Arghya Chattaraj, T. Palani Selvam, D. Datta ICRR-HHE – 2016 Monte Carlo Calculation of microdosimetric quantities for photons of different energies Arghya Chattaraj, T. Palani Selvam, D. Datta Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai-400094. Email: arghyachattaraj@ymail.com; Phone: +91-22-25598654 INTRODUCTION Microdosimetry is a technique for measuring the microscopic distribution of energy. It is useful with different types of radiations and mixed radiation field. This is performed in cellular or sub-cellular level. It provides scientific underpinnings of measurement of radiation and very much important to both Radiotherapy and Radiation protection. Tissue Equivalent Proportional Counter (TEPC) is widely used for characterizing the radiation quality in radiation protection and radiotherapy environment. The aim of the present study, is to investigate the microdosimetric spectra of different photon energies for various cell diameters by using Monte Carlo techniques. Effects of collimator and phantom material of Bhabatron Telecobalt unit on the microdosimetric spectra and beam quality are also investigated. MATERIALS AND METHODS A spherical TEPC of internal diameter 5 cm filled with tissue- equivalent Propane gas having density corresponding to site sizes of 2, 5 & 10µm and covered with A-150 tissue equivalent plastic shell of 3 mm thickness was designed by using Monte Carlo code FLUKA. The TEPC was exposed by parallel photon beams from a disk source of 3 cm radius. For investigating the effect of Collimator, Jaw and Phantom material on Microdosimetric spectra, detailed geometry of Bhabatron unit was modeled. The TEPC was kept at source to axis distance(SAD)=80 cm in air. Simulation was done by replacing Collimator and Jaw material with air. Then it was simulated with Tungsten collimator and jaw of proper dimensions. Finally, TEPC was centered at 5 cm depth of a 30x30x30 cm3 water phantom with SAD=80 cm and along with Collimator and Jaw material was simulated. Cylindrical Co60 source was used for these simulations. Simulation done for 2 µm site size. For scoring purpose, DETECT card was used. Energy deposition in the gas cavity event-by event in a selected energy range over 1024 channels in linear scale was scored. Both the energy threshold for electron and photon were set to 1 keV in all the materials. Frequency distributions of energy deposition at different simulated diameters were calculated by activating the single-scattering mode everywhere. The energy frequency spectra were then converted to lineal energy (y) by dividing energy deposited in each channel with the mean chord length of spherical TEPC. Lineal energy spectra [yd(y) vs y] carries all the information regarding energy deposition. The frequency-mean lineal energy (yF) and dose-mean lineal energy (yD) were calculated from spectra. GRAPHICAL REPRESENTATION RESULTS Fig. : Pulse height spectra of an array of HPGe Detectors Table 1:******* Table 2: ***** TABLE FOR MONOENERGETIC PHOTON BEAMS Photon Energy (keV) Site Size (µm) Freq-mean lineal energy(YF )(keV/µm) Dose-mean lineal energy( yD )(keV/µm) 100 2 5 10 1.94±0.005 1.54±0.003 1.15±0.002 3.56±0.013 2.53±0.008 1.81±0.001 662 0.45±0.0003 0.43±0.0003 0.43±0.0002 1.8±0.046 1.35±0.002 1.07±0.001 1250 0.33±0.0003 0.31±0.0002 1.51±0.003 1.11±0.002 0.86±0.001 Table 2: Retained activity of 241 Am in lungs and liver at different times post exposure for acute inhalation of activity equivalent to 20 mSv CED TABLE BHABATRON TELECOBALT UNIT Air Collimator +J aw Collimator + Jaw+ Phantom yF (keV/µm) 0.44±0.0003 0.43±0.001 0.45±0.001 yD (keV/µm) 1.62±0.004 1.58±0.018 1.70±0.02 RESULTS & DISCUSSIONS The area under the yd(y) vs y curve is proportional to absorbed dose. The lower energy photons deposit more energy compared to higher energy for fixed site. Both yF & yD varies with cell diameter and photon energies. Beam quality of any photon energy is cell size dependent. For Bhabatron, Jaw, Collimator and Phantom material have marginal effect on the microdosimetric spectra and on the beam quality parameters (yF & yD). CONCLUSION Conventional radiation quality parameters ,like half value thickness (HVL), linear energy transfer ( LET), etc do not account for the fluctuation of energy transfer due to radiation interactions. This is very much taken care by the yD value derived from spectra. The yF is proportional to energy imparted to a volume and is also used for characterizing radiation quality. Not only absorbed dose, but number of energy deposits, their magnitudes and spatial distributions are expected to influence effect of radiation on biological or other targets. Microdosimetric Spectrum carries all these information.