ESTRO 2021 Abstract Book

S1286

ESTRO 2021

PO-1561 In-vivo treatment verification by TOF-PET for multiple proton fields: a Monte Carlo simulation study P. Rato Mendes 1 , P. Arce 1 , J.I. Lagares 1 , O. Vela 1 , J. Marín 1 , B. Aguilar 2 , L. Irazola 2 , J.D. Azcona 2 1 CIEMAT, Technology Department, Madrid, Spain; 2 Clínica Universidad de Navarra, Servicio de Radiofísica y Protección Radiológica, Madrid, Spain Purpose or Objective In-vivo treatment verification in charged particle radiotherapy by positron emission tomography (PET) is a well-established technique used to estimate particle range that can be used to assess inter-fraction movement. For a single irradiation field the implementation of this technique is quite straightforward, however in the case of multiple irradiation directions there are situations in which the determination of range is difficult due to the overlap of the distributions of positron-emitting nuclei generated by each irradiation field. To address this problem, we propose a novel approach based on analysis of PET images acquired immediately at the end of treatment delivery and present the results of its implementation by Monte Carlo simulations of real patient treatments using multiple proton fields. Materials and Methods We have used the Geant4-based GAMOS framework to perform Monte Carlo simulations of real patient head and neck treatments with 2 and 3 proton irradiation directions. The accelerator and the proton beams were simulated using the same energy and shape parameters used in planning, and delivered by beam scanning with a 360 ° rotating gantry. For each treatment the phase space of all generated positron-emitting nuclei was stored and later used to simulate the radioactive decays in the few minutes after irradiation and the detection of annihilation gammas by a time-of-flight PET detector. The detector consists of two face-to-face planar detectors with 20x20 cm 2 area, based on the HRFlexToT ASIC developed by our group which provides 200 ps time resolution. The position and face-to-face distance of the PET detectors was selected individually for each patient case. We have simulated the original treatments and several others with the patient artificially displaced up to 10 mm along different directions, in order to check the sensitivity of our method to identify such displacements. PET images were analysed by comparing regions with large variations in positron activity, such as air-tissue interfaces, as those regions allow a better assessment of patient positioning than homogeneous structures. Results Using our method we were able to correctly detect patient displacements on the order of 1-2 mm along different directions. The use of time-of-flight in PET image reconstruction played a determinant role in the achievement of these results. Conclusion We have performed a Monte Carlo simulation study of a dual planar head TOF-PET image-based method for treatment verification in multiple field proton therapy, capable of detecting patient displacements on the order of 1-2 mm. The proposed method may be used to assess inter-fraction movement in-vivo. PO-1562 Analysis of interfractional displacements for rectal cancer patients positioned on Belly Board L. Kharsun 1 , A. Chukhlantseva 1 , S. Bagova 1 , O. Kozlov 1 , E. Petukhov 2 , M. Chernykh 1 , V. Ivanov 1 1 Oncoradiologichesky Center "PET-Technology Podolsk", Radiation Oncology, Podolsk, Moscow Region, Russian Federation; 2 Oncoradiologichesky Center "PET-Technology Podolsk", Radiation Oncology, Podolsk, Moscow Region, Russian Federation Purpose or Objective To evaluate interfractional displacements that occur during radiotherapy by analyzing CBCT data to calculate the optimal PTV margin in case of locally advanced rectal cancer patients prone-positioned on the Belly Board device.

Materials and Methods Couch coordinates before and after CBCT based alignment of patient position from Offline Review were examined in order to determine PTV margin. Patients were divided into three groups with no regard to the stage of the disease. So two of them included ones prone-positioned on the Belly Board device, 20 had daily CBCT control and 36 had CBCT control twice a week, the third group included 30 supine-positioned patients on