A Monte Carlo study of the variation of electron fluence in water from a 6 MV photon beam outside of the field

Linear energy transfer (LET) distribution outside small radiotherapy field edges produced by 6 MV X-rays

In modern radiotherapy with photons, the absorbed dose outside the radiation field is generally investigated. But it is well known that the biological damage depends not only on the absorbed dose but also on LET. This work investigated the dose-average LET (LΔ,D) outside several small radiotherapy fields to provide information that can help for better evaluating the biological effect in organs at risk close to the tumour volume. The electron fluences produced in liquid water by a 6 MV X-rays Varian iX linac were calculated using the EGSnrc Monte Carlo code. With the electron spectra, LΔ,D calculations were made for eight open small square fields and the reference field at water depths of 0.15 cm, 1.35 cm, 9.85 cm and 19.85 cm and several off-axis distances. The variation of LΔ,D from the centre of the beam to 2 cm outside the field's edge depends on the field size and water depth. Using radiobiological data reported in the literature for chromosomal aberrations as an endpoint for the induction of dicentrics determined in Human Lymphocytes, we estimated the maximum low-dose relative biological effectiveness, (RBEM) finding an increase of up to 100% from the centre of the beam to 2 cm from the field's edge.

Cellular Damage in the Target and Out-Of-Field Peripheral Organs during VMAT SBRT Prostate Radiotherapy: An In Vitro Phantom-Based Study

Simple Summary

New developments show that patients with prostate cancer can benefit from radiotherapy delivered with a hypo-fractionated regimen. The aim of our study was to investigate the effect of hypo-fractionated stereotactic body radiation therapy (SBRT) of prostate cancer on out-of-field organs. We used a humanoid phantom to irradiate prostate cells in conditions similar to patient therapy, using SBRT planning. Our results show that radiation doses in the location of the intestine and lung resulted in significantly higher radiation doses than the further locations. We observed a high radiotoxic effect in the cells irradiated in the prostate, and a small increase in DNA damage and cell killing in the intestine location. Gene expression analysis revealed significant enrichment of the biological processes related to the radiation response in the prostate. In the lung and thyroid, the enrichment of several gene groups was revealed, however the processes were not clearly related to the response to radiation. Our study provides extensive data on out-of-field safety of prostate SBRT.

Abstract

Hypo-fractionated stereotactic body radiation therapy (SBRT) is an effective treatment for prostate cancer (PCa). Although many studies have investigated the effects of SBRT on the prostate and adjacent organs, little is known about the effects further out-of-field. The aim of this study was to investigate, both in vitro and in a quasi-humanoid phantom, the biological effects (using a dose-scaling approach) of radiation in the out-of-field peripheral organs delivered by 6 MV volumetric modulated arc therapy (VMAT) SBRT in a prostate cancer model. Healthy prostate cells were irradiated in a phantom at locations corresponding to the prostate, intestine, lung, thyroid, and brain. Seven 10 Gy fractions of VMAT SBRT were delivered to the target in a single session without intermission (scaled-up method). Radiochromic films were used to measure the doses. The radiobiological response was assessed by measuring DNA breaks, the cell survival fraction, and differences in gene expression profile. Our results showed a strong, multiparametric radiobiological response of the cells in the prostate. Outside of the radiation field, the highest doses were observed in the intestine and lung. A small increase (not statistically significant) in DNA damage and cell death was observed in the intestines. Several gene groups (cell cycle, DNA replication) were depleted in the lung and thyroid (DNA replication, endocytosis), but further analysis revealed no changes in the relevant biological processes. This study provides extensive evidence of the types and extent of radiobiological responses during VMAT SBRT in a prostate cancer model. Additional research is needed to determine whether the radiobiological effects observed in the peripheral organs are validated in a clinical context.

