ESTRO 38 Abstract book

S916 ESTRO 38

which is important since dose-rate constants calculated using the egs_brachy Monte Carlo application of EGSnrc (Med Phys 35 (2008) 4228) make use of this correction factor and these values have been clinically recommended for a variety of seeds by the AAPM's TG-43 (Med Phys 44 (2016) e297). EP-1702 Examination of the real-time exposure dosimetry system using synthetic ruby on the radiation therapy A. Maruyama 1 , T. Yamaguchi 1 , D. Ono 1 , T. Kikuchi 1 , Y. Kikuchi 1 , R. Sato 1 , S. Watanabe 2 , Y. Hosokai 1 1 International University of Health and Welfare, Department of Radiological Sciences, Otawara-city- Tochigi, Japan ; 2 South Miyagi Medical Center, Radiology, Shibata- Miyagi, Japan Purpose or Objective It is part of reason in the occurrence of medical accidents on the radiation therapy that the local exposure dose cannot be direct monitoring in real-time. Recently, we developed a real-time exposure dosimetry system using synthetic ruby for interventional radiology (IVR). This time, we discusses some character of the developed real- time radiation dosimetry system for use in radiation therapy condition. Material and Methods A synthetic ruby scintillator with a diameter of ~2mm was attached to the tip of a 10 m long optical acrylic fiber using quick setting adhesive. The emitted light from the synthetic ruby was measured in real time in terms of the number of photons recorded using a photon counter head. A calibrated ionization chamber with Roos Electron Chamber dosimeter was used to a reference dosimeter. The aims of this study were to measure the following items with synthetic ruby dosimeter. 1. Characteristic features of the percentage depth dose (X-ray) The measuring devices were irradiated using 10 MV X-ray, at the center of 10×10 cm 2 and 15×15 cm 2 field, and monitor unit (MU) value was fixed 200 MU. 2. Characteristic features of the percentage depth dose (electron beam) The measuring devices were irradiated using 6, 9 and 12MeV electron beam, at the center of 10×10 cm 2 field, and MU value was fixed 100 MU.. 3. Characteristic features of coefficient of variance of MU value The characteristic features of coefficient of variance (%CV) of MU value in terms of dose and number of photon were measured. The depth of measuring was 10 cm, the measuring number was 10 times. 4. Linearity of the number of photons in terms of MU value The measuring devices were irradiated 10 MV X-ray, at the center of 10×10 cm 2 field, the depth of measuring was 10 cm. The MU value changed to 10 - 800 MU, the dose rate was fixed 500 MU/min. Results 1, 2. The percentage depth dose (PDD) was same as the reference dosimeter. The dose maximum depth was no clear difference between the reference dosimeter and the ruby dosimeter. 3. The %CV of MU value in terms of dose and number of photon was ±0.67%. 4. There is a significant correlation between the MU value and the number of photons (r2=0.9997).

unshielded diode detector corrected with the published field-size dependency factors. Results The reference output factor data for conical collimators has a very close agreement, on average only 0.1 % larger, with output factor data obtained by the integral method. A difference minimum -0.8 % is found with a 25 mm collimator and maximum +0.9 % with a 12.5 mm cone. Conclusion We have demonstrated that the integral dosimetry can be used to determine the output factors of the CyberKnife conical collimators. The method seems suitable for verification the output factor results of the standard formalism. EP-1701 Inverse square corrections for WAFACs D. Rogers 1 1 Carleton University, Physics Department, Ottawa, Canada Purpose or Objective To derive systematically the corrections needed when using Wide Angle Free Air Chambers (WAFACs) to convert a value measured or calculated for a finite solid-angle- detector to the corresponding value on axis assuming an isotropic point source and no attenuation. Secondly, to verify these corrections using Monte Carlo simulations. Material and Methods Using published techniques, corrections for cylindrical, conical and square prisms are derived to give: where L = detector thickness, R = its front radius, d = its distance from point source, α = R/d, β = R/(L+d), θc = arctan(α), θm = arctan(β) and w is the width of a square prism. Monte Carlo simulations are performed for a variety of detector geometries, (e.g., R large and R very small) using the egs_brachy application based on EGSnrc (PMB 61 (2016) 8214) for a point source in vacuum and low density dry air in the detector volume. The average air kerma in the detector volume is calculated. Values are converted to the on-axis air-kerma values using the above corrections and compared to expected values. Results The corrections are dominated by the longitudinal 1/r^2 effects, e.g., the correction is 1.46 for a conical NIST-like WAFAC geometry. However, in practice NIST and other standards labs eliminate this longitudinal effect by using an effective detector volume rather than the true detector volume (J Res NIST 108 (2003) 337) and only the L = 0 correction is required. That said, for the cylindrical and conical corrections derived above, the Monte Carlo simulations verify their accuracy within the typical statistical uncertainty of 0.04%. However, for the square prism, the correction breaks down by up to 16% for values of L typical of actual detectors (e.g., 10 cm). However, for a detector with L=0.05 cm or less, the square prism corrections are accurate within the statistical uncertainties. Conclusion By explicit derivations and simulations, the standard correction factors for 1/r^2 effects with various WAFAC detectors have been shown to be accurate with the exception of that for thick square prism detectors. However, the corrections for square prism detectors using L=0.05 cm are shown to be accurate at the 0.04% level