Room: AAPM ePoster Library
Purpose: quantitatively assess the production of radioactive materials during a yearlong clinical operation of a Mevion hyperscan proton therapy system. Cyclotron components, beam transport line and other room structures become radioactive by direct irradiation with proton beam and indirect irradiation with secondary particles. In this study, production of accumulated radioactive materials was assessed.
Methods: yearlong weekly room survey starting from 01/28/2019 to 02/08/2020 was conducted for the production of accumulated radioactive materials during normal clinical operation. The residual radioactive level was monitored in the treatment room on every Sunday morning after roughly 40 hours of normal weekly clinical operation. A hand-held gamma spectrometer, identiFINDER-U (Thermo Scientific, serial#: 04u3/2664) was used to record the radiation dose level and gamma ray spectra at three different locations: room corner, room isocenter, and nozzle surface. A few outlier data point readings were removed from analysis due to insufficient machine cool down time arising out of weekend patient treatments and maintenance work.
Results: yearlong measurement showed that the room activation level gradually increased in the first nine months and slowly plateaued thereafter. From these data, we could safely assume an average activation level of 35 urem/hr at room isocenter with normal treatment operation. If we assume that the measured dose level was contributed mainly by Na-22 from secondary neutron activated structural aluminum three meters away from isocenter in the beamline, then the accumulated activity of Na-22 was calculated to be less than 300 uCi.
Conclusion: have reported for the first time the residual radioactivity from a clinical operation of a compact proton therapy system. We do not anticipate the accumulated radiation level to rise in future and safe clinical operation will thus continue.