| ObjectiveWith the diagnostic Fan beam CT and planning system provided by the CT-Linac,the variation of bladder volume in postoperative radiotherapy of cervical cancer was studied.The correlation between bladder volume and setup error and target dose as well as OARs dose in different immobilization positions were analyzed.MethodsA total of 22 patients with cervical cancer who received postoperative radiotherapy in Sun Yat-sen University Cancer Center were analyzed in this study.Among them,10 cases were fixed in prone position with Orfit frame combined with thermoplastic sheet and 12 cases were fixed in supine position with negative pressure vacuum bag.Before the immobilization,the patient was instructed to empty the bladder as much as possible and then drink 500-1000 ml of water.After patients felt the urinary urgency,a portable 3-dimensional ultrasound bladder scanner was used to measure the bladder.If the patient lost of urine feeling,the bladder was measured after 40 minutes to 60 minutes.When the bladder volume measured reached 200-350 ml,the patient was fixed with different immobilization device and this indication was repeated in three times:during the immobilization,CT scan and simulation.Before each treatment,Fan beam CT was performed using a kV-level diagnostic CT machine on the uRT-506C accelerator to obtain FBCT images.Image registration online and translational correction was performed,while six-dimensional direction setup error data was recorded.In the treatment planning system(TPS)of UIH,the registered FBCT image was opened and the bladder,small bowel and rectum were delineated.The original planning parameters were copied for dose calculation.Dmean,V100%,V95%of PTV,bladder volume,Dmax,Dmean,V45,V40,V30 of bladder,small bowel volume,Dmax,V45,V40,V30 of small bowel,rectum volume,Dmax,Dmean,V45,V40,V30 of rectum were calculated.Compared with the original plan,the correlation between bladder volume and inter-fractional setup error and target dose as well as OARs dose were analyzed.Results1.Consistency of bladder fillingThe bladder volume of 22 patients during radiotherapy changed to varying degrees compared with the original plan.With the treatment process,the urine volume showed a weakening trend.From the 8th treatment,the overall mean bladder volume began and remained lower than the mean bladder volume in treatment plan.2.Correlation between bladder volume and setup errorPatients using two immobilization devices both have large setup errors in the sagittal rotation direction.In patients with prone position,there was a low negative correlation between volume change and transverse rotation error and a low positive correlation between bladder volume and sagittal rotation error.And there was a low positive correlation between bladder volume and the sum of errors in six directions.In patients with supine position,there was a low positive correlation between volume change and sagittal rotation error and a low negative correlation between bladder volume and head-foot direction error and left-right direction error.There was a low positive correlation between bladder volume and sagittal rotation error.3.Correlation between volume change of bladder and dose change of target and OARIn patients with prone position,the change of bladder volume was positively correlated with the change of Dmax of bladder and negatively correlated with the change of V30.The change of bladder volume was moderately negatively correlated with the change of V45 and V40 of small bowel and highly negatively correlated with the change of V30.The change of bladder volume was moderately negatively correlated with the change of rectal Dmean,V40 and V30 and low negatively correlated with the change of V45.In patients with supine position,the change of bladder volume was low negatively correlated with the change of Dmean and V100%of PTV.The change of bladder volume was highly negatively correlated with the change of Dmean and V30 of bladder.The change of bladder volume was moderately negatively correlated with the change of V40 and low negatively correlated with the change of V45.There was a high negative correlation with the change of V45 and V40 of the small bowel and a very high negative correlation with the change of V30.There was a moderate negative correlation with the change of Dmean,V45 and V40 of the rectum and a low negative correlation with the change of V30.Conclusions1.Patients with postoperative radiotherapy for cervical cancer are affected by surgery and the variability of bladder filling may increase.It is hard to maintain the stability of bladder volume during radiotherapy by relying on urinary habit training during the location.2.In the postoperative radiotherapy for cervical cancer,whether it is fixed in the supine position with negative pressure vacuum bag or in the prone position with Orfit frame combined with thermoplastic sheet,the change of bladder volume not only affects the accuracy of inter-fractional setup error,but also affects the actual dose of bladder,small bowel and rectum.When patients are fixed in supine position,the change of bladder volume has an impact on PTV dose and a more obvious impact on OARs dose. |
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