Adaptive versus non-adaptive MRI-guided radiation treatments for pancreatic cancer: A dosimetric study.

Abstract

336 Background: Combination MRI and radiation therapy systems enable magnetic resonance image guided radiation therapy (MR-IGRT). MR-IGRT allows clear visualization of the target and organs at risk (OARs) allowing for dose adaptation. Using adaptive MR-IGRT with Cobalt-60 for stereotactic body radiation therapy (SBRT) in locally advanced pancreatic cancer (LAPC), we hypothesized that MR-IGRT would improve dose to pancreatic tumor without increasing doses to OARs. Methods: Ten LAPC patients received five fraction SBRT with a total dose of 33-40 Gy. For each fraction, the original plan dose was compared to the dose that would be delivered if the original radiotherapy plan was applied to the anatomy that day (non-adaptive). An adaptive plan was then created for each fraction. The plan was re-optimized based on the anatomy as seen on the daily MRI and re-normalized so the volume of the PTV receiving 100% of the prescription (PTV100) would be 90%. Both the non-adaptive and adaptive doses to the target volume and the OARs were recorded to evaluate the value of adaptation. We used a paired t-test to compare PTV100 between the adaptive and non-adaptive techniques and Chi2 tests to compare the probability of dose constraint failures for OARs. Results: Adaptive MR-IGRT improved target coverage. Mean PTV100 for adaptive and non-adaptive techniques was 89.9% [88.4-90.4] and 78.4% [27.3-96.6] respectively, p = 0.0017. There were no statistically significant differences for violations of dose constraints of OARs using adaptive vs. non-adaptive techniques. Point maxima violations above 35 Gy to duodenum occurred in 6 adaptive fractions (renormalized to 90%) vs. 12 non-adaptive fractions (p = 0.118); to stomach in 8 adaptive fractions vs. 9 non-adaptive fractions (p = 0.790), and to bowel in 9 adaptive fractions vs. 6 non-adaptive fractions (p = 0.401). When adapting, attention must be paid to other OARs in the area: Spinal cord point maxima were violated in 4 adaptive fractions. Conclusions: This study demonstrates that adaptive techniques significantly increase SBRT dose delivered to LAPC without significantly increasing dose constraint violations to OARs. Adaptive MR-IGRT may allow for further SBRT dose escalations.