Patient-Specific Energy Verify Using the Multilayer Acrylic Disk Detector for Proton Pencil Beam Scanning

Abstract

The total dose distribution of the proton pencil beam scanning (PBS) is constructed by individually delivered Bragg peaks and its uniformity depends directly on the accuracy of the delivery (i.e., the beam energy and the spot location). This study is the major reason for verifying the layer-by-layer energy along the scan path of PBS and was newly constructed the acrylic-disk radiation sensor (ADRS) of 20-layers to compare efficiency with the commercial multilayer ionization chamber (MLIC). We analyzed the properties of a multilayer ADRS to evaluate that it is suitable for the new QA system, and to compare the measured patient-specific energy with those predicted by the treatment planning system (TPS). The final goal of this study is to suggest optimized systems and methods for quickly and accurately carry out existing QA processes in PBS proton therapy. The measurements were obtained from a clinical proton pencil beams (IBA Proton Therapy System-Proteus 235). The multilayer ADRS is a series of transmitter disk (BC-800) with 150 mm diameter surrounded by thin optical fiber (PGS-CD1001-13-E). This disk is an ultraviolet-transparent methyl methacrylate polymer and shows water equivalency (mass density of 1.19 gcm-3) and has many other advantages; easy processing, remote sensing, and no interference from electromagnetic fields. It is now improved the efficiency of the measurement by arranging the ADRS of 20-layers and fabricating black plastic cases that allow for flexibility in placement of the solid slabs phantom, including in the extra room. It is also designed with a 1 mm thick disk which is as thin as can be reliably manufactured for verifying the layer-by-layer energy. The new is a significant step forward towards high spatial resolution compared to the previous version, which was made with a thickness of 2 mm. We investigated the feasibility of the multilayer ADRS to verify the layer-by-layer energy along the scan path for patients treated with PBS in three different sites (prostate, head & neck, and lung). The inaccuracy effect of the beam energy creates an uneven distance between adjacent layers in depth and thus cause ripples in depth, particularly at the shallower part where the Bragg peak is distinctively narrow. To evaluate the patient-specific energy, results calculated by the TPS and measured by the multilayer ADRS showed significant differences in the integrated energy of each layer depending on the treatment site. Therefore, the patient-specific energy calculated by the TPS must be compared and verified by the measured value prior to treatment. In this study, we believe that the multilayer ADRS has considerable potential to verify the energy for PBS proton therapy system.