The biological effectiveness of antiproton irradiation

Comparison of 126 MeV antiproton and proton—a FLUKA-based microdosimetric approach

Objective. This study aims at comparing dosimetric parameters of 126 MeV antiprotons and protons using microdosimetric approach. Approach. Microdosimetric distributions of 126 MeV proton and antiproton beams at 1 μm site size are calculated using the Monte Carlo-based FLUKA code. The distributions are calculated at various depths along the central axis in water phantom as well as at different off-axis locations. The study also includes calculations of secondary radiations produced by antiprotons and protons. Mean quality factor, Q ¯ is calculated using the ICRP 60 and ICRU 40 recommendations. The Relative Biological Effectiveness (RBE) of HSG tumour cell at 10% survival level is calculated based on Microdosimetric Kinetic Model. Main results. Q ¯ I C R P , Q ¯ ICRU and RBE for antiprotons are higher by a factor of about 3.60, 3.41 and 1.24, respectively, at Bragg-peak and higher by a factor of about 1.41, 1.76 and 1.05, respectively, at plateau region of depth-dose profile when compared to protons. At 15 cm depth along the central axis, Q ¯ ICRP , Q ¯ ICRU and RBE for protons are higher by a factor of about 1.42, 1.66 and 1.26, respectively, when compared to antiprotons. At the off-axis distance (Ld ) of 6 cm (at 11.5 cm depth in water), Q ¯ ICRP and Q ¯ ICRU of protons are higher than that of antiproton whereas the trend is opposite at off-axis distance of 4 cm. At Ld = 4 cm (at 11.5 cm depth in water), RBE of antiprotons is higher by about 4% than protons whereas at Ld = 6 cm, RBE of protons is higher by about 13% than antiprotons. Significance. The study shows that antiproton radiotherapy is advantageous as compared to protons considering enhancements in the absorbed dose and RBE-weighed dose values at the Bragg-peak.

Antiproton-nuclei cross sections with Woods-Saxon potential at low energies

The present knowledge of the antinucleons elastic scattering and annihilation processes in matter at low energies is limited to a few nuclei data in a small phase-space. Optical potential models are useful tools for modelling nuclear strong interaction of antinucleons with matter providing predictions at very low energies where data are missing. New calculations of elastic and annihilation cross sections for antiproton with nuclei using an optical potential of Woods-Saxon (WS) shape are presented. Preliminary predictions at low energies for carbon and calcium show clearly-measurable nuclear effects for nuclear elastic cross sections at large angles and momenta greater than 50 MeV/c. Some discrepancies in annihilation cross section comparing predictions and data are present using the same fitting parameters.

Neutron activation of gadolinium for ion therapy: a Monte Carlo study of charged particle beams

This study investigates the photon production from thermal neutron capture in a gadolinium (Gd) infused tumor as a result of secondary neutrons from particle therapy. Gadolinium contrast agents used in MRI are distributed within the tumor volume and can act as neutron capture agents. As a result of particle therapy, secondary neutrons are produced and absorbed by Gd in the tumor providing potential enhanced localized dose in addition to a signature photon spectrum that can be used to produce an image of the Gd enriched tumor. To investigate this imaging application, Monte Carlo (MC) simulations were performed for 10 different particles using a 5-10 cm spread out-Bragg peak (SOBP) centered on an 8 cm3, 3 mg/g Gd infused tumor. For a proton beam, 1.9 × 106 neutron captures per RBE weighted Gray Equivalent dose (GyE) occurred within the Gd tumor region. Antiprotons ([Formula: see text]), negative pions (- π), and helium (He) ion beams resulted in 10, 17 and 1.3 times larger Gd neutron captures per GyE than protons, respectively. Therefore, the characteristic photon based spectroscopic imaging and secondary Gd dose enhancement could be viable and likely beneficial for these three particles.

