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, which characterizes the relative particle flux density of the photon beam. The measurement of TPR has to be performed in homogenous photon beams of size 10×10cm 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 was determined from field sizes 5×5cm, 10×5cm, 20×5cm and 40×5cm based on dosimetric measurements and Monte Carlo simulations. The mean value from all experimental values of TPR resulted in TPR¯=0.635±0.4%. The impact of the non-homogenous field due to the flattening-filter-free beam was negligible for field sizes below 20×5cm. The beam quality specifier calculated by Monte Carlo simulations was TPR=0.628 and TPR=0.631 for two different calculation methods. The stopping power ratio water-to-air s directly depends on the beam quality specifier. The value determined from all experimental TPR data was s=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.