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Comparability of Four Different Planar IMRT QA Methods

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V Keeling

V Keeling*, S Ahmad, H Jin, University of Oklahoma Health Science Center, OKLAHOMA CITY, AA

TH-C-BRB-11 Thursday 10:30:00 AM - 12:30:00 PM Room: Ballroom B

Purpose: To investigate comparability of four different planar IMRT QA Methods: Patient Gantry Angle Composite (PGAC), Single Gantry Angle Composite (SGAC), Field by Field (FBF), and Patient Gantry Angle Composite using film (FPGAC).

Methods: IMRT verification plans were generated by Varian Eclipse TPS for twelve heavily modulated head and neck (H&N) and twelve moderately modulated Prostate treatment plans. Sun Nuclear MapCHECK2 (2D diode array with detector spacing of 7.07 mm for SGAC, PGAC, and FBF) and GAFCHROMIC EBT2 film (0.34 mm/pixel for FPGAC) respectively sandwiched with 10 cm thick water equivalent phantom were used for the IMRT plan verification. All measurements were analyzed with Sun Nuclear 'Patient' software using the gamma criteria (MapCHECK2/Film) of 1%-1mm/2%-1mm (G1), 2%-2mm/3%-2mm (G2), and 3%-3mm/4%-3mm (G3) considering known 1% inherent dosimetric uncertainty of MapCHECK2. The Student's t-test was used to compare passing rates with 'Absolute Dose' comparison and 10% threshold.

Results: The average passing rates (H&N/Prostate) were 98.9/99.7% (PGAC), 99.6/100.0% (SGAC), 99.5/100.0% (FBF), and 92.0/95.8% (FPGAC) with the G3 criteria. Three MapCHECK2 QAs had statistically comparable passing rates for all comparisons (p-value>0.05), while they did not show any significant difference based on the plan modulation. FPGAC showed systematically lower passing rates than the MapCHECK2 methods and statistically distinguished the degrees of modulation with all gamma criteria (p-values of H&N against Prostate: 0.037 (G1), 0.012 (G2), and 0.016 (G3)).

Conclusions: Three IMRT QA methods using MapCHECK2 are found to be comparably used for routine IMRT QA for both H&N and Prostate plans. This is against the belief that PGAC is more effective IMRT QA than FBF or SGAC. FPGAC showed significantly different passing rate based on the degree of plan modulation. These findings imply that the gamma passing rate heavily depends on the spatial resolution of detectors and there are intrinsic limitations of the gamma analysis.

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