Direct evidence for the spatial correlation between individual particle traversals and localized CDKN1A (p21) response induced by high-LET radiation

Integrating microdosimetricin vitroRBE models for particle therapy into TOPAS MC using the MicrOdosimetry-based modeliNg for RBE ASsessment (MONAS) tool

Objective.In this paper, we present MONAS (MicrOdosimetry-based modelliNg for relative biological effectiveness (RBE) ASsessment) toolkit. MONAS is a TOPAS Monte Carlo extension, that combines simulations of microdosimetric distributions with radiobiological microdosimetry-based models for predicting cell survival curves and dose-dependent RBE.Approach.MONAS expands TOPAS microdosimetric extension, by including novel specific energy scorers to calculate the single- and multi-event specific energy microdosimetric distributions at different micrometer scales. These spectra are used as physical input to three different formulations of themicrodosimetric kinetic model, and to thegeneralized stochastic microdosimetric model(GSM2), to predict dose-dependent cell survival fraction and RBE. MONAS predictions are then validated against experimental microdosimetric spectra andin vitrosurvival fraction data. To show the MONAS features, we present two different applications of the code: (i) the depth-RBE curve calculation from a passively scattered proton SOBP and monoenergetic12C-ion beam by using experimentally validated spectra as physical input, and (ii) the calculation of the 3D RBE distribution on a real head and neck patient geometry treated with protons.Main results.MONAS can estimate dose-dependent RBE and cell survival curves from experimentally validated microdosimetric spectra with four clinically relevant radiobiological models. From the radiobiological characterization of a proton SOBP and12C fields, we observe the well-known trend of increasing RBE values at the distal edge of the radiation field. The 3D RBE map calculated confirmed the trend observed in the analysis of the SOBP, with the highest RBE values found in the distal edge of the target.Significance.MONAS extension offers a comprehensive microdosimetry-based framework for assessing the biological effects of particle radiation in both research and clinical environments, pushing closer the experimental physics-based description to the biological damage assessment, contributing to bridging the gap between a microdosimetric description of the radiation field and its application in proton therapy treatment with variable RBE.

Results of a prospective randomized trial on long-term effectiveness of protons and carbon ions in prostate cancer: LEM I and α/β = 2 Gy overestimates the RBE

AIM

To analyze the long-term effectiveness of carbon ions relative to protons in the prospective randomized controlled ion prostate irradiation (IPI) trial.

METHODS

Effectiveness via PSA assessment in a randomized study on prostate irradiation with 20x3.3 Gy(RBE) protons versus carbon ions was analyzed in 92 patients. Proton RBE was based on a fixed RBE of 1.1 while the local effect model (LEM) I and an α/β = 2 Gy was used for carbon ions. The dose in the prostate was recalculated based on the delivered treatment plan using LEM I and LEM IV and different α/β values.

RESULTS

Five-year overall and progression free survival was 98% and 85% with protons and 91% and 50% with carbon ions, respectively, with the latter being unexpectedly low compared to Japanese carbon ion data and rather corresponding to a photon dose <72 Gy in 2 Gy fractions. According to LEM I and the applied α/β-value of 2 Gy, the applied carbon ion dose in 2 Gy(RBE) fractions (EQD2) was 87.46 Gy(RBE). Recalculations confirmed a strong dependence of RBE-weighted dose on the α/β ratio as well as on the RBE-model.

CONCLUSION

The data demonstrate a significant lower effectiveness of the calculated RBE-weighted dose in the carbon ion as compared to the proton arm. LEM I and an α/β = 2 Gy overestimates the RBE for carbon ions in prostate cancer treatment. Adjusting the biological dose calculation by using LEM I with α/β = 4 Gy could be a pragmatic way to safely escalate dose in carbon ion radiotherapy for prostate cancer.

Effectiveness of fractionated carbon ion treatments in three rat prostate tumors differing in growth rate, differentiation and hypoxia

PURPOSE

To quantify the fractionation dependence of carbon (¹²C) ions and photons in three rat prostate carcinomas differing in growth rate, differentiation and hypoxia.

