ESTRO 2023 - Abstract Book

S1521

Digital Posters

ESTRO 2023

The treatment was adapted and delivered while acquiring MR image with the scintillator in place. This same treatment was delivered and measured with a PTW semiflex 3D ionization chamber instead of the scintillator and without MR imaging for comparison. Results Table 1 shows calculated and measured dose with the scintillator and the semiflex. The measured dose with the scintillator was a 0.51% higher than the calculated, while the measured with the semiflex resulted 1% higher.

Table 1 also shows the differences in percentage between the calculated and the measured dose with the semiflex with respect to measured with the scintillator. These differences are compensated at the end of the treatment. Figure 3 shows how the differences follow almost the same trend with angle. Conclusion These results demonstrate that it is possible to measure the dose to a point with the Blue Physics Model 10 scintillation detector while acquiring MR images quite accurately on an Elekta Unity MR-Linac. This could solve the impossibility of performing a dosimetric QA for each adapted plan, as this detector allows to measure the delivered dose in real time. Moreover, due to its small and its magnetic field independent response, this detector seems very interesting for in vivo dosimetry and could be used in several applications.

Poster (Digital): Dose calculation

PO-1797 Dynamic backlash of a multileaf collimator

A. Kulmala 1 , J. Tarvonen 2 , T. Paasonen 2 , A. Rintala 2 , L. Koivula 2 , M. Tenhunen 2

1 Clinical Research Institute HUCH Ltd., Radiotherapy, Helsinki, Finland; 2 Comprehensive Cancer Center, Radiotherapy, Helsinki, Finland Purpose or Objective With increasing complexity and detailing of radiotherapy treatments, mechanical imperfections, in part caused by multileaf collimator (MLC), are becoming significant for treatment delivery validity. A built-in feature of MLC robotics is backlash, which causes a drive position, defined by a control point, and an actual position of a collimator to differ. Advancing the determination of the backlash may provide more knowledge of the actual leaf positions during treatment delivery and hence enable enhancing of treatment modelling. To determine the backlash of static collimator leafs as in 'step & shoot' delivery, a robust measurement method has been introduced [1]. The method has also been used to study a variety of MLCs [2]. However, since most of the high-end radiotherapy treatments have a dynamic collimation as in 'sliding window' delivery, there is yet a need to further characterize the backlash. In this work we present a vendor independent method for determining a backlash of a dynamic multileaf collimation. In addition, the method is applied to two technically different commercial MLCs and to a variety of collimator leaf speeds. Materials and Methods In this study the backlash is described as a difference in a radiation fluence width when a collimator leaf is pushed in to a radiation beam and when the collimator leaf is pulled out from the beam. Dynamic test fields were designed to highlight the width difference.

The method was used to define backlash for three 'Varian Standard 120M'/TrueBeam (Varian Medical Systems Inc., Palo