ESTRO 2020 Abstract Book

S354 ESTRO 2020

theoretical Boag model of ion recombination. The experiment was repeated with increased polarizing voltages (-640 V and -960 V), to investigate any possible improvements of the chamber’s ion collection efficiency at ultra-high dose rates. Results The ion collection efficiency of the transmission chamber decreased with increasing DPP values. However, by increasing the voltage applied over the chamber from 320 V to 640 V and 960 V (negative polarity), the ion collection efficiency at the highest dose rate was increased by a factor 1.8 and 2.7, respectively. The logistic model accurately described the measured data, with R 2 -values of 0.995, 0.998 and 0.996 for -320 V, -640 V and -960 V, respectively (Figure 1a). In comparison, the Boag model was not as good at describing the drop in ion collection efficiency with increasing DPP, with corresponding R 2 - values of 0.989, 0.972 and 0.971 (Figure 1b).

safety and QC radiation measurement system based on a 2D radioluminescence (RL) technique for real time IVD. Our goal is to develop a system capable of accurately measuring the two dimensional (2D) real time dose rate received by the patient and account for the proper patient positioning relative to the linear accelerator (LINAC). In this work we present the feasibility study for the idea, presenting the dosimetric characterization of the system and some QA tests. Material and Methods ImageDosis is based on the RL signal from Al 2 O 3 :C,Mg micro-crystals mixed with a polymer binder, coated as a thin layer on a plastic substrate that can be put on top of a patient/phantom. The system consists of three main components: (1) the radiation detector, (micron-size Al 2 O 3 :C,Mg matrix); (2) the readout using a high-resolution scientific camera; and (3) proprietary image and signal processing software. The camera, together with the system's signal processing, measures the time-dependent luminescence and translate this signal to a high-resolution frame-by-frame sequence showing the evolution of the dose delivered to the patient/object. The external beam irradiations were performed using both an Elekta-Synergy and Varian LINACs, with photons (6, 10 and 15 MV) in FF and FFF mode with dose rates from 0.05 to 24 Gy/min. Results Fig 1 shows the real time RL coming from the Al 2 O 3 :C,Mg shhet irradiated with 6 MV and two IMRT geometries. The quantization of homogeneity and variation in the radiation beam as a function of time gives 0.1 mm 2 spatial resolution, linear dose rate range from 0.05 to 24 Gy/min (FF and FFF modes, fig 2), time resolution of 10 ms, good agreement (1%) with the reference ion chamber. The system provides the option to make the plot of intensity values across features in any image, from a point, a line, or area of interest from the image selected. QA test results, using different irradiation modalities, such as Alternating leaves, Picket Fence, Pyramid and detection of errors in phantom position showed good agreement with delivered geometries.

Conclusion In this study, the drop in ion collection efficiency with increasing DPP of a linear accelerator’s built-in transmission chamber has been measured and modelled. By increasing the polarizing voltage, the drop in ion collection efficiency could be reduced, improving the response of the chamber at FLASH dose rates by a factor 2.7 when compared to the standard operational mode. The suggested model allows correction for ion recombination, making the transmission chamber useful for online dosimetry at ultra-high dose rates. OC-0635 ImageDosis: 2D real-time dosimetry system L. De Freitas Nascimento 1 , D. Verellen 2 , J. Goossens 2 , F. Vanhavere 1 , M. Akselrod 3 1 Belgian Nuclear Research Centre - SCK-CEN, Radiation Protection Dosimetry and Calibration, Mol, Belgium ; 2 Iridium Kankernetwerk, Department of Radiotherapy, Antwerpen, Belgium ; 3 Landauer Inc., Stillwater Crystal Growth Division, Stillwater, USA Purpose or Objective Radiotherapy (RT) is a highly complex, multi-step process that requires the input of different experts in planning and delivering the treatment. It is important to consider in- vivo dosimetry (IVD) as part of RT safety and quality control (QC) measures; several public health committees, societies and associations recommend measuring the dose received by each patient routinely. We suggest a novel technique we called ImageDosis aimed to be a patient

Fig 1. RL emission during irradiations (field size 8 x 8 cm 2 , 600 MU/min) and two IMRT geometries

Fig 2. RL vs dose rate for 6, 10 and 15 MV. Conclusion

We demonstrate the feasibility of a system capable of real time in-vivo visual assessment of the radiation 2D map and absorbed dose rate, using a luminescent sheet. In this work we provide the prove of concept, showing linear response with dose rate, high spatial and time resolution and good signal to noise ratio in main beam and penumbra region