The out-of-field dose in radiation therapy induces delayed tumorigenesis by senescence evasion

A rare but severe complication of curative-intent radiation therapy is the induction of second primary cancers. These cancers preferentially develop not inside the planning target volume (PTV) but around, over several centimeters, after a latency period of 1–40 years. We show here that normal human or mouse dermal fibroblasts submitted to the out-of-field dose scattering at the margin of a PTV receiving a mimicked patient’s treatment do not die but enter in a long-lived senescent state resulting from the accumulation of unrepaired DNA single-strand breaks, in the almost absence of double-strand breaks. Importantly, a few of these senescent cells systematically and spontaneously escape from the cell cycle arrest after a while to generate daughter cells harboring mutations and invasive capacities. These findings highlight single-strand break-induced senescence as the mechanism of second primary cancer initiation, with clinically relevant spatiotemporal specificities. Senescence being pharmacologically targetable, they open the avenue for second primary cancer prevention.

On calculation of the average linear energy transfer for radiobiological modelling

Applying the concept of linear energy transfer (LET) to modeling of biological effects of charged particles usually involves calculation of the average LET. To calculate this, the energy distribution of particles is characterized by either the source spectrum or fluence spectrum. Also, the average can be frequency-or dose-weighted. This makes four methods of calculating the average LET, each producing a different number. The purpose of this note is to describe which of these four methods is best suited for radiobiological modelling. We focused on data for photons (x-rays and gamma radiation) because in this case differences in the four averaging methods are most pronounced. However, our conclusions are equally applicable to photons and hadrons. We based our arguments on recently emerged Monte Carlo data that fully account for transport of electrons down to very low energies comparable to the ionization potential of water. We concluded that the frequency average LET calculated using the fluence spectrum has better predictive power than does that calculated using any of the other three options. This optimal method is not new but is different from those currently dominating research in this area.

A simple model for calculating relative biological effectiveness of X-rays and gamma radiation in cell survival

OBJECTIVES

The relative biological effectiveness (RBE) of X-rays and γ radiation increases substantially with decreasing beam energy. This trend affects the efficacy of medical applications of this type of radiation. This study was designed to develop a model based on a survey of experimental data that can reliably predict this trend.

METHODS

In our model, parameters α and β of a cell survival curve are simple functions of the frequency-average linear energy transfer (L_F) of delta electrons. The choice of these functions was guided by a microdosimetry-based model. We calculated L_F by using an innovative algorithm in which L_F is associated with only those electrons that reach a sensitive-to-radiation volume (SV) within the cell. We determined model parameters by fitting the model to 139 measured (α,β) pairs.

RESULTS

We tested nine versions of the model. The best agreement was achieved with [Formula: see text] and β being linear functions of [Formula: see text] .The estimated SV diameter was 0.1-1 µm. We also found that α, β, and the α/β ratio increased with increasing [Formula: see text] .

CONCLUSIONS

By combining an innovative method for calculating [Formula: see text] with a microdosimetric model, we developed a model that is consistent with extensive experimental data involving photon energies from 0.27 keV to 1.25 MeV.

ADVANCES IN KNOWLEDGE

We have developed a photon RBE model applicable to an energy range from ultra-soft X-rays to megaelectron volt γ radiation, including high-dose levels where the RBE cannot be calculated as the ratio of α values. In this model, the ionization density represented by [Formula: see text] determines the RBE for a given photon spectrum.

Average stopping powers for electron and photon sources for radiobiological modeling and microdosimetric applications

This study concerns calculation of the average electronic stopping power for photon and electron sources. It addresses two problems that have not yet been fully resolved. The first is defining the electron spectrum used for averaging in a way that is most suitable for radiobiological modeling. We define it as the spectrum of electrons entering the sensitive to radiation volume (SV) within the cell nucleus, at the moment they enter the SV. For this spectrum we derive a formula that combines linearly the fluence spectrum and the source spectrum. The latter is the distribution of initial energies of electrons produced by a source. Previous studies used either the fluence or source spectra, but not both, thereby neglecting a part of the complete spectrum. Our derived formula reduces to these two prior methods in the case of high and low energy sources, respectively. The second problem is extending electron spectra to low energies. Previous studies used an energy cut-off on the order of 1 keV. However, as we show, even for high energy sources, such as 60Co, electrons with energies below 1 keV contribute about 30% to the dose. In this study all the spectra were calculated with Geant4-DNA code and a cut-off energy of only 11 eV. We present formulas for calculating frequency- and dose-average stopping powers, numerical results for several important electron and photon sources, and tables with all the data needed to use our formulas for arbitrary electron and photon sources producing electrons with initial energies up to  ∼1 MeV.