The relative biological effectiveness of antiprotons

BACKGROUND AND PURPOSE

Aside from the enhancement of physical dose deposited by antiprotons annihilating in tissue-like material compared to protons of the same range a further increase of biological effective dose has been demonstrated. This enhancement can be expressed in an increase of the relative biological effectiveness (RBE) of antiprotons near the end of range. We have performed the first-ever direct measurement of the RBE of antiprotons both at rest and in flight.

MATERIALS AND METHODS

Experimental data were generated on the RBE of an antiproton beam entering a tissue-like target consisting of V79 cells embedded in gelatin with an energy providing a range of approximately 10cm.

RESULTS

The RBE in the entrance channel (the "plateau") is only slightly above the value for a comparable proton beam, and remains low until the proximal edge of the spread-out Bragg peak (SOBP). A steep increase of RBE is seen starting from the onset of the SOBP.

CONCLUSIONS

This paper reports the final results of the experiment AD-4/ACE at CERN on the first-ever direct measurement of RBE of antiprotons and constitutes the first step toward developing treatment plans.

Antiproton stopping power data for radiation therapy simulations

Stopping powers of H, He, H₂, and H₂O targets for antiprotons have been calculated using a convergent close-coupling method. For He and H₂ targets electron-electron correlations are fully accounted for using a multiconfiguration approximation. Two-electron processes are included using an independent-event model. The water molecule is described using a neon-like structure model with a pseudo-spherical potential. Results are tabulated for the purpose of Monte Carlo simulations to model antiproton transport through matter for radiation therapy.

210Po microsphere radiological design for tumor vascular disruption

The feasibility of disrupting a tumor’s vascular structure using ²¹⁰Po microspheres is investigated using standard ion and photon absorbed dose methodologies. Calculated absorbed dose profiles for ²¹⁰Po alpha particles are sufficient to disrupt a tumor’s arteriole structure while minimizing the dose outside the blood vessel wall. ²¹⁰Po photons contribute minimal dose to healthy tissue. The requisite activity of ²¹⁰Po to facilitate vascular disruption is calculated.

Comparison of electromagnetic and hadronic models generated using Geant 4 with antiproton dose measured in CERN

After proposing the idea of antiproton cancer treatment in 1984 many experiments were launched to investigate different aspects of physical and radiobiological properties of antiproton, which came from its annihilation reactions. One of these experiments has been done at the European Organization for Nuclear Research known as CERN using the antiproton decelerator. The ultimate goal of this experiment was to assess the dosimetric and radiobiological properties of beams of antiprotons in order to estimate the suitability of antiprotons for radiotherapy. One difficulty on this way was the unavailability of antiproton beam in CERN for a long time, so the verification of Monte Carlo codes to simulate antiproton depth dose could be useful. Among available simulation codes, Geant4 provides acceptable flexibility and extensibility, which progressively lead to the development of novel Geant4 applications in research domains, especially modeling the biological effects of ionizing radiation at the sub-cellular scale. In this study, the depth dose corresponding to CERN antiproton beam energy by Geant4 recruiting all the standard physics lists currently available and benchmarked for other use cases were calculated. Overall, none of the standard physics lists was able to draw the antiproton percentage depth dose. Although, with some models our results were promising, the Bragg peak level remained as the point of concern for our study. It is concluded that the Bertini model with high precision neutron tracking (QGSP_BERT_HP) is the best to match the experimental data though it is also the slowest model to simulate events among the physics lists.

Radiobiological intercomparison of the 160 MeV and 230 MeV proton therapy beams at the Harvard Cyclotron Laboratory and at Massachusetts General Hospital

