Study On The Change Of Target And Organs At Risk During Image-guided Volumetric Modulated Arc Radiotherapy For Cervical Cancer Patients

Objective:This paper aims to monitor the deformation and displacement of target and organs at risk within the period volumetric modulated arc radiotherapy(VMAT)for cervical cancer by cone-beam computed tomography(CBCT)guided and its setting errors,explore the opportunity of changing radiotherapy plan and calculate the suitable CTV-PTV margin.Methods:Total 18 patients with stage IB-IVA diagnosed with cervical cancer,each patient received comprehensive treatment in our department including VMAT technology,192 Ir high dose rate brachytherapy,and standardization of synchronous chemotherapy during radiotherapy.CBCT was scanned at first three days.Then one time weekly during radiotherapy,each patient received CBCT san 6-7 times and the total were 119 times.The inter-fraction setting errors and the tendency were leveraged to compare the deformation and displacement of the GTV,CTV,the rectum and bladder;then calculate the setting error.Result:1.Volume change: After treatment,the volume of GTV was importantly smaller than that before treatment and the volume of GTV reduced gradually with the growth of the number of radiotherapy weeks showing a negative correlation(P<0.05).The volume of GTV volume before radiotherapy was 114.85±58.29cm3,and the mean volume of radiotherapy to the fifth week was 33.34±15.52cm3.Comparing the volume of GTV before radiotherapy with that of each week respectively was statistically significant(P<0.05).The volume of CTV change little and the average CTV volume before radiotherapy was 780.61 ±144.84cm3,and the mean volume of radiotherapy to the fifth week was 757.39 ±111.62cm3.The volume of CTV changes relatively stable and the volume of CTV before radiotherapy compared with that of each week respectively,there was no statistically significant difference(P>0.05).The volume of rectum and bladder changes large,especially the volume of the bladder,comparing the volume of rectum and bladder before radiotherapy with that of each week respectively and there was no statistically significant difference(P>0.05).2.Volume change rate(?V%): GTV?V%1w,?V%2w,?V%3w,?V%4w,?V%5w was respectively:(10.07±4.51)%,(24.83±6.02)%,(45.28±6.83)%,(62.54±6.72)%,(70.20±6.25)%,with the growth of radiation weeks,GTV?V% increase gradually.Comparing the GTV?V%(1w)with that of each week respectively was statistically significant(P<0.05).Radiotherapy to third week(11-15f)was the most obvious followed by the fourth week(16-20f).The fifth week(21-25f)was minimized.The ?V% of CTV was small,with an average value of 4.62%.Comparing the CTV?V%(1w)with that of each week respectively was not statistically significant(P>0.05).The average volume change rate of the bladder was 46.37% and the average change rate of the rectal weeks was 39.95%.Bladder volume change rate is greater than the rectum in the period of the radiotherapy.Comparing the ?V%(1w)of rectum and bladder with that of each week respectively was no statistically significant difference(P>0.05).3.The correlation of the displacement of anterior and posterior wall of cervical and that of the bladder and rectum: the displacement of cervical posterior wall and the displacement of the rectum anterior wall were positively correlated(P<0.05);the displacement of cervical anterior wall and the displacement of the bladder posterior wall were positively correlated(P<0.05).4.Setup error and CTV-PTV margin.The average setting errors of patients on the left-right,superior-inferior and anterior-posterior were(0.39±1.24)mm,(-0.14±3.57)mm,(1.23±1.51)mm.From these results,we can find that the setting error Y(superior-inferior)> Z(anterior-posterior)>X(left-right),CTV-PTV margin were calculated on the basis of the Van Herk boundary formula,i.e.2.5∑+0.7σ.The results were 4.59 mm on left-right direction,7.24 mm on superior-inferior direction and 4.82 mm on anterior-posterior direction.Conclusion:1.The volume of GTV was gradually shrink with the growth of radiation weeks.The ?V% of GTV was gradually added with the growth of the radiation weeks and radiotherapy to the third week(11-15f)tumor regression was the most obvious.Therefore,the radiation to 11-15 f may be the best opportunity to change the radiotherapy plan and CTV volume is relatively stable in the process of radiotherapy.2.Though the bladder and rectum were prepared before radiotherapy,the rate of change was large especially the bladder and the volume changes of the bladder and rectum were not stable.3.The displacement of cervical anterior wall and the displacement of the bladder posterior wall were positively correlated.Moreover,the displacement of the rectum anterior wall and the displacement of cervical posterior wall were positively correlated.4.The setting errors of X,Y and Z axes were(0.39±1.24)mm,(-0.14±3.57)mm and(1.23±1.51)mm respectively.The CTT-PTV margin were 4.59 mm,7.24 mm and 4.82 mm respectively.