ESTRO 2021 Abstract Book

S739

ESTRO 2021

17 Trans Tasman Radiation Oncology Group (TROG) / Calvary Mater Newcastle, Radiation Oncology, Newcastle, Australia

Purpose or Objective To promote consistency in clinical trials by recommending a uniform framework as it relates to radiation transport and dose calculation in water versus in medium. The current status of clinical practice is very inconsistent on this topic, for example: doses have been historically reported as a mixture of D w,w and D m,m ; different algorithms compute doses differently and often with little transparency; the introduction of D w,m has added further complication as it agrees with neither D w,w or D m,m . While the differences between these approaches are typically small, in order to maximize the consistency and interpretability of clinical trial data, it is important that doses be reported in as uniform a manner as possible. This work represents a consensus report on this topic. Materials and Methods The Global Quality Assurance of Radiation Therapy Clinical Trials Harmonisation Group (GHG; www.rtqaharmonization.org ) is comprised of 5 clinical trial quality assurance groups (IROC, EORTC, JCOG, RTTQA, TROG) and 4 observer groups with an interest in radiotherapy QA (IAEA, RDS, ACDS, NPL). The GHG compared the numerical and conceptual differences between dose to water in water ( D w,w ), dose to water in medium ( D w,m ), and dose to medium in medium ( D m,m ). This was done based on a review of historical frameworks, existing literature and standards, clinical issues in the context of clinical trials, and the trajectory of radiation dose calculations. Based on these factors, recommendations were developed with the goal of maximizing long term consistency in dose reporting for clinical trials. Results No framework was found to be ideal or perfect given the history, complexity, and current status of radiation therapy. Nevertheless, based on the evidence available, the GHG established a recommendation preferring dose to medium in medium ( D m,m ) as the preferred framework for radiation transport and dose calculation. Conclusion Dose to medium in medium ( D m,m ) is the preferred dose calculation and reporting framework for clinical trials. If an institution’s planning system can only calculate dose to water in water ( D w,w ), this is acceptable. Vendors should continue to develop algorithms that inherently transport and calculate dose in tissue to allow for consistency in multi-institutional clinical trials as well as clinical practice. PD-0900 Improving the modelling of Elekta Agility MLC in RayStation M. Hussein 1 , A. Angerud 2 , J. Saez 3 , E. Bogaert 4 , M. Barry 1 , I. Silvestre Patallo 1 , D. Shipley 1 , C. Clark 5,6,7 , V. Hernandez 8 1 National Physical Laboratory, Metrology for Medical Physics Centre, Teddington, United Kingdom; 2 RaySearch Laboratories AB, Research Department, Stockholm, Sweden; 3 Hospital Clínic de Barcelona, Department of Radiation Oncology, Barcelona, Spain; 4 Ghent University Hospital, Department of Radiation Oncology, Ghent, Belgium; 5 National Physical Laboratory, Metrology for Medical Physics Centre, London, United Kingdom; 6 University College London Hospital, Medical Physics, London, United Kingdom; 7 University College London, Medical Physics and Bioengineering, London, United Kingdom; 8 Hospital Sant Joan de Reus, Department of Medical Physics, Tarragona, Spain Purpose or Objective Robust tuning of MLC TPS modelling parameters is crucial for creating an optimal beam model, particularly with the ever-increasing accuracy required for advancing techniques. Challenges can arise from balancing the trade-off between multiple parameters and therefore the quality of the parameter tuning will depend on the experience of the physicist. This is in part due to limitations of the MLC modelling within the TPS. As a result, actual MLC parameter values used have been shown to vary widely between centres [1]. A novel methodology for robust convergence to an optimised MLC model and standardising the procedure has previously been proposed based on measurements of a set of dynamic fields with a Farmer-type chamber to simplify this process [2]. This study investigates the impact of these improvements using measured clinically relevant plans. Materials and Methods This study focused on the RayStation modelling of the Elekta Agility MLC. Three MLC models were assessed: the existing model in v10B, two prototype models with discrete (P1) and continuous (P2) transmission assigned to the Tongue & Groove (TG) and leaf tip (LT) regions. A set of synchronous (SG) and asynchronous (aSG) sweeping gap fields [2] were measured using a Farmer chamber in a solid water phantom at 6MV. The aSG fields characterize increasing distances between adjacent leaves (S) [2]. In the v10B model, MLC parameters were iteratively fine-tuned using LT width and TG static fields. P1 and P2 algorithms were tuned against the SG fields. Comparison was made between the 3 tuned models and the measured aSG fields. Four VMAT plans representing a wide range of geometrical and planning complexity were calculated with the 3 models. These included two vertebra SABR cases (one of which was pushed to high MU), a head & neck and a prostate case. Dose calculations used the CCC algorithm and 2mm dose grid spacing. Measurements were performed using EBT3 film for the vertebra cases to investigate fine differences, and OctaviusII-729 for the prostate and H&N cases to mimic typical device use in the clinic. Analysis was made using γ index with settings: global γ with dose difference relative to prescription dose, 20% lower dose threshold, 2%/2mm criteria. γ passing rate as % of points with γ<1 and mean γ were recorded. Results Figure 1 shows the measured and calculated dose for the aSG fields from the three algorithms. For S > 15mm the differences in the algorithms diverge and P2 has the closest match to the measured data even at the highest interdigitation. Table 1 shows the 2%/2mm γ pass rates and mean gamma for the measured plans.