The method implemented in Monte Carlo (MC) algorithm to convert dose-to-medium (Dm) to dose-to-water (Dw) is usually based on the Bragg-Gray cavity theory. Acuros XB (AXB) reports also Dm and Dw but the method to calculate Dw is based on the energy deposition cross sections for water in place of those for the local media. For both algorithms, the calculation of Dw in non-water media is similar to the dose received in a small volume of water, small enough not to disturb the fluence of charged particles. Recently, two new methods revised the Bragg-Gray cavity theory, one proposed by Andreo and the other by Reynaert et al. In this context, comparisons between AXB and MC were carried out in terms of dose-to-medium () and dose-to-water (), respectively. Multilayer slab heterogeneous phantoms made of lung, bone and polytetrafluoroethylene (PTFE) were investigated and measurements were carried out using radiochromic films. These latter were then compared to and to Dw which would be obtained according to the conversion methods proposed by Andreo and Reynaert et al agreed with for all cases (±1%). In lung, all Dw calculations and film measurements were in agreement. By contrast, and differed notably in bone (4.5%) and PTFE (3.5%), and both algorithms overestimated film measurements. These findings demonstrate that the conversion method is different between AXB and MC. Furthermore, films were not able to give dose in a small volume of water according to the definition of and . Applying either the fluence correction factor suggested by Andreo or the mass energy absorption ratios proposed by Reynaert et al, resulted in a good agreement (<1%) with film measurements. According to the method used for the conversion, different Dw could be obtained which might lead to several issues in clinical context.

The benefits of using an algorithm that reports absorbed dose-to-medium have been jeopardized by the clinical experience and the experimental protocols that have mainly relied on absorbed dose-to-water. The aim of the present work was to investigate the physical aspects that govern the dosimetry in heterogeneous media using Monte Carlo method and to introduce a formalism for the experimental validation of absorbed dose-to-medium reporting algorithms. Particle fluence spectra computed within the sensitive volume of two simulated detectors (T31016 Pinpoint 3D ionization chamber and EBT3 radiochromic film) placed in different media (water, RW3, lung and bone) were compared to those in the undisturbed media for 6 MV photon beams. A heterogeneity correction factor that takes into account the difference between the detector perturbation in medium and under reference conditions as well as the stopping-power ratios was then derived for all media using cema calculations. Furthermore, the different conversion approaches and Eclipse treatment planning system algorithms were compared against the Monte Carlo absorbed dose reports. The detectors electron fluence perturbation in RW3 and lung media were close to that in water (≤1.5%). However, the perturbation was greater in bone (∼4%) and impacted the spectral shape. It was emphasized that detectors readings should be corrected by the heterogeneity correction factor that ranged from 0.932 in bone to 0.985 in lung. Significant discrepancies were observed between all the absorbed dose reports and conversions, especially in bone (exceeding 10%) and to a lesser extent in RW3. Given the ongoing advances in dose calculation algorithms, it is essential to standardize the absorbed dose report mode with absorbed dose-to-medium as a favoured choice. It was concluded that a retrospective conversion should be avoided and switching from absorbed dose-to-water to absorbed dose-to-medium reporting algorithm should be carried out by a direct comparison of both algorithms.

Objective. The aim of this study was to determine field output correction factors kfclin, fref Qclin,Qref and electron fluence perturbation for new PTW unshielded microSilicon and shielded microSilicon X silicon diode detectors. Approach. kfclin, fref Qclin,Qref factors were calculated for 6 and 10 MV with and without flattening filter beams delivered by a TrueBeam STx. Correction factors were determined for field sizes ranging from 0.5 × 0.5 cm2 to 3 × 3 cm2 using both experimental and numerical methods. To better understand the underlying physics of their response, total electron (+positron) fluence spectra were scored in the sensitive volume considering the various component-dependent perturbations. Main results. The microSilicon and microSilicon X detectors can be used down to the smallest studied field size by applying corrections factors fulfilling the tolerance of 5% recommended by the IAEA TRS483. Electron fluence perturbation in both microSilicon detectors was greater than that in water but to a lesser extent than their predecessors. The main contribution of the overall perturbation of the detectors comes from the materials surrounding their sensitive volume, especially the epoxy in the case of unshielded diodes and the shielding for shielded diodes. This work demonstrated that the decrease in the density of the epoxy for the microSilicon led to a decrease in the electron fluence perturbation. Significance. A real improvement was observed regarding the design of the microSilicon and microSilicon X detectors compared to their predecessors.

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