Dosimetry with gafchromic films based on a new micro-opto-electro-mechanical system.

This work presents the first tests performed with radiochromic films and a new Micro‒Opto‒Electro-Mechanical system (MOEMS) for in situ dosimetry evaluation in radiotherapy in real time. We present a new device and methodology that overcomes the traditional limitation of time-delay in radiochromic film analysis by turning a passive detector into an active sensor. The proposed system consists mainly of an optical sensor based on light emitting diodes and photodetectors controlled by both customized electronic circuit and graphical user interface, which enables optical measurements directly. We show the first trials performed in a low‒energy proton cyclotron with this MOEMS by using gafchromic EBT3 films. Results show the feasibility of using this system for in situ dose evaluations. Further adaptation is ongoing to develop a full real‒time active detector by integrating MOEM multi‒arrays and films in flexible printed circuits. Hence, we point to improve the clinical application of radiochromic films with the aim to optimize radiotherapy treatment verifications.

Experimental validation of an analytical microdosimetric model based on Geant4-DNA simulations by using a silicon-based microdosimeter.

PURPOSE: To study the agreement between proton microdosimetric distributions measured with a silicon-based cylindrical microdosimeter and a previously published analytical microdosimetric model based on Geant4-DNA in-water Monte Carlo simulations for low energy proton beams. METHODS AND MATERIAL: Distributions for lineal energy (y) are measured for four proton monoenergetic beams with nominal energies from 2.0 MeV to 4.5 MeV, with a tissue equivalent proportional counter (TEPC) and a silicon-based microdosimeter. The actual energy for protons traversing the silicon-based microdosimeter is simulated with SRIM. Monoenergetic beams with these energies are simulated with Geant4-DNA code by simulating a water cylinder site of dimensions equal to those of the microdosimeter. The microdosimeter response is calibrated by using the distribution peaks obtained from the TEPC. Analytical calculations for y ¯ F and y ¯ D using our methodology based on spherical sites are also performed choosing the equivalent sphere to be checked against experimental results. RESULTS: Distributions for y at silicon are converted into tissue equivalent and compared to the Geant4-DNA simulated, yielding maximum deviations of 1.03% for y ¯ F and 1.17% for y ¯ D . Our analytical method generates maximum deviations of 1.29% and 3.33%, respectively, with respect to experimental results. CONCLUSION: Simulations in Geant4-DNA with ideal cylindrical sites in liquid water produce similar results to the measurements in an actual silicon-based cylindrical microdosimeter properly calibrated. The found agreement suggests the possibility to experimentally verify the calculated clinical y ¯ D with our analytical method.

Impact of charge collection efficiency and electronic noise on the performance of solid-state 3D microdetectors.

Microdosimetry has been traditionally performed through gaseous proportional counters, although in recent years different solid-state microdosimeters have been proposed and constructed for this task. In this paper, we analyze the response of solid-state devices of micrometric size with no intrinsic gain developed by CNM-CSIC (Spain). There are two major aspects of the operation of these devices that affect the reconstruction of the probability distributions and momenta of stochastic quantities related to microdosimetry. For micrometric volumes, the drift and diffusion of the charge carriers gives rise to a partial charge collection efficiency in the peripheral region of the depleted volume. This effect produces a perturbation of the reconstructed pulse height (i.e. imparted energy) distributions with respect to the actual microdosimetric distributions. The relevance of this deviation depends on the size, geometry and operating conditions of the device. On the other hand, the electronic noise from the single-event readout set-up poses a limit on the minimum detectable lineal energy when the microdosimeter size is reduced. This article addresses these issues to provide a framework on the physical constraints for the design and operation of solid-state microdosimeters.

Segment-averaged LET concept and analytical calculation from microdosimetric quantities in proton radiation therapy.

