To Evaluate the Feasibility of Minimizing Ovarian Dose in Irradiation of Abdominal and Pelvic Malignancies by applying Optimal Margin in Helical Tomotherapy

Abstract

This study aimed to evaluate the dosimetric impact on target volume and organs at risk (OARs) in irradiation of abdominal and pelvic malignancies with and without the addition of block and margin (B&M). Based on the results, optimal margins for ovaries will be recommended. The ovaries of 16 patients were contoured by tracing the ovarian veins in CTs. Reference plans (Plan(ref)) were generated following the original physician dose prescriptions. Another 16 plans (Plan(B&M)) with Peters N., suggested margins of 2.2cm and 2.9cm for left (Lt) and right (Rt) ovaries respectively were generated by adding directional or complete block to the ovarian margin. Dose constraints of maximal dose(Dmax) <2Gy and <3Gy were applied to ovaries and ovarian margins respectively in order to achieve the ovarian dose of 2Gy while maintaining the original plan quality. The differences between Plan(ref) and Plan(B&M) in ovary Dmax, ovarian margin Dmax, target Dmax, Dmin,homogeneity index (HI), conformation index(CI) and OARs doses were compared by 2 tailed paired t-tests. A one tailed one sample t-test was used to test if the mean ovary Dmax could be lowered than 2Gy with p<0.05. If not, the lowest achievable ovarian dose after B&M would be found out by subsequent one-tailed one sample t-tests. Pearson correlation test were performed to investigate the correlation between a)ovarian dose and target-ovary distance; b) ovarian dose and target dose; and c) ovarian dose and target volume after B&M was added. Optimal ovarian margins for ovaries <3Gy based on the ovarian margin doses calculation in Plan(B&M) would be estimated. After B&M was added, 75% of patients showed at least one ovary had the Dmax <2Gy while 50% of them had both the Lt and Rt ovaries Dmax less than 2Gy; Also, there were 74.6% and 48.5% average Lt and Rt ovarian doses reduction after the application of B&M respectively. Statistical analysis showed that there were significant difference between the 2 sets of plans in Lt ovary Dmax(p=0.000), Rt ovary Dmax(p=0.037), Lt ovarian margin Dmax(p=0.034), Rt ovarian margin Dmax(p=0.014), target Dmin(p=0.039) and CN(p=0.046), but not for the target Dmax(p=0.077) and HI(p=0.685) as well as all OARs. One-tailed one sample t-test failed to show significant difference between 2Gy and Lt/Rt ovarian Dmax with B&M. Subsequent t-tests showed that the lowest achievable dose of at least one ovary was 5Gy in Plan(B&M). Pearson correlation tests showed significant correlation between target volume and Lt ovary dose (p=0.011) as well as Rt ovary dose (p=0.042). The suggestion of optimal ovarian margins in Plan(B&M) was 1.91cm and 2.57cm for Lt and Rt ovaries respectively. After B&M added, the ovaries doses could be significantly reduced without much affecting target quality. The ovarian dose ≤3Gy could be achieved with the suggested ovarian margin. B&M application offered advantage of lowering the ovarian dose in unilateral small volume target.