Understanding Outcome Beyond EQD2 in Tandem and Ovoid Cervical Cancer Brachytherapy Treatments

Abstracts / Brachytherapy 16 (2017) S14eS118 imported into OcP TPS, a requirement for brachytherapy procedures. The module offers Dice and Hausdorff metric with TRE calculation to assess the accuracy of the registration. Fig. 1 shows a representative registration between MRI (blue) and US (red) contours for a) rigid and b) deformable registration. The deformable registration allows a better representation of the prostate at the time of brachytherapy as it can correct the US endorectal probe deformation. Dice indices were found to be 0.93  0.01 and 0.87  0.05 for the deformable and rigid registration, respectively. Fig. 1 shows c) maximum, mean and 95% confidence interval Hausdorff value for rigid and deformable registration. Rigid and deform MRI volumes (39.6  12.1, 39.4  12.2 cm3) were not statistically different (p O 0.05) from reference US volumes (38.6  11.9 cm3). TRE between centroid position was 2.10  0.98 and 0.37  0.14 mm for the rigid and deformable registration, respectively. Fig. 1 shows d) TRE found between common points identified in US and rigid or deformable MRI images. Deformable registration was found significantly better than rigid registration in terms of Dice, Hausdorff and TRE (p ! 0.01). Conclusions: In conclusion, deformable registration was significantly more accurate than rigid registration for brachytherapy MRI-US fusion. In average, both Hausdorff distance and TRE for common points are within 2mm. This study demonstr ates that the deformable registration is sufficiently accurate and precise for use in focal HDR prostate brachytherapy.

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PP48 Presentation Time: 11:15 AM Automated Prostate Brachytherapy Seed Detection on Post-Implant MRI Using Novel Martin Algorithm Geoffrey V. Martin, MD1, Jingfei Ma, PhD2, Rajat J. Kudchadker, PhD3, Jihong Wang, PhD3, Teresa L. Bruno, BS, CMD1, Pierre Blanchard, MD1, Steven J. Frank, MD1. 1Department of Radiation Oncology, Division of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, TX, USA; 2Department of Imaging Physics, Division of Diagnostic Imaging, University of Texas MD Anderson Cancer Center, Houston, TX, USA; 3Department of Radiation Physics, Division of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, TX, USA. Purpose: Post-prostate brachytherapy MRI provides improved anatomical details, but lacks efficient methods for identification of seed locations compared to CT. Given the poor soft tissue contrast with CT alone and the potential for errors in the MRI-CT coregistration process, the use of a brachytherapy seed detection algorithm using only MRI would provide accuracy for both seed identification and organ contouring, hence improving the reliability of post implant dosimetry The purpose of this study, therefore, is to present a novel method of automated seed detection using only MRI. Materials and Methods: Twenty-three consecutive patients undergoing permanent prostate implant with stranded seeds were included in this study. Each strand contained seeds and positive MRI contrast markers that replaced spacers. Post-implant MRI included a modified T2 weighted, constructive interference in steady state (CISS) MRI sequence developed for brachytherapy seed identification. Morphological image analysis using diffusion enhanced edge detection was used to extract brachytherapy strands followed by registration to 3D reconstruction of the brachytherapy pre-plan. Brachytherapy seeds were then assigned and identified within each strand. Seed locations on MRI detected by the automated algorithm were compared to those identified manually by a brachytherapy dosimetrist. The algorithm was implemented using MATLAB. Results: The average (S.E.) number of brachytherapy strands detected by the algorithm was 96% (2%), and the average (S.E.) number of seeds detected was 89% (1%). The average (S.E.) distance between seeds detected via the algorithm compared to manual identification was 2.2 mm (0.2 mm). The average time to perform the analysis per patient was 48 minutes on a standard desktop computer compared to 2-3 hours for manual detection of seeds. Conclusions: Automated detection of post-implant prostate brachytherapy seeds on MRI is feasible with good accuracy enabling MRI only postimplant dosimetry and reduces the number of brachytherapy seeds which need to be identified manually. Future refinements of the algorithm will focus on improved seed detection and reduced computational time.

SCIENTIFIC SESSION: GYN SNAP ORALS (E-POSTER) Saturday, April 22, 2017 11:30 AM - 12:30 PM GSOR1 Presentation Time: 11:30 AM Understanding Outcome Beyond EQD2 in Tandem and Ovoid Cervical Cancer Brachytherapy Treatments Emma Fields, MD1, Michael Waters, PhD2, Rakhi Melvani, MD2, Nitai Mukhopadhyay, PhD3, Dorin Todor, PhD1. 1Radiation Oncology, Virginia Commonwealth University, Richmond, VA, USA; 2Virginia Commonwealth University, Richmond, VA, USA; 3BioStatistics, Virginia Commonwealth University, Richmond, VA, USA. Purpose: The current ABS and GEC-ESTRO recommendations for reporting and adding the effects of BT and external beam (EB) RT doses are based on BED and EQD2, both of which are implicitly considering

