Simulation And Study Of Stereotactic Breast Irradiation Imaging System

Breast cancer is one of the most common malignant tumors in women,and its incidence and mortality rank first among women in the world.In order to reduce the risk of recurrence and metastasis of breast cancer patients,patients need radiotherapy for subclinical lesions after mastectomy.Radiotherapy,as a major cancer treatment,mainly uses sufficient energy provided by ionizing radiation to kill tumors.Prospective stereotactic radiotherapy?SBRT?,an emerging technology,is used to deliver high-dose radiation to breast cancer patients in divided doses to protect the surrounding normal organs while killing tumors.Due to the existence of systematic and human errors,misalignments often occur,so dosimetry during radiotherapy is an important step to ensure that radiotherapy proceeds as planned.This paper aims to design an imaging system for breast cancer radiotherapy to reduce the patient's radiation dose as much as possible to achieve the goal of precise radiotherapy.X-ray induced acoustic computed tomography?XACT?is a novel imaging method that can produce the dose-related image in the irradiated object after irradiation with ionized photons or electron beams.Using the principle of dose deposition caused by radiation pulses,which is proportional to the acoustic signal,we collect acoustic pressure signals by simulating the ultrasound transducer array and reconstruct the XACT image to evaluate XACT as a dosimeter for feasibility of x-rays location tracking and real-time dose monitoring during stereotactic partial breast irradiation?SPBI?.First,we designed an XACT imaging device for SPBI monitoring in prone position,to verify the possible of XACT for achieving auxiliary radiotherapy?monitor the x-ray beam energy in real time and track the beam position?.We developed an in-house simulation workflow first.For a set of breast CT data,image segmentation?separation of skin,fat,and glandular tissues?was performed first,and then the seroma after tumor resection was manually simulated and placed in the 3D breast phantom.In order to simulate the treatment planning of SPBI,we simulated x-ray energy deposition and attenuation when the beams irradiated into the breast in Matlab.Next,K-wave toolbox was employed to simulate the propagation of the acoustic signal when the breast was irradiated,and the signal was detected by a designed 3D hemispherical ultrasound transducer array,and then the time reversal algorithm was utilized to reconstruct the time-varying pressure signal.A dose-related XACT image is obtained,and finally it is fused with the original breast CT image to get a dose distribution map in the breast.Since image reconstruction can be completed in a few minutes,it is possible to monitor the patient's in vivo dose in real time and make timely adjustments for radiotherapy.The results show that XACT has the feasibility to reconstruct the 3D dose distribution during SPBI.In the reconstructed dose distribution,the average doses in the GTV?gross target volume?and PTV?planning target volume?were 86.15%and 80.89%,respectively.Compared with the treatment planning,the RMSE?root mean square error?of GTV and PTV in the reconstructed XACT were 2.408%and 2.299%,respectively.However,the series of simulations above are all based on planned CT images,that is,prior to radiotherapy,the CT image of the patient needs to be collected to determine the location of the target volume and develop a radiotherapy plan,whereas CT imaging will increase the patient's additional radiation dose and increase the risk of organ damage.As a non-destructive detection technology,photoacoustic imaging can be used for breast cancer imaging before radiotherapy,and the choice of excitation source is of great significance for stable imaging.In order to further improve the above radiotherapy imaging system,we then studied fiber lasers based on 2D materials as saturable absorbers?SAs?in order to provide a suitable excitation source for photoacoustic imaging.In this thesis,we used the new 2D material zirconium selenide?ZrSe₂?and polyvinyl alcohol?PVA?to prepare films for the first time.After measuring its nonlinear and saturable absorption characteristics and verifying its potential in ultrafast laser applications,the ZrSe₂-PVA film is added into an erbium-doped fiber laser.By adjusting the pump power and the position of the polarization controller?PCs?,the dispersion in the cavity is changed.Under the multiple factors such as fiber birefringence and nonlinearity of the saturable absorber,stable dark solitons and dark-bright solitons obtained.It is hoped that a stable laser output with high energy and narrow pulse width could be achieved in an appropriate polarization state,which is conducive to promote the further development of photoacoustic imaging for image acquisition before breast radiotherapy.