MATERIAL AND METHODS

Three sublines (AT1, HI, H) of syngeneic rat prostate tumors (R3327) were treated with six fractions of either ¹²C-ions or 6 MV photons. Dose-response curves were determined for the endpoint local tumor control within 300 days. The doses at 50% control probability (TCD₅₀) and the relative biological effectiveness (RBE) of ¹²C-ions were calculated and compared with the values from single and split dose studies.

RESULTS

Experimental findings for the three tumor sublines revealed (i) a comparably increased RBE (2.47-2.67), (ii) a much smaller variation of the radiation response for ¹²C-ions (TCD₅₀: 35.8-43.7 Gy) than for photons (TCD₅₀: 91.3-116.6 Gy), (iii) similarly steep (AT1) or steeper (HI, H) dose-response curves for ¹²C-ions than for photons, (iv) a larger fractionation effect for photons than for ¹²C-ions, and (v) a steeper increase of the RBE with decreasing fractional dose for the well-differentiated H- than for the less-differentiated HI- and AT1-tumors, reflected by (vi) the smallest α/β-value for H-tumors after photon irradiation.

CONCLUSION

¹²C-ions reduce the radiation response heterogeneity between the three tumor sublines as well as within each subline relative to photon treatments, independently of fractionation. The dose dependence of the RBE varies between tumors of different histology. The results support the use of hypofractionated carbon ion treatments in radioresistant tumors.

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.

Carbon ion radiotherapy in pancreatic cancer: A review of clinical data

Despite all efforts, pancreatic cancer remains a highly lethal disease. Only surgical resection offers a realistic chance of survival. But at diagnosis the majority of patients suffer from unresectable disease. Whereas guidelines clearly recommend systemic treatments in metastatic disease, data is limited to support a specific treatment option for locally advanced or borderline resectable pancreatic cancer. Therefore, there is an urgent need to improve treatment schemes addressing patients that suffer from unresectable pancreatic cancer. Chemotherapy, photon radiotherapy and combinations of both have shown improved local control rates but there is still a lack of evidence demonstrating an overall survival benefit of photon radiotherapy if no surgical resection is achieved. Impressive results of Japanese Phase I/II-trials investigating carbon ion radiotherapy in pancreatic cancer attracted global attention. Several studies have been initiated to validate and intensify this promising issue. This review gives an overview of the evidence and current use of carbon ion radiotherapy in pancreatic cancer.

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.

Co-visualization of DNA damage and ion traversals in live mammalian cells using a fluorescent nuclear track detector

The geometric locations of ion traversals in mammalian cells constitute important information in the study of heavy ion-induced biological effect. Single ion traversal through a cellular nucleus produces complex and massive DNA damage at a nanometer level, leading to cell inactivation, mutations and transformation. We present a novel approach that uses a fluorescent nuclear track detector (FNTD) for the simultaneous detection of the geometrical images of ion traversals and DNA damage in single cells using confocal microscopy. HT1080 or HT1080-53BP1-GFP cells were cultured on the surface of a FNTD and exposed to 5.1-MeV/n neon ions. The positions of the ion traversals were obtained as fluorescent images of a FNTD. Localized DNA damage in cells was identified as fluorescent spots of γ-H2AX or 53BP1-GFP. These track images and images of damaged DNA were obtained in a short time using a confocal laser scanning microscope. The geometrical distribution of DNA damage indicated by fluorescent γ-H2AX spots in fixed cells or fluorescent 53BP1-GFP spots in living cells was found to correlate well with the distribution of the ion traversals. This method will be useful for evaluating the number of ion hits on individual cells, not only for micro-beam but also for random-beam experiments.

Using imaging methods to interrogate radiation-induced cell signaling

There is increasing emphasis on the use of systems biology approaches to define radiation-induced responses in cells and tissues. Such approaches frequently rely on global screening using various high throughput 'omics' platforms. Although these methods are ideal for obtaining an unbiased overview of cellular responses, they often cannot reflect the inherent heterogeneity of the system or provide detailed spatial information. Additionally, performing such studies with multiple sampling time points can be prohibitively expensive. Imaging provides a complementary method with high spatial and temporal resolution capable of following the dynamics of signaling processes. In this review, we utilize specific examples to illustrate how imaging approaches have furthered our understanding of radiation-induced cellular signaling. Particular emphasis is placed on protein colocalization, and oscillatory and transient signaling dynamics.

