and so lead to unexpected clinical outcomes. Late reacting normal tissues (with low α/β values), might be overdosed if RBE >1.1; very radiosensitive tumors (with high α/β), might be under-dosed if RBE <1.1. Some physicists recommend ignoring RBE in favor of a LET × dose product to predict effects.

MATERIAL AND METHODS

Extensive linear-quadratic based modeling is scaled between a standard hospital megavoltage photon reference radiation (low LET of 0.22 keV μm^-1) α and β values and their values at higher LETs, representative of the middle and end of the SOBPs. A previously published energy-efficiency model provide RBE estimates for different α/β (2-27 Gy). The concept of using a LET × dose product is assessed by comparing it with surviving fraction and the equivalent dose in 2 Gy fractions (EQD-2).

RESULTS

Low α/β value biosystems have the widest RBE ranges with dose per fraction changes and increasing LET, often above 1.1 even within the SOBP LET range, with lower values at higher dose per fraction. Highly radiosensitive tumors (α/β 10-27 Gy) have the lowest RBEs, often below 1.1, and are not fraction-sensitive. RBE's generally increase with LET, so curtailment of LET in normal tissues is important. The LET × dose product is insufficiently discriminating when compared with surviving fraction and biological effective dose (BED) or EQD-2.

CONCLUSIONS

An overall research framework is suggested. Proton therapy advantages will only be fully realized if reasonably correct RBE values are used.

Amino acid transport system - A substrate predicts the therapeutic effects of particle radiotherapy

L-[methyl-¹¹C]Methionine (¹¹C-Met) is useful for estimating the therapeutic efficacy of particle radiotherapy at early stages of the treatment. Given the short half-life of ¹¹C, the development of longer-lived ¹⁸F- and ¹²³I-labeled probes that afford diagnostic information similar to ¹¹C-Met, are being sought. Tumor uptake of ¹¹C-Met is involved in many cellular functions such as amino acid transport System-L, protein synthesis, and transmethylation. Among these processes, since the energy-dependent intracellular functions involved with ¹¹C-Met are more reflective of the radiotherapeutic effects, we evaluated the activity of the amino acid transport System-A as an another energy-dependent cellular function in order to estimate radiotherapeutic effects. In this study, using a carbon-ion beam as the radiation source, the activity of System-A was evaluated by a specific System-A substrate, alpha-[1-¹⁴C]-methyl-aminoisobutyric acid (¹⁴C-MeAIB). Cellular growth and the accumulation of ¹⁴C-MeAIB or ¹⁴C-Met were evaluated over time in vitro in cultured human salivary gland (HSG) tumor cells (3-Gy) or in vivo in murine xenografts of HSG tumors (6- or 25-Gy) before and after irradiation with the carbon-ion beam. Post 3-Gy irradiation, in vitro accumulation of ¹⁴C-Met and ¹⁴C-MeAIB decreased over a 5-day period. In xenografts of HSG tumors in mice, tumor re-growth was observed in vivo on day-10 after a 6-Gy irradiation dose, but no re-growth was detected after the 25-Gy irradiation dose. Consistent with the growth results, the in vivo tumor accumulation of ¹⁴C-MeAIB did not decrease after the 6-Gy irradiation dose, whereas a significant decrease was observed after the 25-Gy irradiation dose. These results indicate that the activity of energy dependent System-A transporter may reflect the therapeutic efficacy of carbon-ion radiotherapy and suggests that longer half-life radionuclide-labeled probes for System-A may also provide widely available probes to evaluate the effects of particle radiotherapy on tumors at early stage of the treatment.

Why RBE must be a variable and not a constant in proton therapy

OBJECTIVE

This article considered why the proton therapy (PT) relative biological effect (RBE) should be a variable rather than a constant.

METHODS

The reasons for a variable proton RBE are enumerated, with qualitative and quantitative arguments. The heterogeneous data sets collated by Paganetti et al (2002) and the more homogeneous data of Britten et al (2013) are further analyzed using linear regression fitting and RBE-inclusive adaptations of the linear-quadratic (LQ) radiation model.

RESULTS

The in vitro data show RBE increasing as dose per fraction is lowered. In the Paganetti et al (2002) data sets, the differences between observed and expected effects are smaller when the LQ model is used, but with such data heterogeneity, firm statistical conclusions cannot be obtained. The more homogeneous data set shows an unequivocal variation in RBE with dose per faction. The in vivo data are inappropriate for assessments of late normal tissue effects in radiotherapy. Also, if there is the same degree of uncertainty in an RBE of 1.1 or in an RBE of 2-3 for C ions, the fractional and biological effective doses can vary considerably and be greater in the proton case. So, errors in RBE assignment are important for protons, just as with C ions.

CONCLUSION

Further experimental programmes are proposed, including late normal tissue end points. Better RBE allocations might further improve PT outcomes.

ADVANCES IN KNOWLEDGE

This study provides a rigorous critique of the 1.1 RBE used for protons, from theoretical and practical standpoints. Data analysis shows that the LQ model is more appropriate than simple linear regression. Comprehensive research programmes are suggested.

Medical Applications at CERN and the ENLIGHT Network

State-of-the-art techniques derived from particle accelerators, detectors, and physics computing are routinely used in clinical practice and medical research centers: from imaging technologies to dedicated accelerators for cancer therapy and nuclear medicine, simulations, and data analytics. Principles of particle physics themselves are the foundation of a cutting edge radiotherapy technique for cancer treatment: hadron therapy. This article is an overview of the involvement of CERN, the European Organization for Nuclear Research, in medical applications, with specific focus on hadron therapy. It also presents the history, achievements, and future scientific goals of the European Network for Light Ion Hadron Therapy, whose co-ordination office is at CERN.

Radiation quality and ion-beam therapy: understanding the users' needs

Ion-beam therapy faces a growing demand of tools able to map radiation quality within the irradiated volume. Although analytical computations and simulations provide useful estimations of dose and radiation quality, the direct measure of those parameters would improve ion-beam therapy in particular when deep-seated tumours are irradiated, tissue composition and density are variable or organs at risk are near the tumour. Several ion-beam therapy facilities are studying detectors and procedures for measuring the radiation quality on a microdosimetric as well as a nanodosimetric scale. Simplicity and miniaturisation of the devices are essential for measurements first in phantoms and thereafter during therapy, particularly for intra-cavity detectors. MedAustron is studying solid-state detectors based on a single crystal chemical vapour deposition diamond. In collaboration with Italian National Institute for Nuclear Physics (INFN), Tor Vergata and Legnaro; INFN-microdosimetry and track structure project; Austrian Institute of Technology, Vienna; and Italian National agency for new technologies, energy and sustainable economic development, Rome, prototypes have been developed to characterise radiation quality in sizes equivalent to one micrometre of biological tissue.

A Simpler Energy Transfer Efficiency Model to Predict Relative Biological Effect for Protons and Heavier Ions

The aim of this work is to predict relative biological effectiveness (RBE) for protons and clinically relevant heavier ions, by using a simplified semi-empirical process based on rational expectations and published experimental results using different ion species. The model input parameters are: Z (effective nuclear charge) and radiosensitivity parameters α_L and β_L of the control low linear energy transfer (LET) radiation. Sequential saturation processes are assumed for: (a) the position of the turnover point (LET_U) for the LET–RBE relationship with Z, and (b) the ultimate value of α at this point (α_U) being non-linearly related to α_L. Using the same procedure for β, on the logical assumption that the changes in β with LET, although smaller than α, are symmetrical with those of α, since there is symmetry of the fall off of LET–RBE curves with increasing dose, which suggests that LET_U must be identical for α and β. Then, using iso-effective linear quadratic model equations, the estimated RBE is scaled between α_U and α_L and between β_U and β_L from for any input value of Z, α_L, β_L, and dose. The model described is fitted to the data of Barendsen (alpha particles), Weyrather et al. (carbon ions), and Todd for nine different ions (deuterons to Argon), which include variations in cell surviving fraction and dose. In principle, this new system can be used to complement the more complex methods to predict RBE with LET such as the local effect and MKM models which already have been incorporated into treatment planning systems in various countries. It would be useful to have a secondary check to such systems, especially to alert clinicians of potential risks by relatively easy estimation of relevant RBEs. In clinical practice, LET values smaller than LET_U are mostly encountered, but the model extends to higher values beyond LET_U for other purposes such as radiation, protection, and astrobiology. Considerable further research is required, perhaps in a dedicated international laboratory, using a basket of different models to determine what the best system or combination of systems will be to make proton and ion beam radiotherapy as safe as possible and to produce the best possible clinical results.

