GSOR3 Presentation Time: 9:10 AM: Adding Free Needles and Optimization to Tandem and Ovoids in High Dose Rate Brachytherapy Treatment for Cervical Cancer: How Much Do They Matter?

Abstract

<h3>Purpose</h3> With increased MRI utilization, targets for cervical cancer are more customized. This in turn makes hybrid applicators more essential, creating treatment plans which deviate significantly from classical pear-shaped dose distributions. This study analyzes the dosimetric impact of integrating free needles into tandem and ovoid (T&O) HDR-brachytherapy treatment for locally advanced cervical cancer. <h3>Materials and Methods</h3> 29 patients with cervical cancers (FIGO stage IB-IIIC2) were treated with 125 fractions. HR_CTV and OARs (bladder, rectum, sigmoid) were contoured by one physician. All delivered plans were optimized using a previously described ‘two-step' optimization procedure, with the goal of maximizing D90(HR-CTV), minimizing dose to OARs, while maintaining the pear aspect of the Prescription Dose iso-surface. These plans were then compared to two additional plans, created using only T&O, re-optimized to either match the original plans for coverage (D90 to HR-CTV) or for OAR sparing (D2cc). Dosimetric information was extracted from TPS and distributions evaluated using histograms and statistical t-test. <h3>Results</h3> The CTV_HR volume for this series was 53.58±23.46 cm3. The dose coverage of the target for the clinical plans was excellent, at 99.14±6.62 %PD. There was a negative correlation between D90 and HR-CTV, in that larger volume of HR-CTV in HDR brachytherapy cervical cancer cases tends to have an outcome of lower dose coverage (shown in figure 1). When clinical plans, using T&O and free needles, were compared with T&O only plans optimized to match D90, the mean D2cc calculated from plans without needles was higher than those from original plans by an average of 29%, 22%, 27% for bladder, rectum and sigmoid, respectively. A t-test between the two populations, with a p-values < 0.00006 shows that they are significantly different. The highest difference of D2cc between original plans and re-optimized plans were 261%, 294%, and 193% for bladder, rectum, and sigmoid, respectively. When clinical plans were compared to re-optimized T&O only, matching OAR toxicity, the D90(HR-CTV) coverage of the plans without needles, was as expected, lower by an average of 21% for the entire series. The T-test between the two populations had a p-value < 2.2e-16. The maximum percentage difference of the dose coverage was 128% compared between re-optimized T&O only and original plans with matched toxicity. <h3>Conclusions</h3> Free handed needles are an easy to implement, inexpensive add-on to a classical T&O, in order to create a ‘hybrid' applicator. Based on this series of patients and the dosimetric data from their clinical plans, we believe that integrating free-handed needles and optimization methods into traditional T&O cervical cancer brachytherapy treatment is highly beneficial for controlling organs-at-risk toxicity while achieving optimal coverage for HR-CTV and preserving an overall ‘pear'-shape-like aspect of the overall dose distribution. Our findings suggest that the use of additional needles should be considered for cases with a larger or asymmetric volume of HR-CTV. This study, is to our knowledge, the first to quantify the dosimetric benefits of free-needles augmented T&O when compared to classical T&O. We recommend in addition to adding free needles the two step optimization method to best take advantage of the degrees of freedom one has by using hybrid applicators.