Numerical simulation design study of a positron camera for heavy-ion radiotherapy

The positron camera system has been designed to measure heavy-ion ranges in patients' bodies. The pencil-like beam of positron emitters, such as /sup 11/C, is used to check the range directly and precisely by detecting pairs of annihilation gamma-rays emitted from the end point of the beam trajectory. The positron camera consists of a pair of Anger-type scintillation cameras. The efficiency and the spatial resolution are modeled and simulated by a Monte Carlo method and a numerical calculation so that the positron camera has a high position accuracy for a small amount of irradiation dose. The simulation shows that the optimum effective diameter of the camera crystal is 500 mm and its thickness is 30 mm. The crystal diameter is concluded to be 600 mm by taking the outermost photomultiplier mounting into account. The simulation moreover, indicates that the range can be measured within an accuracy of 1 mm under the limitation that the irradiation dose has to be less than a few percent of the therapeutic one.