Track-average LET of secondary electrons generated in LiF:Mg,Ti and liquid water by 20-300 kV x-ray, 137Cs and 60Co beams

Electrons generated in matter by photons could be a fundamental basis for an adequate analysis of radiation effects and damage. We have studied separately the 'primary electrons' generated directly by photons from the 'secondary electrons' (SE) produced by electron-electron interactions. In this work, track-average linear energy transfer, [Formula: see text], of SE in LiF:Mg,Ti and liquid water produced by twelve photon energy beams from 20 kV x-ray to ⁶⁰Co gamma rays have been investigated using the EGSnrc Monte Carlo Code. The exposure of LiF:Mg,Ti in different phantom materials has been considered. Depending on the photon energy, SE represent 40%-90% of the total electron fluence (TEF) between 1 keV and 10 keV, being higher when the photon energy increases. Independent of the medium, [Formula: see text] versus mean photon energy displays a local minimum at around 40 keV, followed by a local maximum at ~80 keV-100 keV. The [Formula: see text] of SE generated by the x-ray beams are of order of 11 keV µm^-1 to 19 keV µm^-1 in LiF:Mg,Ti and 5 keV µm^-1 to 9 keV µm^-1 in liquid water which represent 3-5 times those produced by ⁶⁰Co gamma rays in both media. These values were considerably greater than those of TEF, by factors of 3-8. Furthermore, [Formula: see text] of SE generated in liquid water by 20 kV-200 kV x-rays are similar to those of 76 MeV-120 MeV ³He ions. Contrary to the TEF, where [Formula: see text] were independent of the phantom material, at low photon energies [Formula: see text] of SE was found to be sensitive to the surrounding medium showing higher values within the phantom than in air. This result, which agrees with published experimental results, implies the importance of the SE ionization density for an understanding of dosimeter response induced by photon beams.

The relative biological effectiveness of out-of-field dose

PURPOSE

using simulations and models derived from existing literature, this work investigates relative biological effectiveness (RBE) for out-of-field radiation and attempts to quantify the relative magnitudes of different contributing phenomena (spectral, bystander, and low dose hypersensitivity effects). Specific attention is paid to external beam radiotherapy treatments for prostate cancer.

MATERIALS AND METHODS

using different biological models that account for spectral, bystander, and low dose hypersensitivity effects, the RBE was calculated for different points moving radially out from isocentre for a typical single arc VMAT prostate case. The RBE was found by taking the ratio of the equivalent dose with the physical dose. Equivalent doses were calculated by determining what physical dose would be necessary to produce the same overall biological effect as that predicted using the different biological models.

RESULTS

spectral effects changed the RBE out-of-field less than 2%, whereas response models incorporating low dose hypersensitivity and bystander effects resulted in a much more profound change of the RBE for out-of-field doses. The bystander effect had the largest RBE for points located just outside the edge of the primary radiation beam in the cranial caudal (z-direction) compared to low dose hypersensitivity and spectral effects. In the coplanar direction, bystander effect played the largest role in enhancing the RBE for points up to 8.75 cm from isocentre.

CONCLUSIONS

spectral, bystander, and low dose hypersensitivity effects can all increase the RBE for out-of-field radiation doses. In most cases, bystander effects seem to play the largest role followed by low dose hypersensitivity. Spectral effects were unlikely to be of any clinical significance. Bystander, low dose hypersensitivity, and spectral effect increased the RBE much more in the cranial caudal direction (z-direction) compared with the coplanar directions.

