The Research Of A Daily QA Instrument Based On Optical Fiber Sensing Technology And Its Performance Analysis

In recent years,more and more people suffer from cancer.As an efficient treatment approach to treating cancers,radiotherapy has attached wide range of concerns.However,if the cancer patient absorbed too much radiation,it will be harmful to their health.Therefore,it is necessary to ensure the safe and effective of treatment when to guarantee the accuracy of radiation dose.This requests us to use the daily quality assurance?QA?instrument to implement quality assurance for linear accelerator?Linac?.In this paper,we presented a kind of fiber X-ray dosimeter which can be used as the radiation detector of daily QA instrument.The fiber dosimeters are fabricated by embedding scintillating material inside the plastic optical fiber core to detect X-ray.This kind of dosimeter has some advantages such as small size,cheap and flexible.It can be put inside the water to measure radiation dose.Some research work has been done to meet the requirement of daily QA.Firstly,we detected the cumulative intensity under the different irradiated doses of the 6MV and 10MV beams and calculated the repeat rate.The deviation was less than 0.3%.Secondly,we detected the linear dose respectively under the energy of 6MV and 10MV,and the linear rate is better than 0.99999.Finally,we rotated the probe of the dosimeter between 0-180°for testing the angle dependence,and the maximum error rate is 0.4%.These experiments show that the dosimeter made by Gd₂O₂S:Tb has excellent repeatability,linearity and low angle dependence,so it can be used to make daily QA instruments.At the same time,we analyzed the factors which can impact the performance of the fiber X-ray dosimeter.At first,we compared the parameter of multi-pixel photon counter?MPPC?,photomultiplier tube?PMT?and photon counting head?PCD?by collecting the fluorescent signals.Then we selected PCD as the most suitable photoelectric detector.After that,we changed the location of the instruments in the therapeutic room for exploring the response in different places.And we tested the influence of insertion loss by doing 10 times experiments.The deviation of the insertion loss is 0.4%.Then,we put the X-ray dosimeter at different water depth positions and compared the results with an ionization chamber.Finally,in order to explore the effect of temperature,we put the dosimeter in the water whose temperature changed between 25?-40?,and the maximum deviation is 2%.Then we measured the radiation dose using the new Linac daily QA instrument,and the result is not consistent with that obtained by an ionization chamber.In order to solve this problem,we proposed to encapsulate the dosimeter by copper,lead.After that,we detected radiation dose in the field inside and outside the radiation field.We found the response curve of the fiber X-ray dosimeter which is encapsulated by lead is the most close to that of the ionization chamber.Finally,we designed two kind of three dimensional structure of daily QA instruments.One can change the position of X-ray dosimeter,the other one can realize the three-dimensional detection by using the multi-layer panel.