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Evaluation of Dose Redistribution Due to Deformation with 3D MRI Gel Dosimetry
C Matrosic*, I Marsh , W Culberson , B Bednarz , University of Wisconsin, Madison, WI
Presentations
SU-I-GPD-T-485 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Recently, many image-guided radiotherapy (IGRT) systems have been developed to manage tumor motion during treatment. One robust validation method is 3D dosimeters that can incorporate dose redistribution due to deformation. This study investigates pairing novel, deformable, polyacrylamide-based gel phantoms with MRI data acquisition to quantify this redistribution due to static deformation.
Methods: nPAG (6%T) was used to fabricate two gel phantoms enclosed by 7.5-cm diameter, 8-cm thick, cylindrical acrylic tubing and nitrile windows at the ends to permit deformation. Identical five-beam, coplanar treatments delivered approximately 6 Gy to the center of each phantom. To represent tissue deformation, a sphere depressed the upper window of one phantom by approximately 1 cm; while the other phantom was unaltered. The undeformed phantom was used to calibrate 3D MRI transverse relaxation rate (R₂) maps to dose. Dose distributions were compared through dose difference maps, dose profiles, and γ-analysis
Results: The deformed phantom showed an offset dose distribution when compared to the reference. This resulted in approximately a 5-Gy underdosing in the inferior section and a 5-Gy overdosing in the superior section, caused by deformation shifting these sections out of and into the irradiated planes. This was confirmed by dose profiles along the deformation axis. Rotation due to deformation caused lateral regions to show 1-Gy underdosing and overdosing. A 3%/3 mm global γ-analysis comparing the deformed phantom to the planned dose showed only a 66.0% pass rate.
Conclusion: This study shows that deformable gel phantoms paired with MRI can be used to quantify the redistribution of dose in treatment plans due to deformation, which can cause dose errors in treatment delivery. With the incorporation of less rigid walls, this system could feasibly be molded into anthropomorphic shapes and incorporated into dynamically deformable phantoms to accurately quantify the redistribution of doses in clinical treatments.
Funding Support, Disclosures, and Conflict of Interest: This work was partially funded by NIH grant R01CA190298.
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