The purpose of this study was to determine the relative biological effectiveness (RBE) along the axis of two range-modulated proton beams (160 and 230 MeV). Both the depth and the dose dependence of RBE were investigated. Chinese hamster V79-WNRE cells, suspended in medium containing gelatin and cooled to 2 °C, were used to obtain complete survival curves at multiple positions throughout the entrance and 10 cm spread-out Bragg peak (SOBP). Simultaneous measurements of the survival response to (60)Co gamma rays served as the reference data for the proton RBE determinations. For both beams the RBE increased significantly with depth in the 10 cm SOBP, particularly in the distal half of the SOBP, then rose even more sharply at the distal edge, the most distal position measured. At a 4 Gy dose of gamma radiation (S = 0.34) the average RBE values for the entrance, proximal half, distal half and distal edge were 1.07 ± 0.01, 1.10 ± 0.01, 1.17 ± 0.01 and 1.21 ± 0.01, respectively, and essentially the same for both beams. At a 2 Gy dose of gamma radiation (S = 0.71) the average RBE values rose to 1.13 ± 0.03, 1.15 ± 0.02, 1.26 ± 0.02 and 1.30 ± 0.02, respectively, for the same four regions of the SOBP. The difference between the 4 Gy and 2 Gy RBE values reflects the dose dependence of RBE as measured in these V79-WNRE cells, which have a low α/β value, as do other widely used cell lines that also show dose-dependent RBE values. Late-responding tissues are also characterized by low α/β values, so it is possible that these cell lines may be predictive for the response of such tissues (e.g., spinal cord, optic nerve, kidney, liver, lung). However, in the very small number of studies of late-responding tissues performed to date there appears to be no evidence of an increased RBE for protons at low doses. Similarly, RBE measurements using early responding in vivo systems (mostly mouse jejunum, an early-responding tissue which has a large α/β ∼ 10 Gy) have generally shown little or no detectable dose dependence. It is useful to compare the RBE values reported here to the commonly used generic clinical RBE of 1.1, which assumes no dependence on depth or on dose. Our proximal RBEs obviously avoid the depth-related increase in RBE and for doses of 4 Gy or more, the low-dose increase in RBE is also minimized, as shown in this article. Thus the proximal RBE at a 4 Gy dose of 1.10 ± 0.01, quoted above, represents an interesting point of congruence with the clinical RBE for conditions where it could reasonably be expected in the measurements reported here. The depth dependence of RBE reported here is consistent with the majority of measurements, both in vitro and in vivo, by other investigators. The dose dependence of RBE, on the other hand, is tissue specific but has not yet been demonstrated for protons by RBE values in late-responding normal tissue systems. This indicates a need for additional RBE determination as function of dose, especially in late-responding tissues.

Tumor vascular disruption using various radiation types

The feasibility of disrupting a tumor's vascular structure with various radiation types and radionuclides is investigated. Calculated absorbed dose profiles for photons and (4)He ions suggest that low-energy beta-gamma and alpha emitting radionuclides can deposit sufficient absorbed dose to disrupt a tumor's vascular structure while minimizing the dose outside the blood vessel. Candidate radionuclides uniformly distributed in microspheres are theoretically investigated with respect to their vascular disruption potential and to offer an alternative to (90)Y microsphere therapy. Requisite activities of candidate low-energy beta-gamma and alpha emitting radionuclides to facilitate vascular disruption are calculated.

Antiproton induced DNA damage: proton like in flight, carbon-ion like near rest

Biological validation of new radiotherapy modalities is essential to understand their therapeutic potential. Antiprotons have been proposed for cancer therapy due to enhanced dose deposition provided by antiproton-nucleon annihilation. We assessed cellular DNA damage and relative biological effectiveness (RBE) of a clinically relevant antiproton beam. Despite a modest LET (~19 keV/μm), antiproton spread out Bragg peak (SOBP) irradiation caused significant residual γ-H2AX foci compared to X-ray, proton and antiproton plateau irradiation. RBE of ~1.48 in the SOBP and ~1 in the plateau were measured and used for a qualitative effective dose curve comparison with proton and carbon-ions. Foci in the antiproton SOBP were larger and more structured compared to X-rays, protons and carbon-ions. This is likely due to overlapping particle tracks near the annihilation vertex, creating spatially correlated DNA lesions. No biological effects were observed at 28–42 mm away from the primary beam suggesting minimal risk from long-range secondary particles.

