PV QA 1 - Poster Viewing Q&A 1
Purpose/Objective(s): The duodenum is often the dose-limiting organ for radiation therapy (RT) of pancreatic cancer, and its separation from the pancreas can vary between daily fractions. Understanding this inter-fractional variation can help in delivering optimal RT that maximizes the dose to the target and/or minimizes the dose to the duodenum by taking advantage of fractions where there is greater separation and using online adaptive re-planning. The aim of this study is to quantitatively characterize inter-fractional variation of the relative motion between the pancreatic head and the duodenum during pancreatic cancer RT.
Materials/Methods: Daily CTs acquired using an in-room CT during routine image-guided RT for 33 patients with pancreas head tumors were analyzed. All patients were treated with chemo-RT of 50.4 Gy in 28 daily fractions. The pancreatic head and duodenum were delineated on the selected daily CTs chosen at regular intervals over the course of treatment for each patient, with a total of 177 daily CTs for all patients studied. The contours were generated by populating the planning contours from the planning CTs to the daily CTs using an auto-segmentation tool with careful editing and independent checking. Spatial separation between the pancreatic head and duodenum was quantified using a modified Hausdorff distance (HD), defined as the maximum distance of a set to the furthest point in the other set. Overlap between the pancreatic head and duodenum was quantified by uniformly expanding the pancreatic head contour by 20 mm and measuring the volume overlap (VO) between the pancreatic-head shell and the duodenum relative to the duodenal volume. The dosimetric benefits of these separations and overlaps were explored.
Results: The inter-fraction variation in the maximum separation between duodenum and pancreatic head for any patient, as measured by HD, was found to be 1.9±0.6 cm. The average HD variation for all patients was 0.9±0.3 cm. The largest HD was 2.4 cm for all the fractions. The minimum VO for any patient was found to be 11.4±4.7%. The average VO for all patients was 25.2±6.4%. The distribution of HD and VO values is presented in the table. For a fraction with HD = 1.7 cm and VO =37%, online re-planning resulted in a reduction in the duodenal volume receiving 50 Gy from 63% to 19%. Such a reduction would allow substantial dose escalation to the target for the fraction with online adaptive RT.
Conclusion: Significant relative movement between the pancreatic head and the duodenum occurs during pancreatic cancer RT and this movement varies from fraction to fraction. In 40% of fractions where HD ≥ 1.0 cm, online adaptive RT may be used to safely escalate the dose to the target by taking advantage of the large separation or small overlap.
|Hausdorff Distance (HD)||Volume Overlap (VO)|
|HD ≥ 1.0 cm||HD ≥ 1.5 cm||HD ≥ 2.0 cm||VO ≤ 15%||VO ≤ 25%||25% < VO ≤ 50%||50% < VO ≤ 75%|
|% of Total Fractions||43.5||15.8||2.3||18.6||57.1||38.4||4.5|
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