The Study Of Influence Factors On Superficial Skin Doses For Patients With Radiotherapy

In the treatment of radiotherapy,the skin radiation damage is a common complication of radiotherapy.The skin superficial doses that the patient finally accepted are higher than the values calculated by the treatment planning system.And it is difficult to figure out the factors that can have impacts on superficial doses,as there are many constraints in measuring the doses during the treatment.In our studies,the Monte Carlo simulation and the Treatment Planning System calculation were adopted to analyze the superficial doses to study the factors influences on the superficial dose distribution.1.The Monte Carlo simulationA 10 cm×10 cm field is simulated to acquire the parameters influences on the dose distributions for Varian IX 6 MV megavoltage electron beams.And a simplified benchmarking procedure was proposed to determine the incident electron beam parameters independently using a Monte Carlo linear accelerator model.Then,with the parameters acquired following the procedure,the dose distributions with off Axis distances of 1.5 cm,2.5 cm,3.5 cm,4.5 cm and 1.5 cm depth were calculated.The results show that:(1)the dose profiles had no significant changes when the electron mean energy varied in small range.But there are errors between measured and calculated percentage depth doses simulated with different energies.It is necessary to determine the optimal electron energy to get accurate depth doses.(2)the percentage depth doses simulated with an electron radial intensity in 0.0 cm-0.4 cm had no difference with the measured doses.But when the radial intensity is above 0.2 cm,the calculated and measured dose profiles have large differences.When the radial intensity varied in 0.0 cm-0.2 cm,there were no difference in the main field while obvious discrepancies in the penumbra region.By matching the penumbra doses,good agreement between measured and calculated dose profiles were achieved.The mean angular spread was found to be 0.3° to get good matching between calculations and measurements at all considered depths.2.The Treatment Planning System calculationFirst,collecting the CBCT images of different times for 16 nasopharyngeal carcinoma patients.Then fusing the CBCT images and location CT images for every patient.Finally,measuring the neck contour shrink values and calculating the superficial dose changes for level A,B and C at different treatment time by using treatment planning system.The results show that:(1)The maximum contour shrink value for A,B,C,levels were 1.44 cm,1.02 cm,0.88 cm respectively;the mean shrink value was 0.98 cm,0.76 cm,0.51 cm,respectively.The empirical curves of dose-shrink value were: A,y=1078x+1768;B,y=3199x+2703;C,y=3877x+3479.The superficial doses were considered to getting bigger as the shrink values.(2)The rank of contour shrink values and superficial skin doses were A>B>C,A<B<C correspondingly.And the superficial skin dose variations increased with the treatment times.The contours of three levels remained stable after 20 treatment times.