Design And Validation Of Fluorescence Detector For Rapid Quality Assurance Of Radiotherapy

Cancer is a major disease that seriously threatens human life and health and society.The situation is particularly acute for China,where cancer cases and deaths are among the highest in the world and show a continuous trend of growth.In the face of such a huge cancer burden,it is imperative to develop effective prevention and treatment approaches.As one of the three mainstay cancer treatments,radiotherapy is an important local treatment.By depositing energy in the target area of the tumor through the radiation therapy rays,the DNA structure of the tumor cells is destroyed,thereby killing the cells.At present,the rays used for radiotherapy are roughly divided into two categories,one is photon beams,including X-rays,γ rays,etc.;Another category is particle beams,including proton beams,heavy ion beams,etc.When photon beams are used in radiation therapy,the dose is mainly deposited closer to the skin,and is therefore suitable for more superficial radiation therapy of tumors.Protons and heavy ions are mainly deposited at the end of the range because of their inverted deep dose distribution and high relative biological effects,and the Bragg peak of protons and heavy ion beams can coincide with the position of the tumor by adjusting the energy of the beam,so as to achieve the ideal therapeutic effect.In radiotherapy,quality assurance of the beam is also necessary to ensure that the dose to the tumor target is adequate and the dose to normal tissues and organs surrounding the target area is reduced.Among these,the energy accuracy of the accelerator output rays is an important test.Currently,the tool used for beam energy measurements is usually a 3D water tank with an ionization chamber.But This approach is time-consuming,labor-intensive and inefficient.In this paper,we propose a fluorescence detector scheme capable of performing fast beam energy detection.Based on the characteristic of scintillator emitting fluorescence under the irradiation of therapy beam,CMOS camera is applied to acquire the image of fluorescent intensity distribution on the side of a thin scintillator,and the depth-dose distribution of the beam in the scintillator material is quickly obtained by analyzing the fluorescent image,and then the energy of the beam is evaluated.First,material selection,size calculations,engineering drawings,and equipment handling were carried out according to the scheme.After determining the device materials and the associated dimensions,the performance of the fluorescence detector was verified by Monte Carlo simulations and irradiation experiments,respectively.Beams also include X-rays from linear accelerators and carbon-ion beams from synchrotron.The Monte Carlo simulations were performed using the Gate simulation software based on the Geant4 core to simulate the response of the fluorescence detector under two kinds of beams: X-ray beams and carbon-ion beams.The depth-dose distributions of two irradiated fields of different size in the scintillator material are simulated under X-rays.The depth-dose distributions of the carbon-ion beams in the scintillator material are simulated under different irradiation methods.The simulation results show that the theoretical results can be acquired by the fluorescence detector under the above conditions.After completing the Monte Carlo simulations,the fluorescence detector was further validated by irradiation experiments.The first is the beam experiment under X-rays,which is divided into two parts,the acquisition of the depth-dose distribution curves of X-rays in the scintillator material and the comparison with the results of Monte Carlo simulations;the other is a gradient dose irradiation experiment and dose rate dependence experiment.The accuracy of the fluorescence detector in obtaining the depth-dose distribution,the linearity of the response to the irradiation dose and independent of dose rate are demonstrated experimentally.Experiment on the reconstruction of the 3D dose distribution under X-ray has also been performed in order to expand the application scenario of fluorescence detector.Tomographic fluorescence images were acquired at different locations in the field by moving the fluorescence detector,and then the full 3D structure was obtained by 3D reconstruction.This experiment developed a new use for the fluorescence detector and provided a new way to obtain the 3D dose distribution of the beam.Finally,irradiation experiments were performed under the carbon-ion beams.The experimental measurements were performed with a fluorescence detector under the irradiation of carbon-ion uniform fields and pencil beams with different energies.The experimental results show that the Bragg peak of the carbon ion can be clearly observed from the fluorescence images obtained by the detector.At the same time,the same experimental conditions of Monte Carlo simulation were compared,and it was found that there was a penetration depth difference between the measured and calculated Bragg peak positions of carbon ion beam in the scintillator material by the fluorescence detector and the Monte Carlo simulation,but the differences under the different irradiation conditions were nearly the same.In this paper,Monte Carlo simulations and experimental verification methods demonstrate that the proposed fluorescence detector scheme can accurately and quickly obtain the depth-dose distribution of beams in scintillator material under two kinds of beams: X-ray and carbon-ion beams.It is also possible to reconstruct the 3D dose distribution under X-ray by acquiring tomographic images.This paper provides a substantial basis for establishing a fast fluorescence detector-based quality assurance measurement method in radiotherapy.