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Comparison of Shallow Fluence to Deep Point Dose Measurements for Spine VMAT SBRT Patient-Specific QA

J Cheung

J Cheung*, M Held , O Morin , B Weethee , C Chuang , A Perez-Andujar , A Sudhyadhom , UC San Francisco, San Francisco, CA


SU-C-BRD-4 (Sunday, July 12, 2015) 1:00 PM - 1:55 PM Room: Ballroom D

Purpose: To investigate the sensitivity of traditional gamma-index-based fluence measurements for patient-specific measurements in VMAT delivered spine SBRT.

Methods: The ten most recent cases for spine SBRT were selected. All cases were planned with Eclipse RapidArc for a TrueBeam STx. The delivery was verified using a point dose measurement with a Pinpoint 3D micro-ion chamber in a Standard Imaging Stereotactic Dose Verification Phantom. Two points were selected for each case, one within the target in a low dose-gradient region and one in the spinal cord. Measurements were localized using on-board CBCT. Cumulative and separate arc measurements were acquired with the ArcCheck and assessed using the SNC patient software with a 3%/3mm and 2%/2mm gamma analysis with global normalization and a 10% dose threshold. Correlations between data were determined using the Pearson Product-Moment Correlation.

Results: For our cohort of patients, the measured doses were higher than calculated ranging from 2.2%-9.7% for the target and 1.0%-8.2% for the spinal cord. There was strong correlation between 3%/3mm and 2%/2mm passing rates (r=0.91). Moderate correlation was found between target and cord dose with a weak fit (r=0.67, R-Square=0.45). The cumulative ArcCheck measurements showed poor correlation with the measured point doses for both the target and cord (r=0.20, r=0.35). If the arcs are assessed separately with an acceptance criteria applied to the minimum passing rate between all arcs, a moderate negative correlation was found for the target and cord (r=-0.48, r=-0.71). The case with the highest dose difference (9.7%) received a passing rate of 97.2% for the cumulative arcs and 87.8% for the minimum with separate arcs.

Conclusion: Our data suggest that traditional passing criteria using ArcCheck with cumulative measurements do not correlate well with dose errors. Separate arc analysis shows better correlation but may still miss large dose errors. Point dose verifications are recommended.

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