Gamma analysis dependence on specified low‐dose thresholds for VMAT QA

Abstract

The American Association of Physicists in Medicine Task Group 119 instructed institutions to use a low‐dose threshold of 10% or a region of interest determined by the jaw setting when they collected gamma analysis quality assurance (QA) data for the planar dose distribution. However, there are no clinical data to quantitatively demonstrate the impact of the low‐dose threshold on the gamma index. Therefore, we performed a gamma analysis with various low‐dose thresholds in the range of 0% to 15% according to both global and local normalization and different acceptance criteria (3%/3 mm, 2%/2 mm, and 1%/1 mm). A total of 30 treatment plans — 10 head and neck, 10 brain, and 10 prostate cancer cases — were randomly selected from the Varian Eclipse treatment planning system (TPS). For the gamma analysis, a calculated portal image was acquired through a portal dose calculation algorithm in the Eclipse TPS, and a measured portal image was obtained using an electronic portal‐imaging device. Then, the gamma analysis was performed using the Portal Dosimetry software (Varian Medical Systems, Palo Alto, CA). The gamma passing rate (%GP) for the global normalization decreased as the low‐dose threshold increased, and all low‐dose thresholds led to %GP values above 95% for both the 3%/3 mm and 2%/2 mm criteria. However, for the local normalization, %GP for a low‐dose threshold of 10% was 7.47%, 10.23%, and 6.71% greater than the low‐dose threshold of 0% for head and neck, brain, and prostate for the 3%/3 mm criteria, respectively. The results indicate that applying the low‐dose threshold to global normalization does not have a critical impact on patient‐specific QA results. However, in the local normalization, the low‐dose threshold level should be carefully selected because the excluded low‐dose points could cause the average %GP to increase rapidly.PACS number: 87.55.Qr

IMRT QA: Selecting gamma criteria based on error detection sensitivity

“…37 Nelms et al hypothesized that decreasing the low dose threshold in conjunction with global normalization of the data may lead to an increase in the passing rate, causing decreased sensitivity of the test 22 and findings from Song et al partially confirm this for VMAT cases measured using EPID and dose thresholds ranging from 0% to 15%. 46 A study by Wu et al also made use of portal dosimetry QA measurements and looked at the sensitivity F. 12.…”

Retrospective analysis of portal dosimetry pre-treatment quality assurance of prostate volumetric-modulated arc therapy (VMAT) plans

“…The %GP does not seem to depend on the low-dose threshold values irrespective of the gamma criteria used. This is because the normalisation of dose difference in improved gamma calculation was with the maximum reference dose which is relatively higher than the dose difference in the low-dose region 17 . The SD values increase by increasing the threshold and tightening the acceptance criteria from 3%/3 mm to 1%/1 mm.…”

“…As the threshold increases from 0 to 15%, the mean %GP increases by 13, 27 and 47% for the 3%/3 mm, 2%/2 mm and 1%/1 mm criteria, respectively. This is due to the fact that the dose difference is normalised to the currently evaluated pixel for local gamma calculation and therefore, the pixels in low-dose regions usually fail the acceptance criteria 17 . After a threshold of 10%, the %GP does not seem to depend on threshold irrespective of the gamma criteria used.…”

Can clinically relevant dose errors in patient anatomy be detected by gamma passing rate or modulation complexity score in volumetric-modulated arc therapy for intracranial tumors?

“…The looser constraint would result in better GPR than more severe criteria such as 2%/2 mm or 1%/1 mm. The value of GPR is also dependent on the specified low-dose threshold, which has an impact when local gamma analysis is performed [ 17 ]. Moreover, Nelms et al .…”