The calculation of absorbed dose near air-tissue interfaces has been an issue for treatment planning systems for some time due to the lack of electronic equilibrium which occurs at the interface. Treatment planning systems often over estimate the absorbed dose close to air-tissue interfaces. It is important to have an understanding of this over estimation, and its affect to patient treatment planning. The purpose of the current study is to investigate the accuracy of a treatment planning system, Pinnacle3, currently utilized for patient treatment planning, in calculating the absorbed dose at air-tissue interfaces, such as that encountered in the treatment of head and neck cancers.
All measurements performed with a 6MV photon beam, with a field size of 5x5cm², delivered from a Varian Clinac ix. Radiographic films Kodak X-Omat V, and extended dose range (EDR2) along with radiochromic film GafChromic EBT, were used to measure absorbed dose profiles at the proximal and distal interfaces of a 2cm diameter cylindrical air-cavity. TLDs were also placed from within the air-cavity to measure the point dose at the air-tissue interface. Dose measurements in the air ways of an anthropomorphic phantom were also conducted with TLDs and EBT film. The measured doses were then compared to those calculated by Pinnacle3.
Results indicate significant over estimating of the absorbed dose by Pinnacle3, where for the cylindrical air cavity phantom and the anthropomorphic phantom, Pinnacle3 over estimates the absorbed by at least 5% and 4% respectively. In some instances Pinnacle3 over estimates by over 10%.