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

The stopping power ratio water-to-air sw,aΔ directly depends on the beam quality specifier. The value determined from all experimental TPR20,10 data was sw,aΔ=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.