Comparison of Monte Carlo calculated electron slowing-down spectra generated by 60Co gamma-rays, electrons, protons and light ions

SEARCH FOR EXPERIMENTAL EVIDENCE OF DOSE-RATE AND WALL SCATTERING EFFECTS IN THE THERMOLUMINESCENCE RESPONSE OF LIF:MG,TI (TLD-100)

An experimental investigation into the possibility of dose-rate effects and wall scatter in the thermoluminescent response of LiF:Mg,Ti (TLD-100) was carried out. The investigation was motivated by theoretical simulations predicting the possible presence of dose-rate effects coupled with the lack of detailed experimental studies. The dose rate was varied by changing the source to sample distance, by the use of attenuators, sources of 137Cs of various activities, filtration and the construction of identical geometrical irradiators of Teflon and stainless steel. Four levels of dose in the linear dose response region were studied at 10-2 Gy, 1.5 × 10-2 Gy, 0.1 Gy and 0.5 Gy to avoid complications in interpretation due to supralinearity above 1 Gy. At the dose of 1.5 × 10-2 Gy, the dose rate was varied by five orders of magnitude from 4.9 × 10-3 Gy s-1 to 4.9 × 10-8 Gy s-1. At the other levels of dose, a one to two orders of magnitude in dose rate was achieved. Within the measurement uncertainty of 5-10%, no dose-rate effects were observed in any of the experimental measurements and no changes in the shape of the glow curve were observed. The maximum wall scatter effect (Teflon to stainless steel) was measured at ~8% within the experimental uncertainty and well below expectations. The results are encouraging with respect to the accurate and reproducible use of LiF:Mg,Ti under various experimental conditions of irradiation.

A DNA Repair-Based Model of Cell Survival with Important Clinical Consequences

This work provides a description of a new interaction, cross-section-based model for radiation-induced cellular inactivation, sublethal damage, DNA repair and cell survival, with the ability to more accurately elucidate different radiation-response phenomena. The principal goal of this work is to describe the damage-induction cross sections, as well as repair and survival, as Poisson processes with two main types of damage: mild damage that can be rapidly handled by the most basic repair processes; and more complex damage requiring longer repair times and the high-fidelity homologous recombination (HR) repair process to ensure accuracy and safety in the survival. This work is unique in its use of Poisson statistics to quantify the main repairable cell compartments that are exposed to simple and more complex sublethal hits, the cross section of which determines what is homologically and non-homologically repairable. The new method is applied to central radiation damage and survival data, such as in vitro cellular repair and survival with key DNA repair genes knocked out, low-dose hypersensitivity (LDHS), change in survival over the cell cycle, and variation with linear energy transfer (LET) for densely ionizing ions, all results supporting our basic assumptions. Among the results, it was shown that less than 1% of the simple DSBs are lethal at approximately 2 Gy and below for sparsely ionizing radiations, but their δ-electron track ends of between 1.5 and 0.5 keV can deliver 0.5 MGy to a few hundred nm3 volumes, mainly due to multiple scatter detours and multiple secondary electrons. They can cause dual double-strand breaks (DSBs) on the periphery of nucleosomes that are the most common multiply damaged sites, with an average of 1-2 δ-electron track ends per cell nucleus at 2 Gy. LDHS is most likely due to the normal lack of fast, efficient repair of sublethal damage below approximately 0.5 Gy, and requires largely intact key DNA repair genes to achieve significant repair recovery at higher doses. The new repair model describes this phenomenon quite accurately. Cells with key non-homologous end joining (NHEJ) genes knocked-out, lose LDHS but provoke HR repair, and cells with HR genes knocked out may lose some LDHS, but provoke NHEJ repair. The DNA duplication during the S phase results in a direct doubling as well of the total and sublethal hit cross sections. For the lowest LET carbon ions, NHEJ is reduced to where it is almost eliminated at maximum relative biological effectiveness (RBE), while HR is induced more than by X rays, due to complex damage and misrepair of DSBs produced by numerous δ electrons. The use of a lower LET such as electrons or photons during the final week of radiation treatment may potentially maximize complication-free cure. Optimally-designed weekly fractionation schedules are proposed to maximize the DNA repair potential in normal tissues. Additionally, the optimal therapeutic ion species, LET, apoptosis and permanent growth arrest/senescence window is identified with helium, lithium and boron ions and LETs at approximately 15-55 eV/nm, to maximize these quantities in the tumor and minimize them in the normal tissues, resulting in a very high probability of complication-free cure.

