ESTRO 2024 - Abstract Book

S3226

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Sweden. 5 Universitat Autonoma de Barcelona, Fisica, Barcelona, Spain. 6 Uppsala University, 6Medical Radiation Sciences, Department of Immunology, Genetics and Pathology, Uppsala, Sweden

Purpose/Objective:

Within the EU-funded project SINFONIA (Radiation risk appraisal for detrimental effects from medical exposure during management of patients with lymphoma or brain tumour), a survey of European Proton Therapy (PT) centres revealed that staff doses were often below the detection limits and always remained below 1 mSv/year, but with scarce description of the neutron detector calibration and corrections. This study describes experimental measurements aiming to confirm these findings and to identify knowledge gaps on detector calibration and corrections. Furthermore, doses from potential accidental exposure situations for staff or comforters staying in the room during paediatric exposures were investigated.

Material/Methods:

Staff and public doses were measured in and around the Skandion PT facility (see figure), including in the control rooms (A: Skandion operating staff, B: IBA staff), inside the treatment rooms of gantry 1 (G1) and gantry 2 (G2) for accidental/comforters irradiations (A*: by the exit door, C: close to patient, D: by the doors to gantry mechanism and E: gantry pit), but also in the garage ramp (F) and outside position close to cyclotron (G). Most irradiations were performed in G1 on a slab phantom while in G2, the Kyoto anthropomorphic phantom allowed to test the impact of phantom selection. Three representative clinical cancer treatments for brain, thorax (Hodgkin's lymphoma - HL) and pelvis (prostate), were investigated (see table) with and without range shifter (RS). Several detectors have been employed for the measurements, including the Wide Energy Neutron Detection Instrument (WENDI), the neutron dose rate probe LB 6411 (Berthold) and 2 DIAMON neutron spectrometers. Furthermore, bubble detectors - personnel neutron dosimeter (BD-PND), LANDAUER Neutrak dosimeters and the neutron dosimeters from Universitat Autónoma de Barcelona (Intercast CR-39 + plastic converters) were also tested. In addition, mono energetic (60, 120, 170 and 226 MeV) irradiations with fields of 10x10 cm2 were used in the research room. No significant neutron dose was measured outside the PT building (G) while in the garage (F), when no stopper was used during mono-energetic irradiations in research room, the dose per monitoring unit (MU) ranged from 2 pSv/MU for 60 MeV to 5000 pSv/MU for 226 MeV. A beam stopper reduced the doses by a factor of almost 15, which confirms the need for the current practice of using a beam stopper. In the table neutron doses from clinical plans show in position A the highest dose for prostate cancer (2.3 nSv/Gy) while the most elevated dose in position B was 0.1 nSv/Gy for HL. A yearly neutron dose to Skandion operating staff of 5 µSv was calculated in position A (control room). In contrast accidental exposure could result in a dose of 24 and 243 nSv/Gy respectively in position D during brain and prostate treatments and for position E, in the gantry pit, in a dose of 0.07 and 256 nSv/Gy. The impact of adding a RS, while leaving the plans unchanged, was investigated for the positions inside the treatment room. This showed a factor of 2 increase for the brain plan in position C, while the RS did not impact the dose for the pelvic plan in position D. On average the dose inside the treatment room increased with 30% when adding a RS to Results: