Abstract
In treatment planning of photon radiation therapy, obtaining accurate attenuation of inphantom primary water collision kerma is important for accurate evaluation of the in-phantom primary and scatter absorbed dose. In-air chamber readings are often taken for evaluation of the attenuation of in-phantom primary water collision kerma. Using the Await concept, theoretical consideration is given to the evaluation method and the following conclusions are drawn:(i) For beams with multiple photon energies, the evaluation method can be perfectly valid only under certain extreme conditions. However, if the build-up cap is made of a low Z material like acrylic or aluminum and if its thickness is approximately equal to what is required for maximum dose build-up, the evaluation method can be valid with a high degree of accuracy.(ii) For beams with a single photon energy, the evaluation method can be perfectly valid without any particular conditions.