Ion beam radiobiology and cancer: time to update ourselves

High-energy protons and carbon ions exhibit an inverse dose profile allowing for increased energy deposition with penetration depth. Additionally, heavier ions like carbon beams have the advantage of a markedly increased biological effectiveness characterized by enhanced ionization density in the individual tracks of the heavy particles, where DNA damage becomes clustered and therefore more difficult to repair, but is restricted to the end of their range. These superior biophysical and biological profiles of particle beams over conventional radiotherapy permit more precise dose localization and make them highly attractive for treating anatomically complex and radioresistant malignant tumors but without increasing the severe side effects in the normal tissue. More than half a century since Wilson proposed their use in cancer therapy, the effects of particle beams have been extensively investigated and the biological complexity of particle beam irradiation begins to unfold itself. The goal of this review is to provide an as comprehensive and up-to-date summary as possible of the different radiobiological aspects of particle beams for effective application in cancer treatment.

DNA damage recognition proteins localize along heavy ion induced tracks in the cell nucleus

To identify the repair dynamics involved in high linear energy transfer (LET) radiation-induced DNA damage, phospho-H2AX (gammaH2AX) foci formation was analyzed after cellular exposure to iron ions (Fe-ions, 500 MeV u(-1), 200 KeV microm(-1)). The foci located at DNA damage sites were visualized using immunocytochemical methods. Since H2AX is phosphorylated at sites of radiation-induced double strand breaks (DSB), gammaH2AX foci were used to detect or illuminate tracks formed by DSB after exposure to various doses of ionizing radiation. Additional DSB-recognition proteins such as ATM phospho-serine 1981, DNA-PKcs phospho-threonine 2609, NBS1 phospho-serine 343 and CHK2 phospho-threonine 68 all co-localized with gammaH2AX at high LET radiation induced DSB. In addition, Fe-ion induced foci remained for longer times than X-radiation induced foci. These findings suggest that Fe-ion induced damage is repaired more slowly than X-radiation induced damage, possibly because Fe-ion induced damage or lesions are more complex or extensive. Antibodies for all these phosphorylated DNA DSB recognition proteins appear to be very effective for the detection and localization of DSB.

A new method for the simultaneous detection of mammalian cells and ion tracks on a surface of CR-39

The geometric locations of ion traversals in mammalian cells constitute important information in the study of heavy ion-induced biological effects. We employed a contact microscopy technique, which was developed for boron imaging in boron neutron capture therapy to the irradiation mammalian cells by low-energy heavy ions. This method enables the simultaneous visualization of mammalian cells as a relief on a plastic track detector, CR-39, and the etch pits which indicate the positions of ion traversals. This technique provides visual geometric information about the cells and ion traversal, without any specially designed devices or microscopes. Only common laboratory equipment, such as a conventional optical microscope, a UV lamp, and commercially available CR-39 is required. To validate this method, CHO-K1 and HeLa cells were cultured on the CR-39 surface and then irradiated with low-energy Ar and Ne ions, respectively. The positions of induced DNA double strand breaks were detected as gamma-H2AX fluorescent spots, which coincided with the positions of the etch pits in the cell relief image.

Targeted Irradiation of Mammalian Cells Using a Heavy-Ion Microprobe

The existing focusing heavy-ion microprobe at the Gesellschaft für Schwerionenforschung in Darmstadt (Germany) has been modified to enable the targeted irradiation of single, selected cells with a defined number of ions. With this setup, ions in the range from helium to uranium with linear energy transfers (LETs) up to approximately 15,000 keV/microm can be positioned with a precision of a few micrometers in the nuclei of single cells that are growing in culture on a thin polypropylene film. To achieve this accuracy, the microbeam traverses a thin vacuum window with minimal scattering. Electron emission from that window is used for particle detection. The cells are kept in a specially designed dish that is mounted directly behind the vacuum window in a setup allowing the precise movement and the imaging of the sample with microscopic methods. The cells are located by an integrated software program that also controls the rapid deflection and switching of the beam. In this paper, the setup is described in detail together with the first experiments showing its performance. We describe the ability of the microprobe to reliably hit randomly positioned etched nuclear tracks in CR-39 with single ions as well as the ability to visualize the ion hits using immunofluorescence staining for 53BP1 as a marker of DNA damage in the targeted cell nuclei.

