Ion therapy within the trimodal management of superior sulcus tumors: the INKA trial

A Consistent Protocol Reveals a Large Heterogeneity in the Biological Effectiveness of Proton and Carbon-Ion Beams for Various Sarcoma and Normal-Tissue-Derived Cell Lines

Simple Summary

Using a consistent experimental protocol, we found a large heterogeneity in the relative biological effectiveness (RBE) values of both proton and carbon-ion beams in various sarcomas and normal-tissue-derived cell lines. Our data suggest that proton beam therapy may be more beneficial for some types of tumors. In carbon-ion therapy, for some types of tumors, large heterogeneity in RBE should prompt consideration of dose reduction or an increased dose per fraction. In particular, a higher RBE value in normal tissues requires caution. Specific dose evaluations for tumor and normal tissues are needed for both proton and carbon-ion therapies.

Abstract

This study investigated variations in the relative biological effectiveness (RBE) values among various sarcoma and normal-tissue-derived cell lines (normal cell line) in proton beam and carbon-ion irradiations. We used a consistent protocol that specified the timing of irradiation after plating cells and detailed the colony formation assay. We examined the cell type dependence of RBE for proton beam and carbon-ion irradiations using four human sarcoma cell lines (MG63 osteosarcoma, HT1080 fibrosarcoma, SW872 liposarcoma, and SW1353 chondrosarcoma) and three normal cell lines (HDF human dermal fibroblast, hTERT-HME1 mammary gland, and NuLi-1 bronchus epithelium). The cells were irradiated with gamma rays, proton beams at the center of the spread-out Bragg peak, or carbon-ion beams at 54.4 keV/μm linear energy transfer. In all sarcoma and normal cell lines, the average RBE values in proton beam and carbon-ion irradiations were 1.08 ± 0.11 and 2.08 ± 0.36, which were consistent with the values of 1.1 and 2.13 used in current treatment planning systems, respectively. Up to 34% difference in the RBE of the proton beam was observed between MG63 and HT1080. Similarly, a 32% difference in the RBE of the carbon-ion beam was observed between SW872 and the other sarcoma cell lines. In proton beam irradiation, normal cell lines had less variation in RBE values (within 10%), whereas in carbon-ion irradiation, RBE values differed by up to 48% between hTERT-HME1 and NuLi-1. Our results suggest that specific dose evaluations for tumor and normal tissues are necessary for treatment planning in both proton and carbon-ion therapies.

The history of ion beam therapy in Germany

The advantageous depth dose profile of ion beams together with state of the art beam delivery and treatment planning systems allow for highly conformal tumor treatments in patients. First treatments date back to 1954 at the Lawrence Berkeley Laboratory (LBL) and in Europe, ion beam therapy started in the mid-1990s at the Paul-Scherrer Institute (PSI) with protons and at the Helmholtz Center for Heavy Ion Research (GSI) with carbon ions, followed by the Heidelberg Ion Therapy Center (HIT) in Heidelberg. This review describes the historical development of ion beam therapy in Germany based on the pioneering work at LBL and in the context of simultaneous developments in other countries as well as recent developments.

The RBE in ion beam radiotherapy: In vivo studies and clinical application

Ion beams used for radiotherapy exhibit an increased relative biological effectiveness (RBE), which depends on several physical treatment parameters as well as on biological factors of the irradiated tissues. While the RBE is an experimentally well-defined quantity, translation to patients is complex and requires radiobiological studies, dedicated models to calculate the RBE in treatment planning as well as strategies for dose prescription. Preclinical in vivo studies and analysis of clinical outcome are important to validate and refine RBE-models. This review describes the concept of the experimental and clinical RBE and explains the fundamental dependencies of the RBE based on in vitro experiments. The available preclinical in vivo studies on normal tissue and tumor RBE for ions heavier than protons are reviewed in the context of the historical and present development of ion beam radiotherapy. In addition, the role of in vivo RBE-values in the development and benchmarking of RBE-models as well as the transition of these models to clinical application are described. Finally, limitations in the translation of experimental RBE-values into clinical application and the direction of future research are discussed.

Determining RBE for development of lung fibrosis induced by fractionated irradiation with carbon ions utilizing fibrosis index and high-LET BED model

BACKGROUND AND PURPOSES

Carbon ion radiotherapy (CIRT) with raster scanning technology is a promising treatment for lung cancer and thoracic malignancies. Determining normal tissue tolerance of organs at risk is of utmost importance for the success of CIRT. Here we report the relative biological effectiveness (RBE) of CIRT as a function of dose and fractionation for development of pulmonary fibrosis using well established fibrosis index (FI) model.

MATERIALS AND METHODS

Dose series of fractionated clinical quality CIRT versus conventional photon irradiation to the whole thorax were compared in C57BL6 mice. Quantitative assessment of pulmonary fibrosis was performed by applying the FI to computed tomography (CT) data acquired 24-weeks post irradiation. RBE was calculated as the ratio of photon to CIRT dose required for the same level of FI. Further RBE predictions were performed using the derived equation from high-linear energy transfer biologically effective dose (high-LET BED) model.

