Experimental and Monte Carlo-based determination of the beam quality specifier for TomoTherapyHD treatment units

Reference dosimetry by means of clinical linear accelerators in high-energy photon fields requires the determination of the beam quality specifier TPR_20,10, which characterizes the relative particle flux density of the photon beam. The measurement of TPR_20,10 has to be performed in homogenous photon beams of size 10×10cm² with a focus-detector distance of 100cm. These requirements cannot be fulfilled by TomoTherapy treatment units from Accuray. The TomoTherapy unit provides a flattening-filter-free photon fan beam with a maximum field width of 40cm and field lengths of 1.0cm, 2.5cm and 5.0cm at a focus-isocenter distance of 85cm. For the determination of the beam quality specifier from measurements under nonstandard reference conditions Sauer and Palmans proposed experiment-based fit functions. Moreover, Sauer recommends considering the impact of the flattening-filter-free beam on the measured data. To verify these fit functions, in the present study a Monte Carlo based model of the treatment head of a TomoTherapyHD unit was designed and commissioned with existing beam data of our clinical TomoTherapy machine. Depth dose curves and dose profiles were in agreement within 1.5% between experimental and Monte Carlo-based data. Based on the fit functions from Sauer and Palmans the beam quality specifier TPR_20,10 was determined from field sizes 5×5cm², 10×5cm², 20×5cm² and 40×5cm² based on dosimetric measurements and Monte Carlo simulations. The mean value from all experimental values of TPR_20,10 resulted in TPR_20,10¯=0.635±0.4%. The impact of the non-homogenous field due to the flattening-filter-free beam was negligible for field sizes below 20×5cm². The beam quality specifier calculated by Monte Carlo simulations was TPR_20,10=0.628 and TPR_20,10=0.631 for two different calculation methods. The stopping power ratio water-to-air s_w,a^Δ directly depends on the beam quality specifier. The value determined from all experimental TPR_20,10 data was s_w,a^Δ=1.126±0.1%, which is in excellent agreement with the value directly calculated by Monte Carlo simulations. The agreement is a good indication that the equations proposed by Sauer and Palmans are able to calculate the beam quality specifier under reference conditions from measurements in arbitrary photon field sizes with high accuracy and are applicable for the TomoTherapyHD treatment unit.