Optimizing normal tissue sparing in ion therapy using calculated isoeffective dose for ion selection

Characterization of the Mixed Radiation Field Produced by Carbon and Oxygen Ion Beams of Therapeutic Energy: A Monte Carlo Simulation Study

Purpose:

The main advantages of charged particle radiotherapy compared to conventional X-ray external beam radiotherapy are a better tumor conformality coupled with the capability of treating deep-seated radio-resistant tumors. This work investigates the possibility to use oxygen beams for hadron therapy, as an alternative to carbon ions.

Materials and Methods:

Oxygen ions have the advantage of a higher relative biological effectiveness (RBE) and better conformality to the tumor target. This work describes the mixed radiation field produced by an oxygen beam in water and compares it to the one produced by a therapeutic carbon ion beam. The study has been performed using Geant4 simulations. The dose is calculated for incident carbon ions with energies of 162 MeV/u and 290 MeV/u, and oxygen ions with energies of 192 MeV/u and 245 MeV/u, and hence that the range of the primary oxygen ions projectiles in water was located at the same depth as the carbon ions.

Results:

The results show that the benefits of oxygen ions are more pronounced when using lower energies because of a slightly higher peak-to-entrance ratio, which allows either providing higher dose in tumor target or reducing it in the surrounding healthy tissues. It is observed that, per incident particle, oxygen ions deliver higher doses than carbon ions.

Conclusions:

This result coupled with the higher RBE shows that it may be possible to use a lower fluence of oxygen ions to achieve the same therapeutic dose in the patient as that obtained with carbon ion therapy.

A comparison of mechanism-inspired models for particle relative biological effectiveness (RBE)

BACKGROUND AND SIGNIFICANCE

The application of heavy ion beams in cancer therapy must account for the increasing relative biological effectiveness (RBE) with increasing penetration depth when determining dose prescriptions and organ at risk (OAR) constraints in treatment planning. Because RBE depends in a complex manner on factors such as the ion type, energy, cell and tissue radiosensitivity, physical dose, biological endpoint, and position within and outside treatment fields, biophysical models reflecting these dependencies are required for the personalization and optimization of treatment plans.

AIM

To review and compare three mechanism-inspired models which predict the complexities of particle RBE for various ion types, energies, linear energy transfer (LET) values and tissue radiation sensitivities.

METHODS

The review of models and mechanisms focuses on the Local Effect Model (LEM), the Microdosimetric-Kinetic (MK) model, and the Repair-Misrepair-Fixation (RMF) model in combination with the Monte Carlo Damage Simulation (MCDS). These models relate the induction of potentially lethal double strand breaks (DSBs) to the subsequent interactions and biological processing of DSB into more lethal forms of damage. A key element to explain the increased biological effectiveness of high LET ions compared to MV x rays is the characterization of the number and local complexity (clustering) of the initial DSB produced within a cell. For high LET ions, the spatial density of DSB induction along an ion's trajectory is much greater than along the path of a low LET electron, such as the secondary electrons produced by the megavoltage (MV) x rays used in conventional radiation therapy. The main aspects of the three models are introduced and the conceptual similarities and differences are critiqued and highlighted. Model predictions are compared in terms of the RBE for DSB induction and for reproductive cell survival.

RESULTS AND CONCLUSIONS

Comparisons of the RBE for DSB induction and for cell survival are presented for proton (¹ H), helium (⁴ He), and carbon (¹² C) ions for the therapeutically most relevant range of ion beam energies. The reviewed models embody mechanisms of action acting over the spatial scales underlying the biological processing of potentially lethal DSB into more lethal forms of damage. Differences among the number and types of input parameters, relevant biological targets, and the computational approaches among the LEM, MK and RMF models are summarized and critiqued. Potential experiments to test some of the seemingly contradictory aspects of the models are discussed.

Sensitivity study of the microdosimetric kinetic model parameters for carbon ion radiotherapy

