[Evaluation of therapeutic carbon-beam attenuation in inhomogeneous layered phantoms: Comparison with the present method using a water phantom.].

Carbon-beam therapy has been successfully carried out at HIMAC, Japan. This treatment offers two advantages over conventional radiation therapy: better dose concentration due to the Bragg peak and higher RBE. In treatment planning at HIMAC, the dose distribution is calculated based on dose measurements in water. We previously made three types of phantoms by using CT images: a liver-cancer phantom and two lung-cancer phantoms (one with bone and one without it). This study evaluates carbon-beam attenuation in inhomogeneous layered phantoms and compares their results with beam attenuation in a water phantom. The phantoms consist of plates of tissue-equivalent materials for the x-rays; these plates are stacked along the beam direction. The beam attenuation in the lung-cancer phantom (with bone) is about 23%, similar to the result in the water phantom, attenuation in the lung-cancer phantom (without bone) is about 25%, which is higher than the result in the water phantom by 2%. Finally, the beam attenuation in the liver-cancer phantom is about 33%, which is lower than the result in the water phantom by 3%. Our evaluation of the carbon-beam attenuation using inhomogeneous layered phantoms is successful and comparison with the results in a water phantom is possible.