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Abstracts / Brachytherapy 16 (2017) S14eS118

homogeneous dose distributions using either the prescription dose (PD) per fraction (typically a minimum dose encompassing the target volume) or D90 to the high-risk (HR)-CTV. Neither of these quantities can fully capture the effects of dose inhomogeneity, inherent and believed critical in brachytherapy. The purpose of this study is to analyze the clinical results of consecutive women with locally advanced cervical cancer, modelling the data for local control. Two radiobiological quantities previously proposed - EUBED and gBEUD - computed for extended ranges of a/b, a and ‘a’ were examined, together with EQD2, Dxx and Vxx, for both CTV and the volume encompassed by PD. Materials and Methods: 31 patients with cervical cancers(FIGO stage IB2-IV) were treated with 142 T&O fractions between 9/2013 and 11/ 2016 and were followed up for an average of 488 days (range 27-1133 days). HR_CTV and OARs (bladder, rectum, sigmoid) were contoured by a single physician. All plans were optimized using a ‘two-step’ optimization procedure, previously described, with the goal of maximizing D90(HR_CTV), minimizing dose to OARs, while maintaining the pear aspect of the PD iso-surface. Dosimetric data, structures and 3D spatial dose distributions were extracted from TPS and radiobiological quantities (BED, EQD2, EUBED and gBEUD) computed offline. Standard EQD2 was evaluated based on D90 to HR_CTV. EUBED and gBEUD were calculated for each fraction after a voxel-based conversion from dose to BED, for both HR_CTV and the volume encompassed by PD. A range of a/b values were considered, from 5.9Gy to 20.9Gy. Additionally, we explored multiple a values (range 0.150.7Gy-1) with smaller values typically associated with radioresistant tumors. Similarly, the free parameter ’a’ in gBEUD allowing a variable emphasis on hot/cold spots was explored in the range [-5,5). Distributions of the two populations (patients with recurrence and disease free) were compared using the 2-sample Kolmogorov-Smirnoff test at 3% significance. Results: The mean CTV was 41.1cm3, with a range from 21.1-84.0cm3. D90 for the series was 98.210.6 %PD showing the use of a consistent planning method and objectives. Among the patients followed up, 8 have recurred (four have deceased) and 23 have no evidence of disease. The average time to recurrence was 265 days (range 10-608 days), while the average disease free survival was 506 days, with a maximum follow-up of 1210 days. EUBED and gBEUD, computed for CTV, for the all a values and ‘a’ values between -5 and 1, produced very similar results, despite the two different methods of voxel-BED ‘integration’. None of the usual dosimetric parameters (Vxx, Dxx, CTV volume, TRAK, etc.) were able to distinguish the recurring patients from the disease free ones, with the exception of gBEUD for a$1.5 (p50.012). Conclusions: It is generally accepted that while there is no consensus on how to report high dose volumes for intracavitary brachytherapy at present, these high dose volumes are regarded as important. Based on our clinical data, we believe that gBEUD, with an ‘a’ exponent larger than 1.5, has to ability to ‘integrate’ high doses delivered with T&O in a manner that is strongly correlated with clinical outcome. Even though historically gBEUD associated negative exponent values with tumor control and positive values with toxicity, in the case of T&O distributions, a positive exponent emphasizes the importance of hot dose volumes, likely to play a significant role in cure. The clinical implications of this finding are huge - instead of focusing on D90 and EQD2 perhaps we should be integrating the inhomogeneity in real time and tracking the gBEUD to ensure the best outcomes for our patients.

GSOR3 Presentation Time: 11:40 AM Sustainable Gynecological Brachytherapy in an Increasingly Cost-Aware Healthcare System: Conversion of Labor-Intense Interstitial Brachytherapy to Hybrid Intracavitary Brachytherapy for Locally Advanced Cervical Cancer Brandon A. Dyer, MD, Stanley Benedict, PhD, Yi Rong, PhD, Sonja Dieterich, PhD, Richard K. Valicenti, MD, Jon Paul Hunt, CMD, Eliseo E. Montemayor, RT, Jyoti S. Mayadev, MD. Radiation Oncology, University of California Davis Comprehensive Cancer Center, Sacramento, CA, USA.

Purpose: Interstitial (IS) and hybrid (H) brachytherapy (BT) techniques allow for improved dose coverage and normal tissue sparing vs standard intracavitary (IC) BT in selected cervical cancer patients (pts), with IS being more complex, labor intensive, and requiring specialized physician and team training. With the decline in brachytherapy utilization trends, to add value and access to patient outcome improvement with BT, simple, efficient, and teachable techniques are needed. We examined our institutional efficiency and utilization trends over time for IC, H, and IS techniques and the theoretical ability for patient-specific conversion from an IS to H implantation for improved value. Materials and Methods: We reviewed cervical cancer pts treated consecutively and definitively at our institution using either IC, H, or IS techniques from 2007 - 2017. For IC cases, we collected process efficiency data related to physician, dosimetry, and physics planning time. For H and IS procedures, we collected patient demographics and radiotherapy (RT) treatment plan information. Additionally, we performed expert review for the potential to convert an IS to H implant. Statistical analyses were performed using two-tailed t-test with significance of p ! 0.05. Results: From 2007 - 2017, 197 cervical cancer pts were treated definitively with EBRT and BT: 158 IC, 14 H, and 25 IS. From 2010 - 2013 there was a significant difference in the number of IS pts vs H pts, 13 vs 3 (p 5 0.03); however, from 2014 - 2017 the number of IS pts vs H pts were equivocal, 12 vs 11 (p 5 0.90). Patient demographics and treatment related specifics are listed in Table 1. The IS pts had higher FIGO stage (p 5 0.03) and more IS pts died from disease failure, Table 1. Average HR-CTV volumes and dimensions were significantly larger in the IS vs H group, Table 1. Dosimetric analysis revealed a higher D2ccB and S in the H vs IS group, 4.9 Gy vs 4.4 Gy (p 5 0.01), 3.8 Gy vs 2.7 Gy (p ! 0.01), respectively. D2ccR was less in the H vs IS group, 3.4 Gy vs 4.0 Gy (p ! 0.01). Retrospective expert review of IS plans revealed that 7 of 25 (28%) pts could have been converted from IS to H implantation, Table 1. For each IC, H, and IS fraction the median physician planning time was 18 min, 28