Relative biological effectiveness of simulated solar particle event proton radiation to induce acute hematological change in the porcine model

The present study was undertaken to determine relative biological effectiveness (RBE) values for simulated solar particle event (SPE) radiation on peripheral blood cells using Yucatan minipigs and electron-simulated SPE as the reference radiation. The results demonstrated a generally downward trend in the RBE values with increasing doses of simulated SPE radiation for leukocytes in the irradiated animals. The fitted RBE values for white blood cells (WBCs), lymphocytes, neutrophils, monocytes and eosinophils were above 1.0 in all three radiation dose groups at all time-points evaluated, and the lower limits of the 95% confidence intervals were > 1.0 in the majority of the dose groups at different time-points, which together suggest that proton-simulated SPE radiation is more effective than electron-simulated SPE radiation in reducing the number of peripheral WBCs, lymphocytes, neutrophils, monocytes and eosinophils, especially at the low end of the 5-10 Gy dose range evaluated. Other than the RBE values, the responses of leukocytes to electron-simulated SPE radiation and proton-simulated SPE radiation exposure are highly similar with respect to the time-course, the most radiosensitive cell type (the lymphocytes), and the shape of the dose-response curves, which is generally log-linear. These findings provide additional evidence that electron-simulated SPE radiation is an appropriate reference radiation for determination of RBE values for the simulated SPE radiations, and the RBE estimations using electron-simulated SPE radiation as the reference radiation are not complicated by other characteristics of the leukocyte response to radiation exposure.

Accurate description of the cell survival and biological effect at low and high doses and LET's

To accurately describe the radiation response over a wide dose and ionization density range Binomial and Poisson statistics have been combined with the recently developed potentially Repairable-Conditionally-Repairable (RCR) damage response model and the combination is shown to have several advantages for the accurate description of the cell survival at both low and very high doses and LET's, especially when compared with the classical Linear and Linear Quadratic cell survival models. Interestingly, the potentially and conditionally repairable damage types of the RCR model may also be linked to the two major radiation damage repair pathways of eukaryotic cells namely Non Homologous End Joining (NHEJ) and Homologous Recombination (HR) respectively. In addition it describes the damage interaction of low and high LET damage in different dose fractions more accurately than any other model (cf. (6) and Fig. 7d). This is of considerable importance when describing the response of tumors and normal tissues during pencil beam scanning with light ion beams where low and high LET dose fractions from the plateau and Bragg peak can interact synergistically when being delivered quasi simultaneously. In conclusion, considering the unique biological properties of light ion beams such as their increased effect on hypoxic tumors, their microdosimetric energy deposition heterogeneity and their pencil beam energy deposition kernels the largest clinical advantages are obtained with medium LET beams (≍ 20-50 eV/nm). This applies even for radiation resistant tumors, at least when the goal is to maximize tumor cure with minimal adverse reactions in normal tissues.

Using electron beam radiation to simulate the dose distribution for whole body solar particle event proton exposure

As a part of the near solar system exploration program, astronauts may receive significant total body proton radiation exposures during a solar particle event (SPE). In the Center for Acute Radiation Research (CARR), symptoms of the acute radiation sickness syndrome induced by conventional radiation are being compared to those induced by SPE-like proton radiation, to determine the relative biological effectiveness (RBE) of SPE protons. In an SPE, the astronaut's whole body will be exposed to radiation consisting mainly of protons with energies below 50 MeV. In addition to providing for a potentially higher RBE than conventional radiation, the energy distribution for an SPE will produce a relatively inhomogeneous total body dose distribution, with a significantly higher dose delivered to the skin and subcutaneous tissues than to the internal organs. These factors make it difficult to use a (60)Co standard for RBE comparisons in our experiments. Here, the novel concept of using megavoltage electron beam radiation to more accurately reproduce both the total dose and the dose distribution of SPE protons and make meaningful RBE comparisons between protons and conventional radiation is described. In these studies, Monte Carlo simulation was used to determine the dose distribution of electron beam radiation in small mammals such as mice and ferrets as well as large mammals such as pigs. These studies will help to better define the topography of the time-dose-fractionation versus biological response landscape for astronaut exposure to an SPE.

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

Existing studies have suggested some debate on whether the quality of radiation that delivers dose outside of the primary field of a radiotherapy photon beam can be considered the same as that inside the primary field. We used a Monte Carlo approach to simulate the electron fluence differential in energy inside a water phantom in response to irradiation by a 6 MV photon beam. The goal was to quantify how significantly the electron fluence changes when moving from a volume exposed to the primary field to one outside of the primary field, and understand any potential biological implications. We scored the electron fluence outwards in annular volumes in response to a 5 cm radius 6 MV beam and at the central axis in response to a rectangular 6 MV beam partially blocked by an MLC. The resulting fluence spectra were compared to different low-LET sources for which biological response in the form of chromosomal aberrations has been published. Our results show a significant increase in the low energy component of the fluence spectra outside of the primary field, which increases the mean LET to values similar to that seen in response to a 137Cs photon source. In turn, it is shown that this has the potential to increase the RBE.