Influence of phantom materials on the energy dependence of LiF:Mg,Ti thermoluminescent dosimeters exposed to 20-300 kV narrow x-ray spectra, 137Cs and 60Co photons

LiF:Mg,Ti, are widely used to estimate absorbed-dose received by patients during diagnostic or medical treatment. Conveniently, measurements are usually made in plastic phantoms. However, experimental conditions vary from one group to another and consequently, a lack of consensus data exists for the energy dependence of thermoluminescent (TL) response. This work investigated the energy dependence of TLD-100 TL-response and the effect of irradiating the dosimeters in different phantom materials for a broad range of energy photons in an attempt to understand the parameters that affect the discrepancies reported by various research groups. TLD-100s were exposed to 20-300 kV narrow x-ray spectra, (137)Cs and (60)Co photons. Measurements were performed in air, PMMA, wt1, polystyrene and TLDS as surrounding material. Total air-kerma values delivered were between 50 and 150 mGy for x-rays and 50 mGy for (137)Cs and (60)Co beams; each dosimeter was irradiated individually. Relative response, R, defined as the TL-response per air-kerma and relative efficiency, RE, described as the TL-response per absorbed-dose (obtained through Monte Carlo (MC) and analytically) were used to describe the TL-response. Both R and RE are normalized to the responses in a (60)Co beam. The results indicate that the use of different phantom materials affects the TL-response and this response varies with energy and material type. MC simulations reproduced qualitatively the experimental data: a) R increases, reaches a maximum at ~25 keV and decreases; b) RE decreases, down to a minimum at ~60 keV, increases to a maximum at ~150 keV and after decreases. Independent of the phantom materials, RE strongly depends on how the absorbed dose is evaluated and the discrepancies between RE evaluated analytically and by MC simulation are around 4% and 18%, dependent on the photon energy. The comparison between our results and that reported in the literature suggests that the discrepancy observed between different research groups appears to be most likely related to supralinearity effect, phantom materials, difference on the energy-spectra and geometry conditions during each experiment rather than parameters such as heating-rate or annealing procedure, which was supported by MC simulation. From the results obtained in this work and the strict analysis performed, we can conclude that for clinical applications of TLD-100, special attention must be taken when published data are used to convert TL calibration curve from (60)Co to low-energy photons. Otherwise, this can lead to incorrect results when later used to measure absorbed dose in human tissue.

Cell survival responses after exposure to modulated radiation fields

In the present study survival responses were determined in cells with differing radiosensitivity, specifically primary fibroblast (AG0-1522B), human breast cancer (MDA-MB-231), human prostate cancer (DU-145) and human glioma (T98G) cells, after exposure to modulated radiation fields delivered by shielding 50% of the tissue culture flask. A significant decrease (P < 0.05) in cell survival was observed in the shielded area, outside the primary treatment field (out-of-field), that was lower than predicted when compared to uniform exposures fitted to the linear-quadratic model. Cellular radiosensitivity was demonstrated to be an important factor in the level of response for both the in- and out-of-field regions. These responses were shown to be dependent on secretion-mediated intercellular communication, because inhibition of cellular secreted factors between the in- and out-of-field regions abrogated the response. Out-of-field cell survival was shown to increase after pretreatment of cells with agents known to inhibit factors involved in mediating radiation-induced bystander signaling (aminoguanidine, DMSO or cPTIO). These data illustrate a significant decrease in survival out-of-field, dependent upon intercellular communication, in several cell lines with varying radiosensitivity after exposure to a modulated radiation field. This study provides further evidence for the importance of intercellular signaling in modulated exposures, where dose gradients are present, and may inform the refinement of established radiobiological models to facilitate the optimization of advanced radiotherapy treatment plans.

Out-of-field cell survival following exposure to intensity-modulated radiation fields

PURPOSE

To determine the in-field and out-of-field cell survival of cells irradiated with either primary field or scattered radiation in the presence and absence of intercellular communication.