RESULTS

The averaged lung fibrosis RBE of 5-fraction CIRT schedule was determined as 2.75 ± 0.55. The RBE estimate at the half maximum effective dose (RBE_ED50) was estimated at 2.82 for clinically relevant fractional sizes of 1-6 Gy. At the same dose range, an RBE value of 2.81 ± 0.40 was predicted by the high-LET BED model. The converted biologically effective dose (BED) of CIRT for induction of half maximum FI (BED_ED50) was identified to be 58.12 Gy_3.95. In accordance, an estimated RBE of 2.88 was obtained at the BED_ED50 level. The LQ model radiosensitivity parameters for 5-fraction was obtained as α_H = 0.3030 ± 0.0037 Gy^-1 and β_H = 0.0056 ± 0.0007 Gy^-2.

CONCLUSION

This is the first report of RBE estimation for CIRT with the endpoint of pulmonary fibrosis in-vivo. We proposed in present study a novel way to mathematically modeling RBE by integrating RBE_max and α/β_L based on conventional high-LET BED conception. This model well predicted RBE in the clinically relevant dose range but is sensitive to the uncertainties of α/β estimates from the reference photon irradiation (α/β_L). These findings will assist to eliminate current uncertainties in prediction of CIRT induced normal tissue complications and builds a solid foundation for development of more accurate in-vivo data driven RBE estimates.

Radiobiological issues in prospective carbon ion therapy trials

Carbon ion radiotherapy (CIRT) is developing toward a versatile tool in radiotherapy; however, the increased relative biological effectiveness (RBE) of carbon ions in tumors and normal tissues with respect to photon irradiation has to be considered by mathematical models in treatment planning. As a consequence, dose prescription and definition of dose constraints are performed in terms of RBE weighted rather than absorbed dose. The RBE is a complex quantity, which depends on physical variables, such as dose and beam quality as well as on normal tissue- or tumor-specific factors. At present, three RBE models are employed in CIRT: (a) the mixed-beam model, (b) the Microdosimetric Kinetic Model (MKM), and (c) the local effect model. While the LEM is used in Europe, the other two models are employed in Japan, and unfortunately, the concepts of how the nominal RBE-weighted dose is determined and prescribed differ significantly between the European and Japanese centers complicating the comparison, transfer, and reproduction of clinical results. This has severe impact on the way treatments should be prescribed, recorded, and reported. This contribution reviews the concept of the clinical application of the different RBE models and the ongoing clinical CIRT trials in Japan and Europe. Limitations of the RBE models and the resulting radiobiological issues in clinical CIRT trials are discussed in the context of current clinical evidence and future challenges.

RBE and related modeling in carbon-ion therapy

Carbon ion therapy is a promising evolving modality in radiotherapy to treat tumors that are radioresistant against photon treatments. As carbon ions are more effective in normal and tumor tissue, the relative biological effectiveness (RBE) has to be calculated by bio-mathematical models and has to be considered in the dose prescription. This review (i) introduces the concept of the RBE and its most important determinants, (ii) describes the physical and biological causes of the increased RBE for carbon ions, (iii) summarizes available RBE measurements in vitro and in vivo, and (iv) describes the concepts of the clinically applied RBE models (mixed beam model, local effect model, and microdosimetric-kinetic model), and (v) the way they are introduced into clinical application as well as (vi) their status of experimental and clinical validation, and finally (vii) summarizes the current status of the use of the RBE concept in carbon ion therapy and points out clinically relevant conclusions as well as open questions. The RBE concept has proven to be a valuable concept for dose prescription in carbon ion radiotherapy, however, different centers use different RBE models and therefore care has to be taken when transferring results from one center to another. Experimental studies significantly improve the understanding of the dependencies and limitations of RBE models in clinical application. For the future, further studies investigating quantitatively the differential effects between normal tissues and tumors are needed accompanied by clinical studies on effectiveness and toxicity.

Effect of Carbon-Ion Radiation on Drug Transporters Organic Anion Transporting Polypeptides in Breast Cancer Cells

Organic anion transporting polypeptides (OATPs) are a family of membrane uptake transporters that play important roles in absorption, distribution, metabolism and excretion of a wide range of endogenous and exogenous compounds. OATP members, such as OATP1A2, 1B1 and 1B3, were found to transport numerous anticancer agents. For this reason, these uptake transporters have been proposed to serve as novel and potential therapeutic targets for chemotherapy. Previously published studies from our laboratory demonstrated that OATP1A2 expression was upregulated in breast cancer MCF7 cells after X-ray irradiation and the transport of its substrate methotrexate was increased. In the current study, we investigated the effect of carbon-ion radiation on MCF7 and MDA-MB231 cells. We observed significant upregulation of OATP1A2 expression in the hormone-dependent MCF7 cells, especially when irradiated with a low dose (0.5 Gy). For the hormone-independent MDA-MB231 cells, while irradiation with a higher dose exerted a greater effect, only a moderate change was observed compared to that of the MCF7 cells. Combined treatments of OATP1A2 substrates 5-fluorouracil, paclitaxel and methotrexate with 0.5 Gy irradiation resulted in greater cytotoxicity toward MCF7 cells than with the treatment of antineoplastic agents and higher doses. Therefore, heavy ions, such as carbon, can affect expression of drug transporters and show promise in facilitating the delivery of antitumor drugs with greater efficiency.