In carbon ion therapy treatment planning, the relative biological effectiveness (RBE) is accounted for by optimization of the RBE-weighted dose (biological dose). The RBE calculation methods currently applied clinically in carbon ion therapy are derived from the microdosimetric kinetic model (MKM) in Japan and the local effect model (LEM) in Europe. The input parameters of these models are based on fit to experimental data subjected to uncertainties. We therefore performed a sensitivity study of the MKM input parameters, i.e. the domain radius (r _d ), the nucleus radius (R _n ) and the parameters of the linear quadratic (LQ) model (α _x and β). The study was performed with the FLUKA Monte Carlo code, using spread out Bragg peak (SOBP) scenarios in water and a biological dose distribution in a clinical patient case. Comparisons were done between biological doses estimated applying the MKM with parameters based on HSG cells, and with HSG parameters varied separately by  ±{5, 25, 50}%. Comparisons were also done between parameter sets from different cell lines (HSG, V79, CHO and T1), as well as versions of the LEM. Of the parameters, r _d had the largest impact on the biological dose distribution, especially on the absolute dose values. Increasing this parameter by 25% decreased the biological dose level at the center of a 3 Gy(RBE) SOBP by 14%. Variations in R _n only influenced the biological dose distribution towards the particle range, and variations in α _x resulted in minor changes in the biological dose, with an increasing impact towards the particle range. β had the overall smallest influence on the SOBPs, but the impact could become more pronounced if alternative (LET dependent) implementations are used. The resulting percentage change in the SOBPs was generally less than the percentage change in the parameters. The patient case showed similar effects as with the SOBPs in water, and parameter variations had similar impact on the biological dose when using the clinical MKM and the general MKM. The clinical LEM calculated the highest biological doses to both tumor and surrounding healthy tissues, with a median target dose (D _50%) of 40.5 Gy(RBE), while the MKM with HSG and V79 parameters resulted in a D _50% of 34.2 and 36.9 Gy(RBE), respectively. In all, the observed change in biological dose distribution due to parameter variations demonstrates the importance of accurate input parameters when applying the MKM in treatment planning.

Carbon ions of different linear energy transfer (LET) values induce apoptosis & G2 cell cycle arrest in radio-resistant melanoma cells

Background & objectives:

The main goal when treating malignancies with radiation is to deprive tumour cells of their reproductive potential. One approach is to induce tumour cell apoptosis. This study was conducted to evaluate the ability of carbon ions (¹²C) to induce apoptosis and cell cycle arrest in human HTB140 melanoma cells.

Methods:

In this in vitro study, human melanoma HTB140 cells were irradiated with the 62 MeV/n carbon (¹²C) ion beam, having two different linear energy transfer (LET) values: 197 and 382 keV/μm. The dose range was 2 to 16 Gy. Cell viability was estimated by the sulforhodamine B assay seven days after irradiation. The cell cycle and apoptosis were evaluated 48 h after irradiation using flow cytometry. At the same time point, protein and gene expression of apoptotic regulators were estimated using the Western blot and q-PCR methods, respectively.

Results:

Cell viability experiments indicated strong anti-tumour effects of ¹²C ions. The analysis of cell cycle showed that ¹²C ions blocked HTB140 cells in G2 phase and induced the dose dependent increase of apoptosis. The maximum value of 21.8 per cent was attained after irradiation with LET of 197 keV/μm at the dose level of 16 Gy. Pro-apoptotic effects of ¹²C ions were confirmed by changes of key apoptotic molecules: the p53, Bax, Bcl-2, poly ADP ribose polymerase (PARP) as well as nuclear factor kappa B (NFκB). At the level of protein expression, the results indicated significant increases of p53, NFκB and Bax/Bcl-2 ratio and PARP cleavage. The Bax/Bcl-2 mRNA ratio was also increased, while no change was detected in the level of NFκB mRNA.

Interpretation & conclusions:

The present results indicated that anti-tumour effects of ¹²C ions in human melanoma HTB140 cells were accomplished through induction of the mitochondrial apoptotic pathway as well as G2 arrest.

Nuclear physics in particle therapy: a review

Charged particle therapy has been largely driven and influenced by nuclear physics. The increase in energy deposition density along the ion path in the body allows reducing the dose to normal tissues during radiotherapy compared to photons. Clinical results of particle therapy support the physical rationale for this treatment, but the method remains controversial because of the high cost and of the lack of comparative clinical trials proving the benefit compared to x-rays. Research in applied nuclear physics, including nuclear interactions, dosimetry, image guidance, range verification, novel accelerators and beam delivery technologies, can significantly improve the clinical outcome in particle therapy. Measurements of fragmentation cross-sections, including those for the production of positron-emitting fragments, and attenuation curves are needed for tuning Monte Carlo codes, whose use in clinical environments is rapidly increasing thanks to fast calculation methods. Existing cross sections and codes are indeed not very accurate in the energy and target regions of interest for particle therapy. These measurements are especially urgent for new ions to be used in therapy, such as helium. Furthermore, nuclear physics hardware developments are frequently finding applications in ion therapy due to similar requirements concerning sensors and real-time data processing. In this review we will briefly describe the physics bases, and concentrate on the open issues.

