ESTRO 2020 Abstract book

S931 ESTRO 2020

also be beneficial for proton therapy by reducing uncertainties for moving targets. Material and Methods Ten left-sided breast cancer patients and ten stage III NSCLC patients were prospectively included and treated with photon radiotherapy during mDIBH using the NHFT device. Heated and humidified airflow of 40 l/min was administered through a nasal interface with 80% oxygen. An optical surface scanning system (CatalystHD, C-RAD®) was used to monitor the voluntary breath holds (BH) with a 3 mm gating window and visual feedback to the patient using augmented reality goggles. Preceded by a training session, a planning CT scan and a treatment plan were performed in mDIBH. Treatment was on a Truebeam linac (Varian) with daily kV-kV imaging for breast patients and daily CBCT imaging in mDIBH for lung patients. Treatment plans consisted of 2 to 3 partial arcs or hybrid technique for lung patients, and 4 to 5 beams using a hybrid technique for breasts. CatalystHD is able to monitor and calculate isocenter shifts instantly during treatment using a non-rigid registration and volumetric deformation model, which we used to assess the intra-fraction motion during mDIBH. Results The first, middle and last NHFT treatment fractions of 3 lung and 3 breast cancer patients (76 BH in total) that completed the treatment with the NHFT device were analyzed. The mean BH length was 52s ± 32s, with a maximum of 135s. Regarding the BH stability, the fluctuation of the respiratory signal within one BH (i.e. standard deviation over one BH) was on average 0.4 mm (range 0.1-2.3 mm). In 92% of the BHs of the breast patients, the median isocenter shift (composite vector) during BH was within 5 mm (setup tolerance); and for the lung patients in 86% was within 8 mm (setup uncertainty). We observed a learning curve because patients get used to the device. In 69% of the fractions, patients were able to maintain the BH during kV or CBCT imaging. In 50% of the fractions, the whole treatment delivery could be given in ≤ two long BHs (Figure 1). The CBCT image quality for matching was judged to be qualitatively higher during BH compared to free breathing CBCT imaging (Figure 2). The patient reported excellent subjective tolerance of the NHFT device.

Conclusion From this first analysis, the use of NHFT in combination with surface scanning monitoring allows safely treating breast and lung cancer patients with prolonged voluntary and stable BH over multiple fractions of treatment. NHFT is therefore a feasible approach for supporting prolonged patient breath holds. PO-1615 Detection and analysis of scattered protons for verification of FLASH lung tumor proton therapy M. Garbacz 1 , R. Schulte 2 , V. Bashkirov 2 , M. Gao 3 , M. Pankuch 3 , C. Sarosiek 4 , R.P. Johnson 5 , J. Ramos Mendez 6 , A. Rucinski 1 , P. Olko 1 1 Institute of Nuclear Physics PAN, Proton Radiotherapy Group, Krakow, Poland ; 2 Loma Linda University, Department of Basic Sciences, Loma Linda, USA ; 3 Northwestern Medicine Chicago Proton Center, Medical