ESTRO 38 Abstract book

S471 ESTRO 38

outside the irradiation field. Confirming the clinical evidence of a systemically mediated effect of radiotherapy, our study contributes to understanding the circumstances of its occurrence and assists in developing a setting for the effective use of the synergistic mechanisms of radioimmunotherapy.

Physics Posters

Poster: Physics track: Basic dosimetry and phantom and detector development

PO-0891 Radiochromic film based output measurement for radio-biological experiments at low energy photons N. Tomic 1 , L. Lieng 1 , M. Lecavalier 1 , J. Seuntjens 1 , R. Apardian 1 , S. Devic 1 1 Jewish General Hospital, Radiation Oncology, Montreal, Canada Purpose or Objective Purpose of radiobiological experiments with small animals is to provide information of the efficacy of dose delivered for the investigated new modalities, for which it is essential to determine exactly the delivered dose. If the experimental setup is simulated before the actual experiment, radiochromic film with high spatial resolution can be used as a calibration tool, providing the spatial output data for future experiments. Material and Methods We designed experimental setup for radiobiological studies in the irradiation of the mouse abdomen (Fig.1a). It consists of the polyactic acid mouse holder and the half cylinder phantom mimicking the mouse during output measurements, both created using 3D printer (MicroBOT). Irradiation was carried by the Axxent -Xoft (Xoft Inc. Sunnyvale, CA) electronic brachytherapy system and Vaginal Applicator. The applicator was covered with the lead collimator having the 7 mm diameter hole, aligned with the source position within the applicator. Dose range used in the described setup is from 2 to 30 Gy. The irradiator has the beam quality of 50 kVp and the measured HVL of 0.81 mm Al. We used EBT3 model GAFCHROMIC TM film to estimate the output and the 2D dose distribution. Output of the irradiator at the reference point of measurement in terms of air kerma in air was measured using a calibrated parallel plate (Exradin A20) ionization chamber following AAPM TG-61 protocol. At the same reference point, film pieces were irradiated to create calibration curve for given beam quality in terms of air kerma in air as a function of film response to radiation: (PV 0 /PV) -1 [Aldelaijan at al.Phys.Med. 49, 112-118(2018)]. Output of irradiator at the phantom surface mimicking mouse was determined by employing previously described film reference dosimetry system, where dose to water is determined by multiplying air kerma in phantom by the mass-energy attenuation coefficient ratio water to air, (m en /r) wat air, for a given beam quality [Tomic at al.Med.Phys.37,1083-1092(2010)].

Results Figure 1.b presents calibration curve for measured beam quality showing linear response in wide dose range. Output of described irradiator at the surface of the animal model phantom in the center of lead opening was found to be 8.4 ± 0.1 Gy/min, and obtained 2D dose distribution was found to be uniform. Profile through the region of irradiation (Fig 1.c) shows dose uniformity to be within 95% from dose in the middle of the profile over 80% of the field diameter defined at 50% of central axis dose. Due to significant energy dependence of the EBT3 film response for effective photon energies bellow 100 keV, the beam quality has to be determined before using described reference dosimetry protocol. Conclusion We described method to measure the output and the 2D dose distribution of low energy photons in radiobiological setups with small animals using the EBT3 radiochromic film dosimetry protocol with the uncertainty in measured dose of the order of 5%. PO-0892 On the orientation of ionization chambers in dosimetry of small photon fields B. Casar 1 , E. Gershkevitsh 2 , I. Mendez 1 1 Institute of Oncology Ljubljana, Department for Dosimetry and Quality of Radiological Procedures, Ljubljana, Slovenia ; 2 North Estonia Medical centre, Medical Physics Service, Tallinn, Estonia Purpose or Objective The new IAEA Code of Practice TRS – 483 for dosimetry of small static photon fields provides advice on the orientation of various detectors for measurements of beam profiles and field output factors (FOFs). For cylindrical ionization chambers (ICs), it is advised to place the IC with its stem perpendicular to the beam axis for the determination of FOFs, while for the measurement of lateral beam profiles, the recommended orientation is parallel to the beam axis. Accurate positioning of the IC in the center of the field is crucial to obtain accurate values for FOFs. As the most accurate location of the center of the field lies in the middle of two orthogonal lateral beam profiles at FWHM, which is supposed to be obtained in a parallel orientation, recommendations from TRS-483 are somewhat incoherent. We aimed to analyze differences in detector specific output correction factors (OCFs) between parallel and perpendicular orientations of IC and to resolve the ambiguity on how to perform measurements of FOFs with ICs.