Towards Achieving the Full Clinical Potential of Proton Therapy by Inclusion of LET and RBE Models

Despite increasing use of proton therapy (PBT), several systematic literature reviews show limited gains in clinical outcomes, with publications mostly devoted to recent technical developments. The lack of randomised control studies has also hampered progress in the acceptance of PBT by many oncologists and policy makers. There remain two important uncertainties associated with PBT, namely: (1) accuracy and reproducibility of Bragg peak position (BPP); and (2) imprecise knowledge of the relative biological effect (RBE) for different tissues and tumours, and at different doses. Incorrect BPP will change dose, linear energy transfer (LET) and RBE, with risks of reduced tumour control and enhanced toxicity. These interrelationships are discussed qualitatively with respect to the ICRU target volume definitions. The internationally accepted proton RBE of 1.1 was based on assays and dose ranges unlikely to reveal the complete range of RBE in the human body. RBE values are not known for human (or animal) brain, spine, kidney, liver, intestine, etc. A simple efficiency model for estimating proton RBE values is described, based on data of Belli et al. and other authors, which allows linear increases in α and β with LET, with a gradient estimated using a saturation model from the low LET α and β radiosensitivity parameter input values, and decreasing RBE with increasing dose. To improve outcomes, 3-D dose-LET-RBE and bio-effectiveness maps are required. Validation experiments are indicated in relevant tissues. Randomised clinical studies that test the invariant 1.1 RBE allocation against higher values in late reacting tissues, and lower tumour RBE values in the case of radiosensitive tumours, are also indicated.

New challenges in high-energy particle radiobiology

Densely ionizing radiation has always been a main topic in radiobiology. In fact, α-particles and neutrons are sources of radiation exposure for the general population and workers in nuclear power plants. More recently, high-energy protons and heavy ions attracted a large interest for two applications: hadrontherapy in oncology and space radiation protection in manned space missions. For many years, studies concentrated on measurements of the relative biological effectiveness (RBE) of the energetic particles for different end points, especially cell killing (for radiotherapy) and carcinogenesis (for late effects). Although more recently, it has been shown that densely ionizing radiation elicits signalling pathways quite distinct from those involved in the cell and tissue response to photons. The response of the microenvironment to charged particles is therefore under scrutiny, and both the damage in the target and non-target tissues are relevant. The role of individual susceptibility in therapy and risk is obviously a major topic in radiation research in general, and for ion radiobiology as well. Particle radiobiology is therefore now entering into a new phase, where beyond RBE, the tissue response is considered. These results may open new applications for both cancer therapy and protection in deep space.

Feasibility study for a biomedical experimental facility based on LEIR at CERN

In light of the recent European developments in ion beam therapy, there is a strong interest from the biomedical research community to have more access to clinically relevant beams. Beamtime for pre-clinical studies is currently very limited and a new dedicated facility would allow extensive research into the radiobiological mechanisms of ion beam radiation and the development of more refined techniques of dosimetry and imaging. This basic research would support the current clinical efforts of the new treatment centres in Europe (for example HIT, CNAO and MedAustron). This paper presents first investigations on the feasibility of an experimental biomedical facility based on the CERN Low Energy Ion Ring LEIR accelerator. Such a new facility could provide beams of light ions (from protons to neon ions) in a collaborative and cost-effective way, since it would rely partly on CERN's competences and infrastructure. The main technical challenges linked to the implementation of a slow extraction scheme for LEIR and to the design of the experimental beamlines are described and first solutions presented. These include introducing new extraction septa into one of the straight sections of the synchrotron, changing the power supply configuration of the magnets, and designing a new horizontal beamline suitable for clinical beam energies, and a low-energy vertical beamline for particular radiobiological experiments.