Monte Carlo Analysis Of The Effects Of Bladder Filling And The Applicator Displacement On Brachytherapy For Cervical Cancer

Cervical cancer is the most common reproductive system tumor in women,and brachytherapy is an indispensable treatment method for it.However,applicator displacement and changes in bladder filling will cause great uncertainty in the dosimetry distribution,the resulting complications such as radiation cystitis and proctitis seriously affect the patient's quality of life.The objective of this research is to investigate the effects of bladder filling and the applicator displacement on the dose distribution of brachytherapy for cervical cancer by using Monte Carlo method.The tasks and methods include:(1)The Nucletron-mHDR-Vl192Ir radioactive source model,cervical cancer patient CT image conversion model and three-dimensional isomerization model USTC-Adult Female were established;(2)The source model was verified by using MCNP5;(3)Doses were calculated for a source inside a three-dimensional voxel model obtained from CT images of a patient with different bladder filling degrees to investigate the effects of bladder filling;(4)The three-dimensional heterogeneous voxel model USTC-Adult Female was used to investigate the effects of applicator displacement.The following major results were obtained:The absorbed dose of the reference point A is 60Gy,the bladder filling degree is 54mL and 328mL respectively,When the bladder filling degree is 54 mL,the absorbed doses of the reference points of the bladder and rectal are 30.21 Gy and 49.05Gy respectively,and for the femoral head the absorbed doses are 6.33Gy(left)and 6.39Gy(right);When the bladder filling degree is 328mL,the absorbed doses of the reference points of the bladder and rectal are 33.59Gy and 50.19Gy respectively,and,for the femoral head,the absorbed doses are 6.15Gy(left)and 6.12Gy(right).More results of absorbed doses are obtained in the voxels of bladder wall and presented on the dose volume histogram of the bladder,when the bladder volume is 54 mL:V20=39.69%,V40=17.89%,V60=11.69%;when the bladder volume was 328 mL:V20=26.53%,V40=9.44%,V60=3.72%.The absorbed doses of the reference points and the important tissues and organs are obtained respectively in the different displacement of the applicator,in the meantime,the absorbed dose variation intervals of each reference point and each vital body organ are compared with the applicator in the original position.These results suggest that following conclusions:Compared with the bladder filling degree of 54mL,the absorbed dose of the reference points of the rectal and the femoral head do not change significantly when the bladder filling degree is 328mL.Although the absorbed dose of the reference point of the bladder is increased(3.38Gy)in the 328mL,it shows better results in the DVHs when compared with the 54mL at all points considered,the irradiated volume of the bladder is significantly reduced.Compared to the original position of the applicator,when the applicator is displaced along the X-axis,Y-axis,and Z-axis,the absorbed doses of various tissues and organs and reference points are changed to varying degrees;The absorbed dose of each organ varies greatly when the applicator is moved along the Z axis;the absorbed dose of point A,the reference points of bladder and rectal are more sensitive to the displacement of the applicator along the X-axis,the Z-axis and the Y-axis.These results suggest that the simulated results in this study can be useful for the clinical brachytherapy of cervical cancer.