Alteration in the expression of signaling parameters following carbon ion irradiation

Ionizing radiation induces DNA damage, which generates a complex array of genotoxic responses. These responses depend on the type of DNA damage, which in turn can lead to unique cellular responses. High LET radiation results in clustered damages. This evokes specific signaling responses, which can be cytotoxic or cytoprotective in nature. In the present study the effect of carbon ion irradiation on p 44/42 MAPK and NF-kappaB, which are essentially survival factors, have been studied. Moreover, the effect of inhibition of DNA-PK, which is an important component of DNA repair mechanism, with wortmanin on these signaling factors has been studied. The expression of p 44/42 MAPK was different at 0.1 Gy and 1 Gy and wortmanin was found to inhibit its expression. NF-kappaB expression was higher at 1 Gy than at 0.1 Gy and its expression is unaffected by inhibition of DNA-PK. The notable findings of this study are that the responses to high and low dose of high LET radiation are essentially different and the 6 h time point post irradiation is crucial in deciding the response and needs further investigation.

The effect of model approximations on single-collision distributions of low-energy electrons in liquid water

The development of cross sections for the inelastic interaction of low-energy electrons with condensed tissue-like media is best accomplished within the framework of the dielectric theory. In this work we investigate the degree to which various model approximations, used in the above methodology, influence electron single-collision distributions. These distributions are of major importance to Monte Carlo track structure codes, namely, the energy-loss spectrum, the inelastic inverse mean free path, and the ionization efficiency. In particular, we make quantitative assessment of the influence of (1) the optical data set, (2) the dispersion algorithm, and (3) the perturbation and exchange Born corrections. It is shown that, although the shape and position of the energy-loss spectrum remains almost fixed, its peak height may vary by up to a factor of 1.5. Discrepancies in the calculated inelastic inverse mean free path are largely within 20-30% above 100 eV; they increase drastically, though, at lower energies. Exchange and perturbation Born corrections increase gradually below 1 keV leading to a approximately 30 to 40% reduction of the inverse mean free path at 100 eV. The perturbation effect contributes more than the exchange effect to this reduction. Similar to the dispersion situation, the effect of Born corrections at lower energies is also unclear since the models examined disagree strongly below 100 eV. In comparison, the vapor data are higher than the liquid calculations by 20 to 50% as the energy decreases from 1 to 0.1 keV, respectively. The excitation contribution is the main cause of this difference, since the ionization efficiency in the liquid levels off at approximately 90%, whereas the plateau value for the vapor is approximately 70%. It is concluded that electron inelastic distributions for liquid water, although in some respects distinctively different from the vapor phase, have associated uncertainties that are comparable in magnitude to the phase differences. The situation below 100 eV is uncertain.

Potential outcomes of modalities and techniques in radiotherapy for patients with hypopharyngeal carcinoma

BACKGROUND AND PURPOSE

To determine potential improvements in treatment outcome for patients with hypopharyngeal carcinoma, T4N0M0, using proton and intensity modulated photon radiotherapy (IMRT) compared to a standard 3D conformal radiotherapy treatment (3D-CRT) in terms of local tumour control probability, TCP, and normal tissue complication probability (NTCP) for the spinal cord and the parotid glands using.

PATIENTS AND METHODS

Using the three-dimensional treatment-planning system, Helax-TMS, 5 patients were planned with protons, IMRT, and 3D-CRT plans. The prescribed dose used was 30 fractions x 2.39 Gy for the protons and IMRT and 35 fractions x 2.00 Gy for 3D-CRT. The treatment plans were evaluated using dose volume data and dose response models were used to calculate TCP and NTCP. The target volumes were delineated to spare the parotid glands. A dose escalation was made for protons and IMRT using NTCP constraints to the spinal cord.

RESULTS

On average, protons and IMRT increase TCP by 17% compared to 3D-CRT. For the spinal cord NTCP values are zero for all methods and patients. Average NTCP values for the parotid glands were >90% for 3D-CRT and significantly lower for protons and IMRT varying from 43-65%. The average parotid gland dose was 33 Gy for the protons, 38 Gy for IMRT and 48 Gy for 3D-CRT.

CONCLUSIONS

Protons and IMRT gave a significant TCP increase compared to 3D-CRT while no significant difference between protons and IMRT was found. Protons generally show lower non-target tissue doses, which indicates a possibility for further dose escalation. Large individual dose differences between protons and IMRT for parotid glands indicate that some patients may benefit more from protons and others from IMRT.