PURPOSE: This work introduces the concept of segment-averaged linear energy transfer (LET) as a new approach to average distributions of LET of proton beams based on a revisiting of microdosimetry theory. The concept of segment-averaged LET is then used to generate an analytical model from Monte Carlo simulations data to perform fast and accurate calculations of LET distributions for proton beams. METHODS AND MATERIAL: The distribution of energy imparted by a proton beam into a representative biological structure or site is influenced by the distributions of (a) LET, (b) segment length, which is the section of the proton track in the site, and (c) energy straggling of the proton beam. The distribution of LET is thus generated by the LET of each component of the beam in the site. However, the situation when the LET of each single proton varies appreciably along its path in the site is not defined. Therefore, a new distribution can be obtained if the particle track segment is decomposed into smaller portions in which LET is roughly constant. We have called "segment distribution" of LET the one generated by the contribution of each portion. The average of that distribution is called segment-averaged LET. This quantity is obtained in the microdosimetry theory from the average and standard deviation of the distributions of energy imparted to the site, segment length, and energy imparted per collision. All this information is calculated for protons of clinically relevant energies by means of Geant4-DNA microdosimetric simulations. Finally, a set of analytical functions is proposed for each one of the previous quantities. The presented model functions are fitted to data from Geant4-DNA simulations for monoenergetic beams from 100 keV to 100 MeV and for spherical sites of 1, 5, and 10 µm in diameter. RESULTS: The average differences along the considered energy range between calculations based on our analytical models and MC for segment-averaged dose-averaged restricted LET are -0.2 ± 0.7 keV/µm for the 1 µm case, 0.0 ± 0.9 keV/µm for the 5 µm case, and -0.3 ± 1.1 keV/µm for the 10 µm case, respectively. All average differences are below the average standard deviation (1σ) of the MC calculations. CONCLUSIONS: A new way of averaging LET for a proton beam is performed to incorporate the effects produced by the variation of stopping power of each individual proton along microscopic biological structures. An analytical model based on MC simulations allows for fast and accurate calculations of segment-averaged dose-averaged restricted LET for proton beams, which otherwise would need to be calculated from exhaustive MC simulations of clinical plans.

Dose-averaged LET calculation for proton track segments using microdosimetric Monte Carlo simulations.

PURPOSE: There is an increasing interest in calculating linear energy transfer (LET) distributions for proton therapy treatments in order to assess the influence of this quantity in biological terms. Microdosimetric Monte Carlo (MC) simulations are useful tools to calculate dose-averaged LET, as this has been broadly proposed as the most adequate quantity to characterize these biological effects. However, a straightforward uniform sampling of the scoring site turns out to be computationally unaffordable. In contrast, some issues have been pointed out with the more efficient weighted sampling approach, frequently used in literature. Here, we address the issues associated with the latter method and propose adequate corrections to achieve reliable calculations of dose-averaged LET values from microdosimetry. METHODS AND MATERIALS: Proton track structures have been simulated with Geant4-DNA considering two different approaches. One version employs a uniform sampling for placing the spherical site and is used as the reference. The other one uses a weighted sampling by considering the spatial distribution of transfer points. Some corrections are proposed for calculating a dose-averaged LET comparable to the reference case. An additional MC approach is proposed to obtain the weighted mean of the energy imparted per electronic collision of the proton within the site, the δ 2 function, related to the straggling distribution, as an intermediate step in the LET calculation. RESULTS: Energy imparted per event distributions are different when employing either sampling methods, due to the different geometrical randomness. We have found an agreement below (0.15 ± 0.05) keV/µm in the worst case for uniform and weighted methods in dose-averaged LET values when the weighted sampling results are corrected according to our proposal. Our analysis is restricted to spherical sites of 1 and 10 µm diameter and monoenergetic beams in the range from 2 to 90 MeV. CONCLUSIONS: This work shows a reliable and computational-efficient method to perform calculations of track segment dose-averaged LET using MC simulations for proton therapy beams, including the necessary considerations for obtaining the straggling distribution characteristics. The validity of this approach remains as long as the stopping power of the proton can be considered as constant along its track within the site.