Monitoring And Assessment Of Residual Radiation In Rotation Of Varian Clinac CX Accelerators

Objective:As a kind of intensity modulated radiotherapy technique,rotational intensity modulated radiotherapy(IMAT)has characters of continuous rotation of gantry and continuous motion of multi-leave collimator during irradiation.The beam intensity adjustment in different directions of field is realized by the rotation of multiple or single arc of the gantry.IMAT has the potential of optimal planning and speed efficiency,as well as the dose distribution advantage of the rotational treatment and the dynamic intensity of the radiation.This study is to monitor and evaluate residual X-ray radiation level near the gantry after the rotary treatment of Varian Clinac CX medical linear accelerator,and to provide advices on radiation protection for medical staff of hospitals,treated patients and the nearby public.Methods:After the examination of the Varian Clinac CX medical linear accelerator,it is used to measure the basic level of radiation levels in the workplace and the ambient radiation levels of the ambient radiation levels,using a THERMO multipurpose radiation measuring device,FH40G.At different time,the X-ray beams with energies of 6MV and 10MV were given at 200MU,400MU,600MU and 800MU,and the radiation fields were 10 X 10cm2,20 X 20cm2,30 X 30cm2,40 X 40cm2,And then the detector was placed in a distance of 5cm from the accelerator head and the distance(100cm and 150cm)away from the center of linear accelerator.The irradiation beam of 6MV energy and 600MU dose rate were conducted in the manners of rotating irradiation and single angle irradiation.After the accelerator irradiation was finished,residual radiation dose levels and cooling time were measured.To ensure the repeatability and accuracy of the data,three measurements were made under each condition.The data collection should be stopped when the monitoring data is three times lower than 0.13?Sv/h.Results:The residual radiation was related to the radiation energy,and the residual radiation gradually increased with the increase of the radiation energy.The residual radiation is related to time.With the extension of time,the residual radiation gradually decreases.The residual radiation is related to the distance.With the increase of the distance,the residual radiation gradually decreases,not according to the rule of varies inversely as the square distance attenuation.The residual radiation is related to the dose of the outgoing beam.Residual radiation is related to the size of the irradiated field.With the gradual increase of the irradiated field,the residual radiation gradually increases.There was no difference between rotational irradiation and fixed angle irradiation in different irradiation modes(P<0.05).Conclusion:The residual radiation level after the treatment surrounding the head of Varian Clinac CX medical linear accelerator is related to irradiation time,distance,exposure field size,beam dose and beam energy.It has nothing to do with the modes of exposure.It is necessary to take measures to protect against the induced radioactivity by 10MV X-ray after treatment on patients with accelerator.The induced radioactivity can be minimized,by shortening the positioning time,delaying entering,reasonable arrangement of high and low radiation machine and effective ventilation.