S3020
Invited Speaker
ESTRO 2026
Medicine, there is currently no standardized method available for the experimental validation of the algorithms reporting Dm,m. This represents a significant limitation for their routine clinical implementation. Measurements in non-water media remain particularly challenging, as detectors are calibrated in terms of dose to water. Moreover, no primary standards dosimetry laboratories currently offer calibration services for non-water media. To overcome this limitation, formalism based on heterogeneity correction factors has been introduced. These heterogeneity correction factors, derived from Monte Carlo simulations, account for both medium and detector-specific perturbation effects. Applying these heterogeneity correction factors to detector readings improves agreement with Dm,m calculations. Across various media (such as bone, lung, adipose tissue, and plastic water), the largest corrections were found in high-density media. The corrections factors were more dependent on the medium than on the detector type, with values reaching up to 9% in bone. Additionally, the correction factors show a clear dependence on the medium composition. An important limitation of the formalism should be noted: the validation assumes that the medium defined in the Treatment Planning System is identical to that used in experimental measurements. In clinical practice, this assumption is rarely satisfied. Therefore, special care is required when performing end-to-end tests, both due to potential mismatches between calculation and experimental media and due to the possible dependence of the size of the heterogeneity on the heterogeneity correction factors. References: Younes T, Chauvin M, Delbaere A, Labour J, Fonteny V, Simon L, Fares G, Vieillevigne L. Towards the standardization of the absorbed dose report mode in high energy photon beams. Phys Med Biol. 2021 Feb 2;66(4):045009. doi: 10.1088/1361-6560/abd22c. PMID: 33296874. Delbaere A, Younes T, Khamphan C, Vieillevigne L. Experimental validation of absorbed dose-to-medium calculation algorithms in heterogeneous media. Phys Med Biol. 2024 Feb 19;69(5). doi: 10.1088/1361- 6560/ad222e. PMID: 38266285. 5375 Let’s get basics right Edmond Sterpin Oncology, KU Leuven, Leuven, Belgium. MIRO lab, UCLouvain, Brussels, Belgium
dose calculation algorithms is not only about improving accuracy, but also represents a conceptual shift in how dose should be understood, prescribed, optimised, and reported in clinical practice. These differences introduce systematic variations in dose distributions that challenge established planning paradigms. We will focus on medium-related dose quantities (dose to medium-in-medium and dose to water-in-medium). On the one hand, prescriptions and constraints applied to volumes that include heterogeneous media may need to be reassessed. On the other hand, target objectives and plan acceptability criteria are also affected. A key issue concerns target coverage and homogeneity. Current PTV-based planning considerations were developed under implicit water- equivalence assumptions. However, this assumption may no longer hold when medium-dependent dose quantities are considered. Media with different dose- response characteristics can lead to significantly and systematically different dose distributions, depending on how dose is defined and reported. As a result, strict enforcement of PTV homogeneity and coverage criteria may not always be appropriate when observed deviations are driven by medium-dependent effects. Enforcing such criteria typically requires introducing local fluence adjustments during optimisation to compensate for medium-dependent dose differences. The often overlooked consequences include a shift away from established clinical practice based on relatively homogeneous fluence patterns, inherent impairment of plan robustness, and an increase in plan complexity. Potential strategies to address these challenges are presented for both plan optimisation and plan evaluation. For each proposed strategy, the potential consequences in terms of clinical outcome, dosimetric accuracy, plan robustness, and plan complexity are analysed. Overall, this work highlights that the adoption of advanced dose calculation algorithms requires not only technical adaptation, but also a redefinition of the conceptual framework underpinning radiotherapy planning and evaluation. 5374 Validation of dose to medium computations Laure VIEILLEVIGNE Medical Physics, IUCT Oncopole, Toulouse, France The American Association of Physicists in Medicine Task Group 329 and the Global Quality Assurance of Radiation Therapy Clinical Trials Harmonisation Group recommend the use of absorbed dose-to-medium Dm,m. However, as highlighted by a survey conducted by the Institute of Physics and Engineering in
The material for reporting absorbed dose has been a subject of debate for several decades, resulting in
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