Influence of different sampling depths in a two-dimensional ionization chamber matrix on the γ pass rate of intensity-modulated radiation therapy

Abstract

In order to study the influence of different sampling depths in a two-dimensional ionization chamber matrix on the γ pass rate, we compared and analyzed the different depth dose in the quality assurance (QA) plan and the measurement levels of the two-dimensional ionization chamber matrix (MatriXX). A total of 21 cases of nasopharyngeal carcinoma IMRT plan were selected, and the CT images of MatriXX and Miniphantom phantoms were imported into the planning system as the QA phantom. We transplanted the intensity-modulated radiation therapy plan to the QA phantom to recalculate the dose and selected the plane dose with 6 dose levels at a height of 5 mm from the top to the bottom of the ionization chamber cavity at an equal interval of 1 mm. Then, we exported the dose distribution file of the corresponding plane to the MatriXX system and analyzed the measured dose results to obtain 6 sets of results, and recorded the γ pass rate under the 3 mm/3%, 2 mm/2%, and 2 mm/2% evaluation standards. The results show that the pass rate of 3 mm/3% at the dose level of Compare4, Compare5, and Compare6 was greater than 95.00%, which met the clinical requirements; the pass rate of 3 mm/2% at the dose level of Compare6 was greater than 95.00%, meeting the clinical requirements. The pass rate was increased from 81.45% of Compare1 to 98.08% of Compare6 by 3 mm/3%, which was an increase of 16.63%; as well as an increase of 21.13% by 3 mm/2% from 74.27% of Compare1 to 95.40% of Compare6; and also an increase of 29.56% by 2 mm/2% from 55.15% of Compare1 to 84.71% of Compare6. The level of Compare3 was the level corresponding to the height of the cross-marker line on the side of MatriXX. The dose at this level was taken as the reference group, and the results of the other groups were statistically significant when compared with the dose at this level (p<0.05). When doing QA of an IMRT plan, the dose plane at a depth of the bottom layer of the two-dimensional ionization chamber matrix was used as the reference dose plane for the verification of the IMRT plan γ pass rate, which has the highest pass rate compared with the actual measured value.

Keywords

• intensity-modulated radiation therapy (imrt)
• two-dimensional ionization chamber matrix
• γ pass rate
• quality assurance