Towards the standardization of the absorbed dose report mode in high energy photon beams

Abstract

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.

Experimental validation of absorbed dose-to-medium calculation algorithms in heterogeneous media

“…The impact of mass density became predominant in non-electronic equilibrium conditions according to the Fano theorem (Fano 1954). This dependency was also stressed in previous studies (Delbaere et al 2019, Younes et al 2021) where differences up to 2.6% were found between the absorbed dose D m m calculated in the Gammex cortical bone and in the ICRP-23 (ICRP 1975) cortical bone. The same findings could be observed on the ] factors and could explain the difference between our film correction factor in the Gammex cortical bone (∼0.934) and the one calculated by Shaw et al (2021) in the CIRS (Norfolk, VA, USA) cortical bone (∼0.875).…”

“…The results of this study demonstrated the validity of the formalism introduced by Younes et al (2021). The application of the heterogeneity correction factor ] factors could be dependent on the physical data of the medium, namely the atomic composition, mean excitation value (I-value) and mass density.…”

“…Regardless of the quantity reported, the limitation of using such advanced algorithms lies in the lack of methodology for validating them experimentally. Medical physicists request further guidance in validating D m m as emphasized in the Scope 2023 of the United Kingdom (UK) Institute of Physics and Engineering in Medicine (IPEM) (Martin et al 2023).Our team has been working on this subject for several years (Delbaere et al 2019) and a formalism was proposed byYounes et al (2021) introducing a heterogeneity correction factor h Q Q…”

Impact of the dose quantity used in MV photon optimization on dose distribution, robustness, and complexity

“…D ref,m* is not related to any radiobiological consideration, nor to the “true” dose received by the voxels ( D m,m ) 13,25 or the target cells ( D w,m ) 9,25,26 . D ref,m* is useful to connect with the clinical data collected through previous algorithms and offers several advantages for planning and evaluation that will be shown in the sections that follow.…”

Guía para el control de calidad y seguridad de los sistemas de planificación y planes de tratamiento de radioterapia externa

“…Existen argumentos a favor y en contra de documentar la dosis usando cada una de estas dos magnitudes, [18][19][20][21][22][23][24][25] pero el uso de dosis de Dm,m cuenta cada vez con un mayor consenso. 26,27 También es importante tener en cuenta las inconsistencias entre estas magnitudes en medios heterogéneos y sus potenciales implicaciones clínicas. [28][29][30][31] Según la encuesta realizada por la SEFM en 2021, 32 los algoritmos más empleados en España son AXB (29.7%) y AAA (26.8%), debido al uso muy extendido del sistema de planificación Eclipse TM .…”