Medical Applications of Accelerators

Particle accelerators play an essential role in the field of medical applications. A large variety of systems is in use for diagnostic purposes, such as the production of radioactive tracers for imaging or x-ray radiography. The dominant application, however, is related to the treatment of cancer patients. This article puts emphasis on cancer treatment, presenting the status and developments of the corresponding technical systems, and gives a brief overview of the biophysical properties and medical aspects of these treatments.

Neutron fluence in antiproton radiotherapy, measurements and simulations

INTRODUCTION

A significant part of the secondary particle spectrum from antiproton annihilation consists of fast neutrons, which may contribute to a significant dose background found outside the primary beam.

MATERIALS AND METHODS

Using a polystyrene phantom as a moderator, we have performed absolute fluence measurements of the thermalized part of the fast neutron spectrum using Lithium-6 and -7 Fluoride TLD pairs. The results were compared with the Monte Carlo particle transport code FLUKA.

RESULTS

The experimental results are found to be in good agreement with simulations. The thermal neutron kerma resulting from the measured thermal neutron fluence is insignificant compared to the contribution from fast neutrons.

DISCUSSION

The secondary neutron fluences encountered in antiproton therapy are found to be similar to values calculated for pion treatment, however exact modeling under more realistic treatment scenarios is still required to quantitatively compare these treatment modalities.

Dose- and LET-painting with particle therapy

Tumour hypoxia is one of the limiting factors in obtaining tumour control in radiotherapy. The high-LET region of a beam of heavy charged particles such as carbon ions is located in the distal part of the Bragg peak. A modulated or spread out Bragg peak (SOBP) is a weighted function of several Bragg peaks at various energies, which however results in a dilution of the dose-average LET in the target volume. Here, we investigate the possibility to redistribute the LET by dedicated treatment plan optimisation, in order to maximise LET in the target volume. This may be a strategy to potentially overcome hypoxia along with dose escalation or dose painting. The high-LET region can be shaped in very different ways, while maintaining the distribution of the absorbed dose or biological effective dose. Treatment plans involving only carbon ion beams, show very different LET distributions depending on how the fields are arranged. Alternatively, a LET boost can be applied in multi-modal treatment planning, such as combining carbon ions with protons and/or photons. For such mixed radiation modalities, significant "LET boosts" can be achieved at nearly arbitrary positions within the target volume. Following the general understanding of the relationship between hypoxia, LET and the oxygen enhancement ratio (OER), we conclude, that an additional therapeutic advantage can be achieved by confining the high-LET part of the radiation in hypoxic compartments of the tumour, and applying low-LET radiation to the normoxic tissue. We also anticipate that additional advantages may be achieved by deliberate sparing of normal tissue from high LET regions. Consequently, treatment planning based on simultaneous dose and LET optimisation has a potential to achieve higher tumour control and/or reduced normal tissue control probability (NTCP).

Feasibility of using internal radiation-generating devices in radiotherapy

The feasibility of using radiation-generating devices located within a tumor mass for radiotherapy applications is investigated. This paper presumes the existence of these devices and develops their requisite characteristics and possible arrangement configurations to permit the selective irradiation of tumors. Calculations are provided for a prototypical proton-generating device.

Real-time imaging for dose evaluation during antiproton irradiation

Online monitoring of the stopping distribution of particle beams used for radiotherapy provides the possibility of detecting possible errors in dose deposition early during a given treatment session, and may therefore help to improve the quality of the therapy. Antiproton annihilation events produce several long-range secondary particles which can be detected in real time by standard high energy particle physics detector systems. In this note, Monte Carlo calculations are performed in order to study the feasibility of real-time imaging by detecting charged pions produced during antiproton irradiation of typical biological targets. A simple treatment plan in a water phantom is simulated and the results show that by detecting pi+/- the position and the size of the planned target volume can be located with precision in the order of 1 mm.

