Region of Interest Analysis as a Tool for Exploring Adaptive IMRT Strategy for Cervix Cancer Patients Young-Bin Cho 1,2, Valerie Kelly 1, Karen Lim 1,2, James Stewart 1, Anthony Fyles 1,2, Kristy Brock 1,2, Jason Xie 1, Anna Lundin 3, Henrik Rehbinder 3, Michael Milosevic 1,2 1 Radiation Medicine Program, Princess Margaret Hospital/Ontario Cancer Institute; 2 University of Toronto; 3 RaySearch Laboratories AB, Stockholm, Sweden Introduction Planning studies of whole pelvis IMRT for cervix cancer have shown that organ motion and deformation can lead to deviations between the original treatment plan and the distribution of dose delivered during treatment. Finding strategies to respond to these anatomical changes using an adaptive planning approach is time consuming, however, due to the large number of variable parameters. Assuming that the dose gradient conforms well to the PTV, and that geometrical expansion of the PTV can be used to represent various isodose levels, a region of interest (ROI) analysis can efficiently explore and evaluate different planning strategies. This study shows (1) the effect of dose conformity on target coverage and OAR sparing and (2) the performance of various adaptive planning strategies. (Results from our previous adaptive planning studies were compared with the estimation from ROI analysis). Materials and Methods A. Adaptive planning and dose accumulation 25 patients had baseline and weekly MR scans performed during radiotherapy. These were fused to the planning CT using bony alignment. Perfect daily setup to bone was assumed. Each image- set was fully contoured and PTV margins generated. PTV D98 was 4750cGy (95% prescription dose). Adaptive IMRT plans were developed by a dosimetrist as well as via an automated method using the ORBIT Workstation from RaySearch laboratories. Dose delivered to the deformed organs in each image-set was then calculated (Fig 1 and Fig 2). Isodose volume at 80-95% of the prescribed dose were then compared with that of the PTV for each plan using ROI analysis. The optimal schedule for one time on-line adaptive planning is during the 1 st or 2 nd week of treatment. Any later would be too late to compensate for areas of target under dosing. This result agrees well with our previous studies. The volume of OARs (rectum, sigmoid, bowel and bladder) in the PTV was also reduced with adaptation and OAR sparing was significantly enhanced with more frequent on-line adaptation. Conclusions We found ROI analysis to be an elegant and efficient tool for exploring and evaluating adaptive planning strategies. Using this method, on-line adaptation proved to be the most important factor for target coverage with OAR sparing. Off-line adaptation did not improve target coverage, but did improve OAR sparing (due to shrinking target volumes). At present, weekly adaptation using the automated process adds an additional 2mm expansion to the 95% isodose line when compared to the single adaptive plan generated by the dosimetrist. Conformality is an important factor for successful adaptation. Less conformal plans are less sensitive to target motion but result in less OAR sparing. Acknowledgements The authors would also like to thank Philip Chan, David Jaffray and Johann Löf. B. ROI analysis Various ROIs were analysed to compare different adaptive planning strategies. CTV expansions of of 3, 5, 7, 10, 15 and 30mm were made. The 3mm expansion was considered to be the PTV. The 5mm expansion was regarded as a 2mm additional expansion from the PTV (PTVexp_2) and so on. Proportions of CTV and OARs lying in/out of the PTV and PTVexp were assessed for each weekly image-set (Fig 3). Various adaptive planning strategies were simulated (no adaptation, single mid-treatment adaptation, and weekly adaptation with on-line and off-line adaptation). Each margin and adaptation schedule was assessed with regards to the competing goals of target coverage and OAR sparing. Results and Discussion Both GTV and CTV shrank to 25% and 65% of their original volumes by the end of treatment. A combination of weekly CTVs into a single CTV union became 113% of the original volume due to the inclusion of inter-fractional motion. This clearly supports the need for planning margins. Volumes of the PTV expansions increase rapidly with margin size. The Volume of PTVexp_2 (2mm expansion from PTV) is 25% larger than the PTV and PTVexp_7 is 81% larger than the PTV. Conformity of adaptive plans done by the dosimetrist as well as the unsupervised automated method using ORBIT Workstation were compared for all 25 patients (Fig 4). The dosimetrist achieved better conformity and less isodose volume (IDV) than automated version in the same dose range. Volumes of PTVexp are shown on the right vertical axis for comparison with the IDV. Although the same PTVs were used for planning, IDV of the automated method was 2mm larger than those of the dosimetrist. Considering IDV95% is 2mm larger than PTV (Fig 4) for IMRT plans by expert, the average volumes of CTV outside of IDV95% were 2.6% (blue dot in Fig 5) and 0.5% (red dot) for the original PTV (IDV95% = PTV+2mm) and enlarged PTV by 5mm (IDV95% = PTV+7mm) when no adaptation was applied. Weekly (full) on-line adaptation with no delay from planning to delivery significantly improved CTV coverage, however off-line adaptation (1 week delay from planning to delivery in this study) could not. Figure 1 Slide from Karen Lim Figure 2. Delivered dose is different from planned dose due to organ deformation & tumour shrinkage over the course of treatment. Planned dose Delivered dose at Week 2 Uterus Cervix/ GTV Vagina PTV Bowel Rectum Bladder Week 0 Week 1 Week 2 Week 3 … No Adaptation week 2 Figure 3. CTV (orange), OAR (green), PTV (sold yellow) and PTVexp (expansion of PTV, dotted yellow) Adapt every week Figure 4. Conformity of IMRT plans. An expert dosimetrist made IMRT plans with better conformity showing less amount of isodose volume than automatic tool does. Lower conformality is believed to be responsible for the inability of OAR sparing for on-line automatic adaptation. PTV 2mm 4mm 7mm 12mm Automatic optimization Expert optimization Ideal dose distribution Added margin from PTV Figure 6. Percentage of OAR inside PTV, IDV95% (47.5Gy, PTV+2mm) and IDV90% (45Gy, PTV+6mm). Adaptation helps to spare OAR for both on-line and off-line adaptation. Effect of PTV margin and conformity on OAR dose sparing is significant. 2 nd week is the best time for single adaptive plan. Figure 5. Larger PTV margin improves target coverage for both on-line and off-line adaptation. Either 1 st or 2 nd week is the best time for single on-line adaptation. Off-line adaptation does not significantly improve target coverage. 1% 2% 3%