ESTRO 2021 Abstract Book

S1559

ESTRO 2021

The clinically obtained D x

differences are all within ±0.3% of the predicted values, except for D 2

to the trachea and

proximal bronchi (-1.2%), probably because of the presence of air, which is not included in the CF set 1 . Conclusion

We validated a direct method for predicting dosimetric differences between AXB Dm and AXB Dw in clinical practice. This would allow approximate mass conversion of Dm distributions to Dw using the DICOM-RT objects without recalculation, which can be helpful to better assess the predictive accuracy of Dw vs Dm for clinical outcomes. 1. Jurado-Bruggeman D, Muñoz-Montplet C, Vilanova JC. A new dose quantity for evaluation and optimisation of MV photon dose distributions when using advanced algorithms: Proof of concept and potential applications. Phys Med Biol . 2020;65(23). doi:10.1088/1361-6560/abb6bc

PO-1828 Comparison of two automated treatment planning solutions for prostate cancer D. Kopeć 1 , A. Zawadzka 2 , D. Bodzak 2 , P. Kukolowicz 2

1 Maria Skłodowska-Curie Memorial Cancer Center and Institute of Oncology,, Medical Physics Department, Warsaw, Poland; 2 Maria Skłodowska-Curie Memorial Cancer Center and Institute of Oncology, Medical Physics Department , Warszawa, Poland Purpose or Objective Automatic tools for treatment planning are currently being developed and implemented in many clinics. They allow to standardize and improve planning quality and are time efficient tools allowing to shift the workload from standard plans to new, more demanding areas of radiotherapy. Two different systems were evaluated: RapidPlan ( Varian Medical Systems, Palo Alto, USA) and multicriteria optimization MCO ( RaySearch Laboratories AB, Stockholm, Sweden) for prostate cancer. Materials and Methods The RapidPlan (RP) and MCO were used for automatic planning for randomly chosen patients with prostate cancer. 75 m anually prepared clinical VMAT plans with prescribed dose of 70,2 Gy deliverd in 27 fraction were used to created a model in RP. Subsequently for 25 new patients plans were created using both RP and MCO. Plans created with MCO were then used to create another RP model (MCO-RP). The last step was to use MCO-RP model to create plans for the same group of 25 patients. The dose distributions of these three automated planning methods were assessed against clinical plans. Doses delivered to CTVs, PTVs, rectum, bladder and femoral heads were compared. In comparison of plans the statistics used in our clinic were used. The statistics were based on QUANTEC.The Wilcoxon signed pairs rank test was used to test the statistical difference between the analyzed parameters. Results For all plans constraints for CTVs, PTVs and organs at risk (OAR) were met. First RP model allowed to achieve lower doses for all organs at risk than in clinical plans (mean doses for rectum, bladder and for left and right femoral heads were: 32,8 ± 6,4 vs 28,8 ± 4,6; 25,6 ± 11,9 vs. 23,5 ± 11,0; 13,2 ± 2,4 vs.11,4 ± 1,8 and 13,3 ± 2,6 vs.11,8 ± 1,6 respectively). MCO planning allowed for even lower doses for rectum and bladder but at the cost of higher doses in femoral heads (mean doses for left and right femoral heads were 15,5 ± 2,6 and 14,9 ± 2,5 respectively). In MCO-RP model doses for organs at risk were similar to MCO plans with exception of lower doses to femoral heads (mean doses for rectum, bladder and for left and right femoral heads were: 24,6 ± 5,5; 22,3 ±10,2; 13,7 ± 2,0 and 13,3 ± 2,0 respectively). MCO planning was more time consuming and dose distributions were less coherent than in both RP models. It also required significantly more experience in treatment planning than using RP models prepared beforehand. Conclusion The minimizing of some of the statistics used to evaluate the dose distributions increased the value of others, eg. doses to femoral heads but in general dose distributions of automated plans prepared with Eclipse (Rapid Plan method) and with RaySearch (Muliticriteria Optimisation) for prostate patients were at least as good or better than clinical plans. PO-1829 Development and optimization of a modulated arc technique for total body irradiation radiotherapy J. Krayenbuehl 1 , V. Gajdos 2 , C. Linsenmeier 1 , M. Guckenberger 1 , S. Tanadini-Lang 1 1 University Hospital Zurich, Radiation Oncology, Zurich, Switzerland; 2 Kantonsspital Aarau, Radiation Oncology, Aarau, Switzerland Purpose or Objective It was the aim of this project to develop a robust treatment technique using dynamic arc therapy for total body irradiation (TBI), which can be used in any standard treatment room without the need for dedicated patient translation devices. Materials and Methods The treatment consist of two arcs ranging between 285° and 75° consisting of 31 sub-arcs (5° per sub-arc). A field size of 40 × 40 cm2 was used with a 6MV photon beam on a conventional linac (TrueBeam, Varian). The distance from the linac isocenter to the couch was 100cm. During the first dynamic arc, patients are lying in supine position with head towards the right. For the second dynamic arc, patients are lying in prone position with head towards the right. The dose fractionation was 6 x 2Gy normalized to patient midline at the umbilicus plane. A homogenous dose distribution of 12Gy should be obtained. The weights of each sub-arcs were optimized in order to obtain a dose comprised between 95% and 105% in patient mid-line and a maximal dose < 110% at 1.5cm from the skin surface. The sub-arc weight optimization was performed with an algorithm minimizing objective function of weighted midline and surface dose differences along z-axis from the prescribed dose using Python 3 library SciPy and can be optimized for the patient dimensions. The weights of the arc segments were then entered into the planning system (Eclipse V16.0, Varian Medical System) and the dose distribution was calculated in Eclipse (Acuros V16.0). A sum plan was then created in order to add the dose delivered in the supine position to the dose delivered in the prone position. The dose homogeneity was calculated and verified in a solid water phantom (RW3, PTW-Freiburg) with thickness of 15cm, 20cm, 25cm and 30cm. The measurements were performed with diodes placed in the middle of the phantom and at the surface, defined at 1.5cm depth. The plan robustness in respect of the patient positioning was evaluated by moving the isocenter 3cm in-plane and 3cm cross-plane. The dose distribution was then compared to the original plan. Results The dose distribution calculated and measured for a solid water phantom of 20cm thickness in the cross-plane are displayed