PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s): Given the proximity of the target volume to critical structures, radiation therapy for pancreatic cancer is challenging. The present study aimed to estimate planning target volume (PTV) margins of pancreatic tumors using daily image guided radiation therapy (IGRT) and to compare the dosimetric outcomes with and without fiducial markers (with-FM and without-FM) during multiple breath-hold segmented volumetric modulated arc therapy (MBHS-VMAT).
Materials/Methods: Based on re-planned VMAT technique, two types of PTV margins, with-FM or without-FM, were retrospectively estimated in 10 patients with unresected pancreatic cancer. Daily IGRT shifts based on MBHS-cone-beam volume CT obtained prior to daily fractionated MBHS-VMAT and cone-beam motion CT under fractionated MBHS-VMAT were recorded and evaluated to estimate the two PTV margin types. Accordingly, 10 patients, 270 3D images, and 1080 2D images were considered. Appropriate margins were estimated using the equation M = 2.5Σ + 0.7σ. Regarding the treatment plans, the organs at risk (OAR) included the kidneys, liver, stomach, duodenum, jejunum, and spinal cord. The prescribed dose was 55 Gy in 25 fractions and was set to encompass 95% of the PTV. For both the plans, the planning objectives used in the optimizer were constant and were based on NCCN guidelines. Homogeneity and conformity indices, doses to OARs with attention to the digestive tract (stomach, duodenum, and jejunum), and PTV doses were evaluated between the two margin types.
Results: The respective PTV margins in the lateral, vertical, and longitudinal directions were 4, 4, and 5 mm with-FM and 7, 6, and 10 mm without-FM. For each patient, treatment planning implied two PTV margin types. For the digestive tract, a consistent trend indicated a significantly (p <0.05) lower V45, V50, and V55 using the with-FM MBHS-VMAT technique; these respective values decreased by 4.9%, 3.8% and 2.0% using with-FM MBHS-VMAT plans. There were no differences (p >0.05) in digestive tract (mean), PTV, liver dose (mean), and total kidney (V18) between the two margin types. Conformity and homogeneity indexes were also similar.
Conclusion: The pancreas carries the potential for significant motion. In the absence of fiducial markers, our data suggest that the PTV margin will increase from 5 to 10 mm. These margins would surely overlap the surrounding bowel, duodenum, and/or stomach, suggesting that the dose coverage of margin-based PTVs is not be feasible. Our comparison shows that with-FM plans provide superior OAR-sparing abilities (particularly for the digestive tract) without decreasing target coverage. Table1 Comparison of various parameters for the two margin types
|Digestive tract||V45||8.4 %||13.3 %||< 0.05|
|V50||4.2 %||8.0 %||< 0.05|
|V55||1.1 %||3.1 %||< 0.05|
|Dmean, digestive tract (Gy)||22.1||25.6||0.118|
|Liver||Dmean, liver (Gy)||8.4||10.3||0.190|
|Kidneys||V18||14.6 %||16.6 %||0.668|
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