V-79 Chinese hamster cells irradiated with antiprotons, a study of peripheral damage due to medium and long range components of the annihilation radiation

PURPOSE

Radiotherapy of cancer carries a perceived risk of inducing secondary cancer and other damage due to dose delivered to normal tissue. While expectedly small, this risk must be carefully analysed for all modalities. Especially in the use of exotic particles like pions and antiprotons, which annihilate and produce a mixed radiation field when interacting with normal matter nuclei, the biological effective dose far out of field needs to be considered in evaluating this approach. We describe first biological measurements to address the concern that medium and long range annihilation products may produce a significant background dose and reverse any benefits of higher biological dose in the target area.

MATERIALS AND METHODS

Using the Antiproton Decelerator (AD) at CERN (Conseil Européen pour la Recherche Nucléaire) we irradiated V-79 Chinese Hamster cells embedded in gelatine using an antiproton beam with fluence ranging from 4.5 x 10(8) to 4.5 x 10(9) particles, and evaluated the biological effect on cells located distal to the Bragg peak using clonogenic survival and the COMET assay.

RESULTS

Both methods show a substantial biological effect on the cells in the entrance channel and the Bragg Peak area, but any damage is reduced to levels well below the effect in the entrance channel 15 mm distal to the Bragg peak for even the highest particle fluence used.

CONCLUSIONS

The annihilation radiation generated by antiprotons stopping in biological targets causes an increase of the penumbra of the beam but the effect rapidly decreases with distance from the target volume. No major increase in the biological effect is found in the far field outside of the primary beam.

Calculated LET spectrum from antiproton beams stopping in water

INTRODUCTION

Antiprotons have been proposed as a potential modality for radiotherapy because the annihilation at the end of range leads to roughly a doubling of physical dose in the Bragg peak region. So far it has been anticipated that the radiobiology of antiproton beams is similar to that of protons in the entry region of the beam, but very different in the annihilation region, due to the expected high-LET components resulting from the annihilation. On closer inspection we find that calculations of dose averaged LET in the entry region may suggest that the RBE of antiprotons in the plateau region could significantly differ from unity, which seems to warrant closer inspection of the radiobiology in this region.

MATERIALS AND METHODS

Monte Carlo simulations using FLUKA were performed for calculating the entire particle spectrum of a beam of 126 MeV antiprotons hitting a water phantom.

RESULTS AND DISCUSSION

In the plateau region of the simulated antiproton beam we observe a dose-averaged unrestricted LET of about 4 keV/microm, which is very different from the expected 0.6 keV/microm of an equivalent primary proton beam. Even though the fluence of secondaries is a magnitude less than the fluence of primary particles, the increased stopping power of the secondary particles causes an increase in the dose averaged LET which is expected to result in a RBE different from unity.

European developments in radiotherapy with beams of large radiobiological effectiveness

This paper reviews the European activities in the field of tumour therapy with beams which have a Radio Biological Effectiveness (RBE) larger than 1. Initially neutron beams have been used. Then charged pions promised better cure rates so that their use was pursued in the framework of the ;Piotron' project at the Paul Scherrer Institute (Switzerland). However both approaches did not meet the expectations and in the 80s the EULIMA project became the flagship of these attempts to improve the effects of the delivery of radiation doses of large RBE with respect to photons, electrons and even protons. The EULIMA ion accelerator was never built and it took more than ten years to see the approval, in Heidelberg and Pavia, of the construction of the HIT and CNAO ;dual' centres for carbon ions and protons. In 2008 they will start treating patients. The developments that brought to these construction projects are described together with the special features of these two facilities. The third European dual centre is being built by Siemens Medical Systems in Marburg, Germany, while other facilities have been approved but not yet fully financed in Wiener Neustadt (Austria), Lyon (France) and Uppsala (Sweden). Finally the collaboration activities of the European Network ENLIGHT are presented together with the recent involvements of European industries in the construction of turn-key dual centres and the development of a new accelerator concept for hadrontherapy, the ;cyclinac'.