Abstract
The radiation positioning system (RADPOS) combines an electromagnetic positioning sensor with metal oxide semiconductor field-effect transistor (MOSFET) dosimetry, enabling simultaneous online measurement of dose and spatial position. Evaluation points can be determined with the RADPOS. The accuracy of in-vivo proton dosimetry was evaluated using the RADPOS and an anthropomorphic head and neck phantom. MOSFET doses measured at 3D positions obtained with the RADPOS were compared with treatment plan values calculated using a simplified Monte Carlo (SMC) method. MOSFET responses, which depend strongly on the linear energy transfer of the proton beam, were corrected using the SMC method. The SMC method was used to calculate only dose deposition determined by the experimental depth-dose distribution and lateral displacement of protons due to the multiple scattering effect in materials and incident angle. This method thus enabled rapid calculation of accurate doses in even heterogeneities. In vivo dosimetry using the RADPOS, as well as MOSFET doses, agreed with SMC calculations in the range of ?3.0% to 8.3%. Most measurement errors occurred because of uncertainties in dose calculations due to the 1-mm position error. The results indicate that uncertainties in measurement position can be controlled successfully within 1 mm when using the RADPOS with in-vivo proton dosimetry.
Highlights
Proton beam therapy (PBT) provides a therapeutic gain for deeply seated tumors because the depth-dose distribution is characterized by a slowly rising dose near the entrance, followed by a sharp increase near the end of the range
We evaluated doses delivered in an anthropomorphic phantom using the radiation positioning system (RADPOS) for PBT
The metal oxide semiconductor field-effect transistor (MOSFET) doses agreed with simplified Monte Carlo (SMC) calculations within the measurement error
Summary
Proton beam therapy (PBT) provides a therapeutic gain for deeply seated tumors because the depth-dose distribution is characterized by a slowly rising dose near the entrance, followed by a sharp increase near the end of the range. To deliver a highly conformal dose to a tumor while sparing surrounding normal tissue, accurate dose delivery is essential. Proton dose distributions must be evaluated accurately, namely by in vivo proton dosimetry. In vivo dosimetry is generally performed by placing some type of detector on the point of interest in the patient anatomy. In vivo dosimetry requires a very small and localized detector, and diode [1], plastic scintillation [2] and thermoluminescent dosimeter [3] are used as an in vivo dosimeter. Metal oxide semiconductor field-effect transistor (MOSFET) [4] is useful for patient dose measurements. Because the MOSFET is direct reading with a very small active area (0.04 mm2), and the physical size of the MOSFET is less than 4 mm. The MOSFET has been examined thoroughly [5]-[7]
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