| Computational human phantom is a computer model which represents anatomical structure and contains the information of the physical properties of the organs and tissues.It has been widely applied in radiation dose calculation.As a typical computational phantom,the Reference Man phantom represents the 50th percentile of the anthropometric values for a specific population.Many countries have constructed their own Reference Man phantoms for the radiological protection of the citizens.However,as a substantial diversity of physique may exist between individuals,the anatomical deviations in body size and organ shape from the Reference Man can cause as much as 100%difference in the radiation dose calculation.Therefore,to reduce the uncertainty in radiation dose calculation and establish a more comprehensive radiological protection scheme,it is necessary to evaluate the influence of physique on the radiation dose by studying computational phantoms with different physiques.Three Chinese male phantoms with 50th percentile height and 10th,50th and 90th percentile BMI have been constructed based on the Deformable Chinese Reference Male(DCRM)phantom to represent the underweight,average weight and overweight Chinese individuals,respectively.This study corrected the fat percentage of each phantom and segmented the red bone marrow(RBM)and yellow bone marrow(YBM)from the site-specific bone tissue.The improved Chinese male phantoms were then applied in simulations of uniform radiation fields and medical radiation fields with photons and protons.The influence of physique on dose distribution in the body were evaluated.The contents of this research are as follows:1)This study adjusted the percentage of fat,which has an unignorable impact on the scattering and attenuation of the radiation particles,based on a predictive regression model of anatomy.Moreover,this study segmented the RBM,which is one of the most radiosensitive organs,based on the coloured cryosection images.The fat percentage and bone marrow compositions in our improved DCRM phantoms were in accordance with the existing Chinese criterions.These improvements made to the phantoms increase the accuracy of radiation dose calculation and ensure feasibility of dose assessment for the RBM.2)The three improved DCRM phantoms with different physiques were exposed to the uniform radiation fields of photons and protons with different energies and directions.The conversion coefficients of each organ,the RBM at each site,and whole body effective dose were calculated and compared.The influence of physique on the dose distribution in each radiation field was analysed in detail.The results of this study reveal that the conversion coefficients provided by the current radiological protection criterions may underestimate the dose for underweight people,and the average conversion coefficient of the whole body RBM may underestimate the doses for the site-specific RBMs.The deviation in radiation dose estimation caused by physique is more severe in the proton radiation field than in the photon radiation field,which can cross 100%in low energy radiation fields.3)Proton therapy in China is in the early stages of development.A pencil beam algorithm and a treatment planning algorithm for computational phantoms were constructed in this study.To improve the accuracy and actuality of simulation,three prostate proton therapy treatment plans were made for the three DCRM phantoms.The treatment plans and DCRM phantoms were then simulated in a Monte Carlo code system to calculate the production and dosimetry of the internal neutrons.The results reveal that internal neutrons are mainly produced around the endpoint of protons and the production of neutrons is lower in thinner patients.Therefore,losing weight to a proper extent before prostate proton therapy can reduce the normal tissue radiation toxicity caused by the internal neutrons.4)Deformation and displacement of organs in radiotherapy can cause severe distortion of dose distribution in the planning target volume(PTV).This study constructed ten computational pelvic phantoms with specific intravesical volumes varying from 100 mL to 700 mL using finite element method.The deformation and displacement of the pelvic organs were verified by reported clinical results.Then,simulations of prostate radiotherapy were performed with these pelvic phantoms with different intravesical volumes,and the influences of the intravesical volume on the radiation dose of the organs at risk(OAR,e.g.bladder wall and rectum)were evaluated.The dosimetry results reveal that an empty bladder with small PTV margins and a filled bladder with normal PTV margins can restrain the dose of the OAR to a similar level.Considering that the shape of a large bladder is more variable than that of a small bladder,this study recommends that a personalized evaluation of the bladder filling capacity of the patients should be performed prior to the prostate radiation treatment.This study used three Chinese male phantoms with different physiques to evaluate the dose in uniform radiation fields and medical radiation fields.The abundant conversion coefficients calculated in this study reveal the insufficiency of the Reference Man phantom in dose evaluation and provide theoretical and data support for radiological protection to individuals,including protection of the RBM.The study of the prostate radiation treatment offers prospective dosimetry data on the influence of body size and organ defomation.The constructed dose algorithms for protons in the computational phantoms are helpful for the development of proton therapy techniques in China. |
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