Can particle beam therapy be improved using helium ions? - a planning study focusing on pediatric patients

Aim To explore the potential of scanned helium ion beam therapy ((4)He) compared to proton therapy in a comparative planning study focusing on pediatric patients. This was motivated by the superior biological and physical characteristics of (4)He. Material and methods For eleven neuroblastoma (NB), nine Hodgkin lymphoma (HL), five Wilms tumor (WT), five ependymoma (EP) and four Ewing sarcoma (EW) patients, treatment plans were created for protons and (4)He. Dose prescription to the planning target volume (PTV) was 21 Gy [relative biological effectiveness (RBE)] (NB), 19.8 Gy (RBE) (HL), 25.2 Gy (RBE) for the WT boost volume and 54 Gy (RBE) for EP and EW patients. A pencil beam algorithm for protons (constant RBE = 1.1) and (4)He was implemented in the treatment planning system Hyperion. For (4)He the relative biological effectiveness (RBE) was calculated with a 'zonal' model based on different linear energy transfer regions. Results Target constraints were fulfilled for all indications. For NB patients differences for kidneys and liver were observed for all dose-volume areas, except the high-dose volume. The body volume receiving up to 12.6 Gy (RBE) was reduced by up to 10% with (4)He. For WT patients the mean and high-dose volume for the liver was improved when using (4)He. For EP normal tissue dose was reduced using (4)He with 12.7% of the voxels receiving higher doses using protons. For HL and EW sarcoma patients the combination of large PTV volumes with the position of the organs at risk (OARs) obliterated the differences between the two particle species, while patients with the heart close to the PTV could benefit from (4)He. Conclusion Treatment plan quality improved with (4)He compared to proton plans, but advantages in OAR sparing were depending on indication and tumor geometries. These first results of scanned (4)He therapy motivate comprehensive research on (4)He, including acquisition of experimental data to improve modeling of (4)He.

New Ions for Therapy

Purpose

Charged particle therapy (CPT) is currently based on the use of protons or carbon ions for the treatment of deep-seated and/or radioresistant tumors, which are known to return poor prognosis in photon treatments. A renovated interest has recently been observed in the possibility of extending the spectrum of ions used in CPT. The potential and limitations of different particle species will be discussed in this work, with special regard to ¹H, ⁴He, ¹²C, and ¹⁶O, that is, those presently available in the most advanced particle therapy clinical centers.

Materials and Methods

Literature information has been screened, as well as additional analysis has been performed, aimed at the comparison of basic physical and biological properties of several ions. The research treatment planning system TRiP98 is also employed to compare the dose distribution resulting from exposure to the different ions in different configurations, including the irradiation of hypoxic targets.

Results

Particles of intermediate charge, such as helium and lithium, offer an increased physical selectivity compared with protons, while having reduced biological effectiveness compared with carbon. The latter aspect translates into a less sensitive biological optimization of CPT treatments, though still more effective than protons in killing cancer cells. At the same time, in view of their increased linear energy transfer, heavier ions, like oxygen, are considered attractive, especially for the treatment of hypoxic tumors. While the higher biological dose released in the entrance dose represents in general a drawback for ions heavier than carbon, for oxygen beam this effect may be balanced by the lower dose increase requested to overcome hypoxia.

Conclusions

The possibility of delivering radiation quality-optimized CPT treatments seems to be the new challenge in heavy ion therapy. The potential and limitations of different particle species, according to different sensitivity and morphological scenarios, makes combined treatments of different ions an intriguing option. New ions could open new scenarios in cancer therapy, but would represent as well an opportunity for the treatment of specific non-cancer disease such as atrial fibrillation.

Rapid MCNP simulation of DNA double strand break (DSB) relative biological effectiveness (RBE) for photons, neutrons, and light ions

To account for particle interactions in the extracellular (physical) environment, information from the cell-level Monte Carlo damage simulation (MCDS) for DNA double strand break (DSB) induction has been integrated into the general purpose Monte Carlo N-particle (MCNP) radiation transport code system. The effort to integrate these models is motivated by the need for a computationally efficient model to accurately predict particle relative biological effectiveness (RBE) in cell cultures and in vivo. To illustrate the approach and highlight the impact of the larger scale physical environment (e.g. establishing charged particle equilibrium), we examined the RBE for DSB induction (RBEDSB) of x-rays, (137)Cs γ-rays, neutrons and light ions relative to γ-rays from (60)Co in monolayer cell cultures at various depths in water. Under normoxic conditions, we found that (137)Cs γ-rays are about 1.7% more effective at creating DSB than γ-rays from (60)Co (RBEDSB  =  1.017) whereas 60-250 kV x-rays are 1.1 to 1.25 times more efficient at creating DSB than (60)Co. Under anoxic conditions, kV x-rays may have an RBEDSB up to 1.51 times as large as (60)Co γ-rays. Fission neutrons passing through monolayer cell cultures have an RBEDSB that ranges from 2.6 to 3.0 in normoxic cells, but may be as large as 9.93 for anoxic cells. For proton pencil beams, Monte Carlo simulations suggest an RBEDSB of about 1.2 at the tip of the Bragg peak and up to 1.6 a few mm beyond the Bragg peak. Bragg peak RBEDSB increases with decreasing oxygen concentration, which may create opportunities to apply proton dose painting to help address tumor hypoxia. Modeling of the particle RBE for DSB induction across multiple physical and biological scales has the potential to aid in the interpretation of laboratory experiments and provide useful information to advance the safety and effectiveness of hadron therapy in the treatment of cancer.