Live cell imaging of heavy-ion-induced radiation responses by beamline microscopy

To study the dynamics of protein recruitment to DNA lesions, ion beams can be used to generate extremely localized DNA damage within restricted regions of the nuclei. This inhomogeneous spatial distribution of lesions can be visualized indirectly and rapidly in the form of radiation-induced foci using immunocytochemical detection or GFP-tagged DNA repair proteins. To analyze faster protein translocations and a possible contribution of radiation-induced chromatin movement in DNA damage recognition in live cells, we developed a remote-controlled system to obtain high-resolution fluorescence images of living cells during ion irradiation with a frame rate of the order of seconds. Using scratch replication labeling, only minor chromatin movement at sites of ion traversal was observed within the first few minutes of impact. Furthermore, time-lapse images of the GFP-coupled DNA repair protein aprataxin revealed accumulations within seconds at sites of ion hits, indicating a very fast recruitment to damaged sites. Repositioning of the irradiated cells after fixation allowed the comparison of live cell observation with immunocytochemical staining and retrospective etching of ion tracks. These results demonstrate that heavy-ion radiation-induced changes in subnuclear structures can be used to determine the kinetics of early protein recruitment in living cells and that the changes are not dependent on large-scale chromatin movement at short times postirradiation.

Accumulation of the cell cycle regulators TP53 and CDKN1A (p21) in human fibroblasts after exposure to low- and high-LET radiation

The accumulation of the cell cycle regulators TP53 and CDKN1A (p21/CIP1/WAF1) was investigated after exposure to X rays and carbon ions (170 keV microm(-1)) and xenon, bismuth and uranium ions (8900-15,000 keV microm(-1)) in normal human fibroblasts. The influence of the overall dose and the LET of these radiation types was studied systematically and the kinetics of the cell response was followed up to 24 h after exposure. The accumulation of TP53 protein was dependent on the dose and the LET, and TP53 levels declined to lower levels for all radiation types within 24 h after exposure. CDKN1A levels increased and peaked at 3 to 6 h after exposure. The persisting level of this protein at 24 h was strongly dependent on the dose and the LET for X rays and carbon ions. The exposure to very high-LET ions (8900-15,000 keV microm(-1)) did not lead to a further increase in CDKN1A, suggesting a saturation effect for the induction of this protein. The cellular effects of elevated CDKN1A after particle irradiation are discussed.

The Physical and Radiobiological Basis of the Local Effect Model:A Response to the Commentary by R. Katz

The physical and biological basis of our model to calculate the biological effects of charged particles, termed the local effect model (LEM), has recently been questioned in a commentary by R. Katz. Major objections were related to the definition of the target size and the use of the term cross section. Here we show that the objections raised against our approach are unjustified and are largely based on serious misunderstandings of the conceptual basis of the local effect model. Furthermore, we show that the approach developed by Katz and coworkers itself suffers from exactly those deficiencies for which Katz criticizes our model. The essential conceptual differences between the two models are discussed by means of some illustrative examples, based on a comparison with experimental data. For these examples, the predictions of the local effect model are fully consistent with the experimental data. In contrast, e.g. for very heavy ions, there are significant discrepancies observed for the Katz approach. These discrepancies can be attributed to the inadequate definition of the target size in this model. Experimental data are thus clearly in favor of the definition of the target as used in the local effect model. Agreement with experimental data is achieved for protons within the Katz approach but at the cost of questionable approximations in combination with the violation of the fundamental physical principle of energy conservation.

Biological imaging of heavy charged-particle tracks

The immunocytochemical response to DNA damage induced by low-energy bismuth and carbon ions was investigated in normal human fibroblasts. Inside the nuclei, the traversing charged particles lead to the accumulation of proteins related to DNA lesions and repair along the ion trajectories. Irradiation under a standard geometric setup with the beam direction perpendicular to the cell monolayer generates spots of these proteins as described previously for MRE11B (hMre11), CDKN1A (p21) and PCNA (Jakob et al., Int. J. Radiat. Biol. 78, 75-88, 2002). Here we present data obtained with a new irradiation geometry characterized by a small angle between the beam direction and the monolayer of cells. This new irradiation geometry leads to the formation of protein aggregates in the shape of streaks stretching over several micrometers in the x/y plane, thus facilitating the analysis of the fluorescence distributions along the particle trajectories. Measurements of fluorescence intensity along the ion tracks in double- and triple-stained samples revealed a strict spatial correlation for the occurrence of CDKN1A and MRE11B clusters. In addition, immunostained gamma-H2AX is used as a marker of double-strand breaks (DSBs) to visualize the localized induction of these lesions along the particle paths. A clear coincidence of CDKN1A and gamma-H2AX signals within the ion-induced streaks is observed. Also for PCNA, which mainly associates with lesions processed by excision repair, a strict colocalization with the MRE11B aggregations was found along the ion trajectories, despite the higher estimated yield of this type of lesions compared to DSBs. Strikingly similar patterns of protein clusters are generated not only for the various proteins studied but also using different ion species from carbon to bismuth, covering LET values ranging from about 300 to 13600 keV/microm and producing estimated DSB densities differing by a factor around 45. The patterns of protein clustering along the very heavy-ion trajectories appear far more heterogeneous than expected based on idealized DSB distributions arising from model calculations. The results suggest that additional factors like compaction or confined movement of chromatin are responsible for the observed clustering of proteins.

Low fluence

The question of the appropriate extrapolation to low dose has long been a subject of controversy. A linear no-threshold model is favored by regulatory bodies as the basis of RBE assignments and estimates of radiation hazards to the general population. This model is largely supported by extensive application of the linear-quadratic survival formula "fitted" statistically to a wide variety of experimental data obtained at doses typically exceeding 1 Gy, and then extrapolated to mGy for practical applications, and even to the prediction of hazards from single electrons. Such extrapolations are questionable at best, and may even prove hazardous for risk evaluations. Fluence and geometry rather than dose based data are proposed as a basis for a limiting "threshold" for a "low dose" extrapolation. The proposed threshold is one where the fluence of particles is one per square micron, where on average only 2/3 of the 1 micrometers2 pixels covering an irradiated area are traversed by one or more particles. The corresponding dose threshold is determined by the LET of the bombarding radiation. For relativistic electrons this dose is about 0.032 Gy.

Detection of DNA damage in individual cells induced by heavy-ion irradiation with an non-denaturing comet assay

Investigating the biological effects of high-LET heavy-ion irradiation at low fluence is important to evaluate the risk of radiation in space. It is especially necessary to detect radiation damage induced by a precise number of heavy ions in individual cells. We thus compared the number of ions traversing a cell and the DNA damage produced by ion hits. We applied a comet assay to measure the DNA damage in individual cells. Cells attached on ion-track detector (CR-39) were irradiated with 17.3 MeV/u 12C, 15.7 MeV/u, 10.4 MeV/u 20Ne ion and 7.2 MeV/u 40Ar beams at TIARA, JAERI-Takasaki. After irradiation, CR-39 was covered with 1% agarose. The agarose was allowed to solidify on a glass slide, and then the electrophoresis was performed. Afterward, the CR-39 was taken off the glass slide. The agarose gel on the CR-39 was stained with ethidium bromide and the opposite side of the CR-39 was etched with a KOH-ethanol solution at 37 degrees C. We observed that heavy ions with higher LET values induced heavier DNA damage, even with the same number of ion hits within the irradiated cells. The result indicated that the amount of DNA damage induced by one particle depended on the LET value of the heavy ions.