Encrypted login | home

Program Information

The Influence of Steep Dose Gradients On the Sensitivity to Errors of IMRT QA Analyzed with Gamma

J Steers

J Steers1,2*, B Fraass1, (1) Department of Radiation Oncology, Cedars-Sinai Medical Center, Los Angeles, CA, (2) Department of Physics and Biology in Medicine, University of California-Los Angeles, Los Angeles, CA


SU-I-GPD-T-266 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the insensitivity of gamma comparisons to induced errors and the alarmingly low number of points contributing to error sensitivity in IMRT QA comparisons.

Methods: MU and MLC errors were induced in plan calculations for test fields and copies of real patient plans. Using an in-house MATLAB program, gamma analyses were performed for comparisons between error-induced calculations and calculations without induced errors, with all plans calculated at 1mm spacing for the ArcCHECK® (Sun Nuclear, Melbourne, FL). Maps of gamma values were compared to the dose and dose-gradient maps to identify regions in comparisons that were sensitive to induced errors. The number of points that reside in more highly sensitive regions of the dose distribution were recorded in order to evaluate the number of useful points for detecting errors in composite dose distributions using the gamma comparison.

Results: High gradient regions in the IMRT QA comparisons were found to often have low gamma values, even in the presence of large MU and MLC errors. While high-dose/low-gradient regions fail the gamma comparison first (and this may be desirable) we found that some plans had as few as 3/1386 in-field/low-gradient diodes (0.2% of all diodes) available for evaluation (average=5.9% of all diodes, n=21 cases). Additionally, as many as 52% of available comparison points fell along high gradients, further limiting IMRT QA sensitivity. Locally normalized comparisons also exhibited low gamma values along steep dose gradients, suggesting this behavior is normalization independent.

Conclusion: Dose comparison points along high gradients tend to fail last when analyzed using the gamma comparison method, which may partially explain the insensitivity of IMRT QA results given the numerous overlapping high dose gradients for composite measurements. Higher spatial sampling in measurements, development of new comparison techniques, or both, may be required for more meaningful IMRT QA comparisons.

Contact Email: