Estimation of Background Radiation from e-LINAC for Direct Confirmation of Irradiated Area by Utilizing the Annihilation Photons.

In radiation therapy, there is no suitable method to confirm either the three-dimensional irradiated area or the absorbed dose during irradiation. If there were such a method it would allow verification of the absorbed dose as well as help to avoid extraneous irradiation. We previously proposed a method utilizing annihilation photons generated by therapeutic photon beams to measure the irradiated area directly during irradiation. In the present study, we designed and fabricated a fast YAP:Ce scintillation counter which uses a compact photosensor to measure annihilation photons. The energy resolution and detection efficiency were obtained experimentally using a (22)Na planar source. We measured count rates of background radiation by using the detector and estimated those of background radiation from an e-LINAC by correcting for detection efficiency. The obtained energy resolution of the YAP:Ce scintillation detector was 12.16+/-3.5% and its detection efficiency was 28.36+/-5.3%. The counting rates of background radiation from e-LINAC were approximately from 10(8) to 10(9) cps, and were proportional to the e-LINAC dose rate. Our developed detector had a high sensitivity, good time resolution and compact size which allows for easy radiation shielding. Therefore, we concluded that it was both practical and effective for the measurement of the position-distribution of annihilation photons.

[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.