Categories:Medical Physics

IQM's superior error sensitivity

Volumetric modulated arc therapy (VMAT) can achieve highly conformal dose distributions with improved target volume coverage and sparing of normal tissues.

Ladies and Gentlemen,

Volumetric modulated arc therapy (VMAT) can achieve highly conformal dose distributions with improved target volume coverage and sparing of normal tissues. However, the use of complicated dose distributions, continually varying dose rates, multi-leaf collimator (MLC) shapes, and gantry speeds, requires an understanding of each parameter’s potential error on the final dosimetric effect.

Masahide Saito and his colleagues at the University of Yamanachi compared the error sensitivity of four systems for VMAT pre-treatment quality assurance: The Scandidos Delta4, the PTW Octavius, and a PTW Farmer Chamber inserted into a solid phantom, as well as the IQM System.

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IQM’s superior error sensitivity

Nine retrospective VMAT plans were used: three head and neck boost plans (30 Gy / 15 fractions), three lung stereotactic body radiation therapy (SBRT) plans (50 Gy / 4 fractions), and three prostate plans (76 Gy / 38 fractions). To investigate the MLC error sensitivities, nine plans with MLC errors were generated for each of the original nine VMAT plans.

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MLC errors introduced into VMAT plans

Because the accuracy of MLC position and movement affects the dose prescription accuracy, MLC QA is required for complex treatment techniques (IMRT, VMAT, etc.).

Several reports have evaluated the sensitivity of different QA devices to MLC errors. For example, Vieillevigne et al. investigated the sensitivity of three gamma-analysis-based devices (ArcCheck, 2D-array, and EPID) to gantry and MLC errors, and concluded that pretreatment QA with these devices may not identify delivery errors, even using strict criteria (2% / 2 mm). In addition, Oliver et al. reported that open/closed MLC errors tended to have a greater effect on the clinical dose, compared to random or systematic MLC shift errors.

“The IQM System has significant advantages in detecting systematic MLC errors”

M. Saito, et al; Comparison of MLC error sensitivity of various commercial devices for VMAT pre-treatment QA

This study by Saito and his colleagues indicates that the IQM System’s large sensitive volume gives the IQM a significant advantage in detecting systematic MLC error. The other devices tested in this study could not detect systematic MLC errors especially in cases with a small range of MLC error. Click here to read the full publication.

 

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