磁共振加速器液氦量变化及其剂量学影响分析OA

Objective:To explore the differences of Cryostat Transmission Characterization(CTC)curves that the beam passes through cryostat under different liquid helium volumes,and confirm the influence law of liquid helium state on beam attenuation and gantry-angle-related transmission characteristics by analyzing the dynamic changes of magnet pressure and liquid helium volume of MR-linac cold head during the full process from abnormal shutdown to recovery,so as to provide references for output calibration correction of MR-linacs,and accurate characterization of beam properties in treatment planning system(TPS).Methods:The relationships of the change of pressure and liquid helium volume of magnetic body,and duration of fault were analyzed after cold head was closed down through log files about sensing of pressure and liquid helium volume of scanning workstation of Elekta Unity MR-linac accelerator.After 96 h that cold head recovered operation,the CTC special measure platform was adopted to take PTW31013 ionization chamber with a water-filled build-up cap at the isocenter.The dose attenuation was measured after the ray of different gantry angle passed through the cryostat,and the results were normalized to 0°,which were compared with the measured results at the time of installation.Results:Within 48 h after cold head was shut down,the pressure of magnet body increased from the 206.8 kPa of baseline to 2 194 kPa,which appeared a trend from fast in the front to slow in the back,in addition,the liquid helium volume cumulatively reduced 3.4%.After cold head recovery,the pressure appeared linear decrease,and it recovered to baseline level after 96 h.The comparative results of CTC measures before and after liquid helium loss showed the change of dose attenuation existed differences between range of 0°-180°,and between range of 180°-360° when the 0° and 180° of gantry angles were used as boundary value,and the average values were respectively 0.52%and-0.48%.The maximum deviations of them were respectively 0.86%and-1.65%,and the occurred gantry angles were respectively 76° and 294°.Conclusion:The pressure of magnet body of Unity MR-linac enhances,and the liquid helium volume of that reduces after the cold head shut down,which exists risk of losing superconductivity.It should be timely found,and the fault of that should be timely eliminated,so as to ensure normal operation of equipment.The dosimetric influence that is caused by loss of liquid helium can exceed 1%,and it should be corrected through measures CTC curve again.