| Objectives:Based on the imaging data of CT simulation with the brachytherapy applicator in place, the DVH parameters of the first intracavitary brachytherapy obtained from three methods:normalizing to Point A, H and target volume were compared and the advantages and disadvantages were analyzed; for the same patient in 1st,2nd intracavitary brachytherapy, the Point A/H reference doses as well as,the DVH parameters obtained from normalizing to Point A were compared,,and the difference was investigated; according to normalizing to Point A,the association among DVH parameters as well as between Point A reference dose and DVH parameters was explored and the possible correlation was discussed.Methods:20 patients with locally advanced cervical cancer treated with high dose rate 192Ir intracavitary brachytherapy in our department from November 2014 to February 2015.The images obtaind from CT simulation with applicators in place were exported to Oncentra treatment planning system and the HR CTV (high-risk CTV) and organs at risk (bladder, rectum,) were contoured,refering to the pelvic MRI prior to the treatment and the pelvic exam before each operation. The plan was made on the Oncentra treatment planning system, including the reconstruction of the applicator,the reasonable arrangement of dwell points according to the target volume and local tumor status, the confirming position of the reference points (Point A and H) as well as the target extent and the dose normalization of reference points or target volume. The Point A was defined according to the No.38 Report of International Commission on Radiation Units and Measurement (ICRU); and Point H was defined according to the recommendations of American brachytherapy society (ABS). The treatment plans were grouped according to different normalization methods and the treatment plans normalized to Point A were set as Group A; the treatment plans normalized to Point H were set as Group H; and the treatment plans normalized to the target volume was set as Group T. Then the DVH parameters of the target and adjacent organs at risk of three normalization groups were recorded respectively, and the 1st and 2nd treatment plan parameters of each patient were selected for the study. Above parameters were compared and studied with the SPSS 17.0 statistical analysis software, and correlation analysis was implemented on relevant parameters.Results:1. Comparison of DVH Parameters of Different Normalization Methods: among Group H, A and T, the mean values of HR CTV D90 were (619.73±153.83)cGy, (581.41±151.34)cGy and (476.10±58.60)cGy respectively; the mean values of bladder D2cc were (610.42±145.35)cGy, (571.05±130.32)cGy and (475.21±78.69)cGy; and the mean values of rectum D2cc were (435.17±127.73)cGy, (407.13±120.19)cGy and (338.96±81.45)cGy. The one-way analysis of variance revealed that the differences in the HR CTV D90, bladder D2cc and rectum D2cc average values of three groups were statistically significant (average P＜0.05). Further pairwise comparisons showed that the differences in the HR CTV D90, bladder D2cc and rectum D2cc of Group H and A had no statistical significance (P=0.352,0.310 and 0.430 respectively), the differences in rectum D2cc of Group A and T had no statistical significance (P=0.058), the differences in the HR CTV D90, bladder D2cc of Group A and T had statistical significance (p=0.012 and 0.016). The differences in the HR CTV D90, bladder D2cc and rectum D2cc of Group H and T had obviously statistical significance (P= 0.001,0.001, and 0.009).2. Dose comparison between reference points of twice intracavitary radiotherapy of the same patient:the Point A and H were defined by referring to the ICRU No.38 report and ABS respectively (the right side was set as Point A1 or H1, the left side was set as Point A2 or H2). The paired t test showed that the differences between two operations of Point A1 were statistically insignificant (P= 0.216); the differences between two operations of Point A2 had statistical significance (P= 0.049); and the differences between two operations of Point H1 and H2 had also no statistical significance(P= 0.733 and 0.453).3. Comparison between DVH Parameters of two intracavitary radiotherapy of the Same Patient:the HR CTV D90, bladder D2cc and rectum D2cc were obtained from two intracavitary radiotherapy plans, and the paired t test showed that the differences in two intracavitary brachytherapy operations of HR CTV D90 were statistically significance (P= 0.040); the differences in two intracavitary brachytherapy operations of bladder D2cc and rectum D2cc had no statistically significance (P= 0.620 and 0.135).4.Relevance study:(1) The relevance analysis between the HR CTV D90 with rectum D2cc and bladder D2cc:Pearson correlation analysis shows that the correlation coefficient r of HR CTV D90 and bladder D2cc was 0.705, two-sided Pearson test P＜0.001; the correlation coefficient r of HR CTV D90 and rectum D2cc was 0.496, two-sided Pearson test P＜0.001. The above results showed that the rectum D2cc and bladder D2cc were all positive related with HR CTV D90.(2) The relevance analysis between rectum D2cc and bladder D2cc:Pearson correlation analysis shows that the correlation coefficient r of rectum D2cc and bladder D2cc was 0.743, two-sided Pearson test P<0.001; row scatterplot showed that they had liner distribution, further linear regression derived regression equation:Y(bladder D2cc)= 221.318+0.840X(rectum D2cc). F test was conducted in the regression model, showed F= 71.337, P=0.000. The difference had statistical significance, which means that the regression equation is sensitive.(3) The relevance analysis between dose of point A and HR CTV D90:Pearson correlation analysis shows that the correlation coefficient r of HR CTV D90 and dose of point A1 was 0.232, two-sided Pearson test P= 0.339; the correlation coefficient r of HR CTV D90 and dose of point A2 was -0.174, two-sided Pearson test P= 0.477. The above results showed that the dose of both point A1 and A2 were not related with HR CTV D90.(4) The relevance analysis between dose of point A and bladder D2cc and rectum D2cc:Pearson correlation analysis shows that the correlation coefficient r of dose of point A1 and A2 with bladder D2cc were 0.347 and - 0.081 respectively, two-sided Pearson test P= 0.145 and 0.743; the correlation coefficient r of dose of point A1 and A2 with rectum D2cc were 0.162 an -0.181 respectively, two-sided Pearson test P= 0.508 and 0.459. The above results showed that the dose of point A1 and A2 were neither related with bladder D2cc nor rectum D2cc.Conclusions:1. By comparing the dose normalization to Point A and H, there are insignificant differences in the DVH parameters of target volume and organs at risk; through comparison between the normalization to the target volume and Point A/H, the DVH parameters of target volume and organs at risk are significantly different, and each parameters reduced significantly, which indicates that the normalization to the target volume can have better protection for the organs at risk, but the dose in the target volume may be insufficient, so the contour of target volume has higher requirements for accuracy.2. Comparing the dose of point A and H in two intracavitary brachytherapy operations, point H showed smaller dose fluctuation range and higher repeatability; HR CTV D90 changed slightly while organs at risk did not show any difference in DVH parameters. The above results showed that target volume may change between two treatments while organs at risk remained steady.3. The relevance analysis revealed obvious positive correlation between DVH parameters (bladder D2cc, rectum D2cc and HR CTV D90), dose of point A was not related to DVH parameters. Therefore increasing the dose of HR CTV could result in the risk of increasing dose of the organs at risk, however the dose of reference point had no significance in the evaluation of DVH parameters. |
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