SU-E-T-386: Gamma Analysis of Normalized and Un-Normalized Dose Distributions

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Current gamma index methods in commercial QA software require selecting a normalization point.
• This normalization point choice leads to multiple possible solutions for dose distribution agreement.
• A unique solution is achieved by omitting the normalization point.

Purpose of the Study:

• To evaluate the impact of using a normalization point versus no normalization point in the gamma index method.
• To assess the agreement between calculated and delivered dose distributions using AAPM TG119 test cases and clinical data.
• To determine the suitability of common gamma index criteria (3%/3 mm) without normalization.

Main Methods:

• AAPM TG119 test cases were planned and delivered using Pinnacle 8.0m and a Varian 21EX linac (6 and 18 MV photons).
• Point and planar dose measurements were performed using ion chamber, film, and MatriXX.
• Gamma index analysis (3%/3 mm) was conducted with and without a normalization point on both benchmark and clinical data.

Main Results:

• With normalization, gamma passing rates for TG119 complex tests were high (95.8±1.6% for 6 MV, 95.6±1.0% for 18 MV).
• Without normalization, gamma passing rates significantly dropped (20.7±6.7% for 6 MV, 13.9±4.0% for 18 MV).
• Clinical data showed a similar trend, with passing rates decreasing from 98.9±0.7% to 33.4±13.1% without normalization.

Conclusions:

• The gamma index method yields a unique gamma passing rate solely when dose distributions are not normalized.
• The standard 3%/3 mm gamma criteria is an insufficient metric when no normalization point is used.
• Further investigation into appropriate gamma criteria for non-normalized dose comparisons is warranted.