Gamma Passing Rate – Correlation with Patient Specific Quality Assurance Devices using Two Photon energies in Pelvic IMRT

Abstract

The effect on Gamma Passing Rate is studied by changing the Photon Energy in the Patient Specific Quality Assurance in Pelvic IMRT using EPID and MatriXX keeping PSQA device at Isocenter. Dose verification of 62 patients, suffering from pelvic malignancies and treated with Intensity modulated radiation therapy (IMRT), with EPID and MatriXX using gamma criteria of 3%/3 mm, 3%/2 mm, 2%/3 mm, 2%/2 mm and investigated the effect on Gamma Passing Rate (%GP) by changing beam energy from 6 MV to 10 MV. The results demonstrated that the behavior of gamma passing rate decreases as criteria is changed from 3%/3 mm to strict criteria. The mean gamma passing rate for 6 MV beam were (%GP±Standard Deviation σ) 99.64±0.81, 99.45±1.08, 97.57±2.12, 96.17±2.63 using the criteria 3%/3 mm, 3%/2 mm, 2%/3 mm, 2%/2 mm respectively in EPID. For MatriXX, the mean %GP were 96.69±4.11, 93.85±5.31, 88.17±10.63, 82.2±12.28 respectively. For 10 MV, the mean (%GP±Std. Dev) were 99.64±0.81, 99.45±1.08, 97.57±2.12, 96.17±2.63 for in EPID, 96.69±4.11, 93.85±5.31, 88.17±10.63, 82.2±12.28 for MatriXX respectively. Difference between %GP for 6 MV and 10 MV beam were -0.01, -0.11, 1.08, 1.61 with gamma criteria of 3%/3 mm, 3%/2 mm, 2%/3 mm, 2%/2 in EPID. For MatriXX, difference between %GP for 6 MV and 10 MV beam were -0.40, -0.24, 0.96, 1.62 respectively. It can be concluded that applying more strict gamma criteria results in low gamma passing rate. There is marked difference in Gamma Passing Rate (%GP) with change in photon energy.