||Effect of asymmetric lateral beam spread of a pencil beam and oscillation of spot position on fluence distribution for particle pencil beam scanning
Tansho, Ryohei ,
Furukawa, Takuji ,
Hara, Yousuke ,
Mizushima, Kota ,
Saotome, NaoyaSaraya, Yuichi
A pencil beam scanning system for carbon ion radiotherapy has operated at National Institute of Radiological Sciences (NIRS) in Japan since 2011. The pencil beam scanning system makes conformal dose distribution to a target by superposition of a pencil beam. Then it is required to accurately irradiate the pencil beam with 2-dimensional Gaussian distribution with symmetric variance in the lateral direction to a planned spot position. However, the actual lateral beam spread of the pencil beam is slightly different and the actual beam irradiation position is also slightly different from the planned spot position. The different lateral beam spread and the different beam irradiation position are suppressed to the acceptance level in the beam commissioning process to remain the uniformity of dose distribution. However, the range of the acceptance level has been unclear and determined experimentally. The purpose of this study is to objectively determine the acceptance level about the symmetry of the lateral beam spread of the pencil beam and the accuracy of the beam irradiation position. We calculated a fluence distribution by superposing the individual pencil beam with asymmetric variances in the lateral direction and/or with oscillation of the beam irradiation position. We changed the difference of the lateral beam spread and the amplitude of the oscillation and studied the effect of these variations on the fluence distribution. We present the calculation results and summarize the acceptance level about the symmetry of the pencil beam spread and the accuracy of the beam irradiation position.
54th Annual Conference of the Particle Therapy Co-Operative Group (PTCOG54)