METHODS AND MATERIALS

Cell survival was determined by clonogenic assay in human prostate cancer (DU145) and primary fibroblast (AGO1552) cells following exposure to different field configurations delivered using a 6-MV photon beam produced with a Varian linear accelerator.

RESULTS

Nonuniform dose distributions were delivered using a multileaf collimator (MLC) in which half of the cell population was shielded. Clonogenic survival in the shielded region was significantly lower than that predicted from the linear quadratic model. In both cell lines, the out-of-field responses appeared to saturate at 40%-50% survival at a scattered dose of 0.70 Gy in DU-145 cells and 0.24 Gy in AGO1522 cells. There was an approximately eightfold difference in the initial slopes of the out-of-field response compared with the α-component of the uniform field response. In contrast, cells in the exposed part of the field showed increased survival. These observations were abrogated by direct physical inhibition of cellular communication and by the addition of the inducible nitric oxide synthase inhibitor aminoguanidine known to inhibit intercellular bystander effects. Additional studies showed the proportion of cells irradiated and dose delivered to the shielded and exposed regions of the field to impact on response.

CONCLUSIONS

These data demonstrate out-of-field effects as important determinants of cell survival following exposure to modulated irradiation fields with cellular communication between differentially irradiated cell populations playing an important role. Validation of these observations in additional cell models may facilitate the refinement of existing radiobiological models and the observations considered important determinants of cell survival.

Relation between lineal energy distribution and relative biological effectiveness for photon beams according to the microdosimetric kinetic model

Our cell survival data showed the obvious dependence of RBE on photon energy: The RBE value for 200 kV X-rays was approximately 10% greater than those for mega-voltage photon beams. In radiation therapy using mega-voltage photon beams, the photon energy distribution outside the field is different with that in the radiation field because of a large number of low energy scattering photons. Hence, the RBE values outside the field become greater. To evaluate the increase in RBE, the method of deriving the RBE using the Microdosimetric Kinetic model (MK model) was proposed in this study. The MK model has two kinds of the parameters, tissue-specific parameters and the dose-mean lineal energy derived from the lineal energy distributions measured with a Tissue-Equivalent Proportional Counter (TEPC). The lineal energy distributions with the same geometries of the cell irradiations for 200 kV X-rays, (60)Co γ-rays, and 6 MV X-rays were obtained with the TEPC and Monte Carlo code GEANT4. The measured lineal energy distribution for 200 kV X-rays was quite different from those for mega-voltage photon beams. The dose-mean lineal energy of 200 kV X-rays showed the greatest value, 4.51 keV/µm, comparing with 2.34 and 2.36 keV/µm for (60)Co γ-rays and 6 MV X-rays, respectively. By using the results of the TEPC and cell irradiations, the tissue-specific parameters in the MK model were determined. As a result, the RBE of the photon beams (y(D): 2~5 keV/µm) in arbitrary conditions can be derived by the measurements only or the calculations only of the dose-mean lineal energy.

Relative biological damage and electron fluence in and out of a 6 MV photon field

Scattered radiation in the penumbra of a megavoltage radiation therapy beam can deposit a non-negligible dose in the healthy tissue around a target volume. The lower energy of the radiation in this region suggests that its biological effectiveness might not be the same as that of the open beam. In this work, we determined the relative biological damage in normal human fibroblasts after megavoltage irradiation in two geometries. The first was an open-beam irradiation and the second was a blocked configuration in which only scattered radiation could reach the target cells. The biological damage was evaluated by the gamma-H2AX immunofluorescence assay, which is capable of detecting DNA double-strand breaks in individual cells. We report that the scattered radiation is more effective at producing biological damage than the open beam radiation. We found a 27% enhancement in the net mean nuclear gamma-H2AX fluorescence intensity at 2 Gy and a 48% enhancement at 4 Gy. These findings are of interest due to the increased doses of penumbral radiation close to target volumes both in dose escalation studies and in IMRT treatment deliveries where high dose gradients exist for the purpose of conformal avoidance of healthy tissues.