The Development Of New Dosimetry System Using An Optic-Disk Radiation Sensor For Pencil Beam Scanning Mode

A measurement of integral depth dose profile (IDD) for the pencil beam scanning (PBS) is very important for the accurate delivery of dose to the target volume with a good conformity. In the beam commissioning process for PBS treatment method, the IDD profiles are commonly acquired for the beam modeling of treatment planning system. In this study, we fabricated an optic-disk radiation sensor (ODRS) and characterized the photoluminescence signal generated from an optical disk as fundamental research for the development and evaluation of a new dosimetric system for pencil beam proton therapy using an ODRS system. The relative IDD curve of proton beam was obtained by ODRS system and analyzed with it measured by ion chamber. We proposed an innovative method using a signal generated in clear plastic optic disk. We fabricated the ODRS system for measuring the photoluminescence signal induced by therapeutic pencil beam proton beams, respectively. Homemade phantoms based optic disk made of poly methyl methacrylate were fabricated to allow the optic disk to be exposed in the direction of the beam inside a water phantom capable of 3D operation. The photoluminescence signal generated from the optic disk by the proton beam was converted to an electric signal by a multi-anode photomultiplier tube (MAPMT: H7546, Hamamatsu Photonics, Shizuoka, Japan) connected to the plastic optic fiber surrounding the disk, NI-DAQ system, which allows us to check in real time through a notebook computer. The basic characteristics as the dosimetry sensor for this system based optic disk are described. The linearity relationship between the output according to the dose and dose rate measured using the ion chamber and the photoluminescence signal obtained by this system was confirmed, and the result was shown to be linear. The calibration factor for the energy was found to be close to zero, and it was confirmed that the energy dependence on the optic disk need not be considered. The relative IDD profiles were obtained using the photoluminescence signal generated by the pencil beam irradiation with two energies of 99.9 and 162.1 MeV in this system, which was compared with the curve measured by the Bragg chamber. The IDD results obtained by optic disk radiation sensor indicated good agreement without any correction process with the one by Bragg chamber. In this study, we developed a clear optic disk-based system to evaluate the characteristics for PBS beam dosimetry. An advantage of this system is that it can be easily extended to a two-dimensional array system for application in the PBS beam dosimetry. As a further study, we are developing the ODRS system which can measure the depth dose at each depth in 2 mm increments with respect to the direction of the PBS beam using a 2 mm thick disk array.