ESTRO 38 Abstract book

S904 ESTRO 38

perspective of trainers and trainees in radiation oncology and input from both stakeholders is needed for development. The survey tool should be validated in an external and large cohort for further evaluation.

Nuclear Medicine, Stockholm, Sweden ; 3 The Skandion Clinic, The Skandion Clinic, Uppsala, Sweden Purpose or Objective To simulate organ doses delivered by a proton gantry- mounted cone-beam computed tomography (CBCT) system using two Monte Carlo codes. Material and Methods The CBCT system at the Skandion Clinic, Uppsala, Sweden, was modelled using MCNP6 and GATE from measurements of depth doses in water and spatial profiles in air. The beam models were subsequently validated against absolute dose measurements in a CTDI phantom. Organ doses in a whole-body phantom from different CBCT scan modes (360° full scan, 190° posterior/anterior scans) and different protocols (Head, Thorax, Pelvis) were then evaluated with the validated Monte Carlo beam models. The influence of repeated imaging on organ doses was also investigated, taking into account different imaging schedules (weekly/daily imaging, based on a standard treatment schedule of 30 fractions). Results An agreement within 4% was observed between MCNP6, GATE and the measurements with regard to depth doses and beam profiles for all protocols. The resultant average agreement in absolute dose validation was 4%. In-field absorbed organ doses after a single 360° scan ranged between 6-8 mGy (Head protocol), 15-17 mGy (Thorax) and 24-54 mGy (Pelvis). The highest absorbed organ dose after a single scan (54 mGy) was deposited in the femoral heads for the Pelvis protocol. Total organ doses after repeated CBCT imaging ranged between 0.04 and 0.32 Gy for weekly imaging and 0.2–1.6 Gy for daily imaging, depending on the organ and protocol considered. An average increase of 24% in dose per mAs to the organs of interest was observed for the anterior scans in comparison to the 360° scans, while the posterior scans showed a 37% decrease. Conclusion An accurate model of a proton gantry-mounted CBCT system was implemented in the MCNP6 and GATE Monte Carlo codes and used to calculate organ doses in a whole- body phantom following CBCT acquisitions. Various protocols and acquisition modes were evaluated. Organ doses varied greatly depending on acquisition mode, favouring posterior scans. EP-1684 Radiation isocontour levels for shielding considerations of the Varian Halcyon linear accelerator K. Caravani 1 , T. Jarema 1 , A. Kazi 1 , R. Murry 1 , G. Godwin 2 1 Radiation Oncology Centres, Physics, Toowoomba, Australia ; 2 Radiation Oncology Centres, Physics, Brisbane, Australia Purpose or Objective The purpose of this study was to create a radiation isocontour plot of the Varian Halcyon linear accelerator (Halcyon), within the bunker, with the aim of creating a Halcyon specific shielding design for future installations. Material and Methods A PTW 30013 Farmer chamber (with the manufacturer’s supplied build-up cap) was positioned at predefined points in 15° increments around the room, in a horizontal plane at the height of the isocentre and at various radial distances. Static cardinal gantry angles were used, with collimator angle 45°, maximum field size 28 cm x 28 cm, 6 MV FFF beam, maximum dose rate 800 MU/minute and 1000 MU/reading. Room angles were defined as per Figure 1.

Physics E-posters

Electronic Poster: Physics track: Radiation protection, secondary tumour induction and low dose (incl. imaging)

EP-1682 Fetal dose from head and neck tomotherapy versus 3D conformal radiotherapy S.H. Park 1 , J. Choi 1 , Y.S. Kim 1 , J. Yoon 2 , S. Ahn 3 , W.H. Choi 4 1 Jeju National University Hospital, Department of Radiation Oncology, Jeju, Korea Republic of ; 2 Seoul National University Hospital, Department of Radiation Oncology, Seoul, Korea Republic of ; 3 Kangwon National University Hospital, Department of Radiation Oncology, Chuncheon, Korea Republic of ; 4 Gangnam Severance Hospital, Department of Radiation Oncology, Seoul, Korea Republic of Purpose or Objective To compare the dose of radiation received by the fetus in a pregnant patient irradiated for head and neck cancer using helical tomotherapy and 3-dimensional conformal radiation therapy (3DCRT). Material and Methods The patient was modeled with a humanoid phantom to mimic a gestation of 26 weeks. Radiotherapy with a total dose of 2 Gy in 20 fractions was delivered with both tomotherapy (2.5- and 5.0-cm jaw size) and 3DCRT. The position of the fetus was predicted to be 45 cm from the field edge at the time of treatment. The delivered dose was measured according to the distance from the field edge and the fetus using optically simulated luminescence dosimeters. Results The accumulated dose to the fetus was 1.6 cGy by 3DCRT and 2.0 and 2.3 cGy by the 2.5-and 5.0 cm jaw tomotherapy plans. For tomotherapy, the fetal dose with the 2.5-cm jaw was lower than that with the 5.0-cm jaw, although the radiation leakage was greater for 2.5-cm jaw plan due to the 1.5-fold longer beam-on time. At the uterine fundus, tomotherapy with a 5.0-cm jaw delivered the highest dose of 2.4 cGy. Considering that the fetus moves up to 35 cm at the 29th week of gestation, the resultant fetal doses for 3DCRT and tomotherapy with 2.5- and 5.0-cm jaws were estimated as 2.1, 2.7, and 3.9 cGy, respectively. Conclusion Fetal radiation dose was about 30% higher with tomotherapy than with 3DCRT, but this observed difference was tiny in absolute terms. For tomotherapy, scattering radiation was more important due to the high monitor unit values. Therefore, selecting a smaller jaw size for tomotherapy may reduce the fetal dose; however, evaluation of risk should be individually performed for each patient. EP-1683 Monte Carlo evaluation of organ doses from a proton gantry-mounted CBCT system T. Henry 1 , O. Ardenfors 1 , I. Gudowska 1 , G. Poludniowski 2 , A. Dasu 3 1 Stockholm University, Medical Radiation Physics Department of Physics, Stockholm, Sweden ; 2 Karolinska University Hospital, Medical Radiation Physics and