Development of Monte Carlo Treatment Planning and Dosimetry System for Small Animal Irradiator

This work utilizes Monte Carlo simulation techniques to build a model of an x-ray tube in order to develop Monte Carlo treatment planning system for a small animal irradiator. To accomplish this, the absolute dose calibration of the irradiator performed in accordance with the recommendations of AAPM TG-61 protocol. Both in-air and in-water calibrations were performed at a 30.5 cm source-to-surface distance (SSD) for the reference applicator 40x40 mm2 square field size. The BEAM/EGS was used to model 225 kV photon beams from a small animal irradiator (Precision XRAD225). The Monte Carlo model was extensively tuned to provide good agreement with achievable measurements of the beam characteristics (e.g. PDD and off-axis ratios). Subsequently, output factors for various square and circular applicators were measured using different dosimeters (ionization chamber, radiochromic film) and compared with MC simulations. The standard gamma index method with AAPM TG 53 recommendations are used to benchmark the measurements (radio chromic film) against planar dose (Monte Carlo simulation) along with isodose lines and profiles in both homogeneous and heterogeneous mediums. The statistical uncertainty on the MC-calculated results is between 0.5% and 2% for most points. The CBCT images obtained on the XRAD 225Cx irradiator were converted to a material /density matrix as an input to DOSXYZnrc a MC dose computation module. The measured and computed point doses and isodose distributions were compared using the gamma index method. The absolute dose measured for reference collimator at 30.5 cm SSD in water and in air is 3.42 and 3.45 Gy/min. The agreement between simulated and measured dosimetric characteristics was excellent. For all fields, a good agreement is observed between measurements and calculations. Finally, a Monte Carlo treatment planning system for heterogeneous media is developed and validated. Monte Carlo simulation provides an indispensible tool for validating measurements of the smallest field sizes used in preclinical small animal irradiation.