Presentation Development of a new ridge filter with honeycomb geometry for a pencil beam scanning system in particle radiotherapy

Tansho, Ryohei  ,  Furukawa, Takuji  ,  Hara, Yousuke  ,  Mizushima, Kota  ,  Saotome, Naoya  ,  Saraya, Yuichi  ,  Shirai, Toshiyuki  ,  Noda, Koji

Thanks to its characteristic depth-dose distribution with a Bragg peak, particle radiotherapy provides high dose localization to a tumor and is an effective tool for cancer therapy. The National Institute of Radiological Sciences (NIRS) in Japan has provided the particle radiotherapy with carbon-ion beam since 1994 using the Heavy-Ion Medical Accelerator in Chiba (HIMAC). To maximize characteristic advantage of the carbon-ion beams, the NIRS introduced a three-dimensional (3D) pencil beam scanning system in 2011. The pencil beam scanning system achieves conformal dose distribution to the tumor by superposing a dose deposit of the individual pencil beam. Since the high irradiation accuracy of beam position is required due to the sharpness of the Bragg peak, a device called as ridge filter is usually used to spread the Bragg peak size. The ridge filter has periodic stepped structure in one direction and a unit of the stepped structure called as a ridge bar. Since the energy loss of the scanned pencil beam depends on the thickness of the ridge bar the beam passed through, the energy distribution of the beam is spread. When the lateral pencil beam size is constant, the ridge filter with densely packed ridge bars is preferred because the particles with different energy is more mixed and the Bragg peak spread as planned. Since the pencil beam with smaller lateral beam size provides higher dose conformation the tumor, the pencil beam size has been improved to be small approximately 1-2 mm. Recently, it is difficult to pack the sufficient number of ridge bar within the lateral pencil beam region due to manufacturing limitation and a new designed ridge filter is required to realize more accurate irradiation by using the pencil beam with smaller beam size. We have designed the new ridge filter with honeycomb geometry. The new ridge bar has a structure stacked a hexagonal plate and periodically arranged the ridge bar to be honeycomb geometry. Since the packed density of the new ridge bar is higher than that of the conventional ridge bar, it is expected that the new ridge filter with honeycomb geometry gives the spread Bragg peak as planned when the beam size is below 1 mm.We have verified the advantages of the new ridge filter with honeycomb geometry by the calculation using particle tracking and the measurement of dose distribution. We will show the measurement and calculation results and that the new ridge filter is more useful for the pencil beam scanning with the smaller beam size.
The 12th European Conference on Accelerators in Applied Research and Technology (ECAART12)

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