Online Parameter Identification and Control in the Commissioning of Nozzle for CIAE Proton Therapy System

A dedicated pencil beam scan system is developed and integrated into the proton therapy (PT) system of China Institute by a joint effort of Pyramid Consult Inc. and China Institute of Atomic Energy (CIAE). In nozzle commissioning, a proton beam with an intensity of tens of nanoamperes is provided by the CIAE beam production system, including a 242-MeV superconducting cyclotron and a fast energy variable beam transportation system. Most scanning devices are commercial products from the pyramid, including the ion chambers, helium beam path, magnet, and amplifier. A scintillator camera system named Lynx from IBA provides position readouts of the beam spot during commissioning. A dedicated controlling software and related strategy for the beam commission of the scanning system is developed by the CIAE team. In the commissioning, a straightforward irradiation consisting of 114 beam spots is carried out to evaluate the scanning magnets’ nonlinearity and crosstalks. Afterward, Lynx generates these coordinates and analyzes them using the reported software. A modified 2-D polynomial fitting, including hyperbolic functions, is invested in the commissioning to yield an open-loop control accuracy in orders of millimeters. An iterative learning algorithm has also been developed to give even better accuracy. A 50-mm separation irradiation map consisting of 37 points is carried out by combining these two corrections on the first commission day, reaching an accuracy of 0.55 mm. An irradiation field of $250\times 250$ mm is also verified at the same time. The method and the first-day result will be reported in this article.