Seiji Tomori, Presenter: Nothing to Disclose

Yuya Sugama, Abstract Co-Author: Nothing to Disclose

Syungo Akita, Abstract Co-Author: Nothing to Disclose

Hidetaka Seto, Abstract Co-Author: Nothing to Disclose

Teiji Nishio PhD, Abstract Co-Author: Nothing to Disclose

Koichi Maruyama PhD, Abstract Co-Author: Nothing to Disclose

Hideyuki Takei, Abstract Co-Author: Nothing to Disclose

Toru Kawabata, Abstract Co-Author: Nothing to Disclose

Proton-beam therapy has become widespread in many countries due to its excellent dose distribution that uses the Bragg peak. The scanning-irradiation method using a pencil beam has been developed to improve three-dimensional conformity. The beam profile of the pencil beam increases as a function of depth due to multiple Coulomb scattering and nuclear scattering. Therefore, it is important to measure and calculate the beam profile for proper scanning irradiation for treatment planning. For that, a standard method is to express the material thickness in terms of a water-equivalent thickness (WET) that gives rise to the same energy loss. The purpose of this study was to quantitatively assess the accuracy of transforming material thickness to WET for beam profile evaluation, by measuring beam profiles for a number of different phantom materials and their corresponding WETs.

Our tool for beam-profile measurement consisted of a plastic-scintillator plate placed on the beam line and a CCD camera that recorded the scintillation light. The spatial resolution of the image recorded by the CCD camera was 0.14mm. We carried out experiments at the National Cancer Center Hospital East using the continuous proton beam with a nominal energy of 150MeV. We used several targets, including tough-bone phantoms of 2cm and 4cm thickness and tough lung phantoms of 10cm, 15cm, and 20cm thickness. In addition, we used water targets with thicknesses corresponding to the WET values of these targets.

The ratio between the width of the beam profile expressed in FWHM in the water targets and that in tough phantom targets were 109.3±0.1%, 114.2±0.1%, 109.9±0.1%, 109.0±0.1%, and 109.4±0.1%, respectively, and the mean value was 110.4±0.1%. Thus, WET gives a 10% wider beam profile. This increase in the width leads to a decrease of 5.8% in the dose obtained by beam superposition. These findings suggest that treatment planning using WET should not be recommended to obtain beam profile variations in depth.

We confirmed that there is a 10% overestimation of the width of beam profile by transforming material thickness into WET in treatment planning for proton-beam therapy.

Treatment planning using WET to obtain beam profile variations in depth should be replaced by the use of more realistic materials.

Tomori, S, Sugama, Y, Akita, S, Seto, H, Nishio, T, Maruyama, K, Takei, H, Kawabata, T, Inaccuracy of the Beam Profile Evaluation Method by Transforming Material Thickness into Water Equivalent Thickness for Proton-beam Therapy.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9010447.html