Modulation index for VMAT considering both mechanical and dose calculation uncertainties

Abstract

The aim of this study is to present a modulation index considering both mechanical and dose calculation uncertainties for volumetric modulated arc therapy (VMAT). As a modulation index considering only mechanical uncertainty of VMAT, MIt has been previously suggested. In this study, we developed a weighting factor which represents dose calculation uncertainty based on the aperture shapes of fluence maps at every control point of VMAT plans. In order to calculate the weighting factor, the thinning algorithm of image processing techniques was applied to measure field aperture irregularity. By combining this weighting factor with the previously suggested modulation index, MIt, comprehensive modulation index (MIc) was designed. To evaluate the performance of MIc, gamma passing rates, differences in mechanical parameters between plans and log files and differences in dose-volume parameters between plans and the plans reconstructed from log files were acquired with a total of 52 VMAT plans. Spearman's correlation coefficients (rs) between the values of MIc and measures of VMAT delivery accuracy were calculated. The rs values of MIc (f = 0.5) to global gamma passing rates with 2%/2 mm, 1%/2 mm and 2%/1 mm were -0.728,-0.847 and -0.617, respectively (p< 0.001). Those to local gamma passing rates were -0.765,-0.767 and -0.748, respectively (p < 0.001). The rs values of MIc (f = 0.5) to multi-leaf collimator and gantry angle errors were 0.800 and -0.712, respectively (p < 0.001). The MIc (f = 0.5) showed a total of 20 rs values (p < 0.05) to the differences in dose-volumetric parameters from a total of 35 tested cases. The MIc (f = 0.5) demonstrated considerable power to predict VMAT delivery accuracy showing strong correlations to various measures of VMAT delivery accuracy.

Correlation analysis between 2D and quasi-3D gamma evaluations for both intensity-modulated radiation therapy and volumetric modulated arc therapy

“…The IMRT technique achieves optimal dose distributions by modulating multi-leaf collimator (MLC) positions while VMAT generates optimal dose distributions by modulating MLC positions, gantry rotation speeds and dose rates, simultaneously [2]. These modulated delivery techniques can involve large uncertainties in the treatment planning process as well as beam delivery [3–5]. Therefore, patient-specific quality assurance (QA) before treatment has been strongly recommended for both IMRT and VMAT to verify plan delivery accuracy [6–10].…”

“…However, this methodology has inherent limitations because the verification system and the beam delivery system are not independent of each other. The other approach is to calculate modulation indices [4, 5, 20–24]. The modulation index has limitations since it is based on the calculation with parameters acquired from treatment plans not the measurement.…”

Gamma Evaluation with Portal Dosimetry for Volumetric Modulated Arc Therapy and Intensity-Modulated Radiation Therapy

“…4,5) Moreover, calculations of the small or irregular fields which are frequently used for both IMRT and VMAT are not accurate even with the state-of-theart dose calculation algorithms. 7,8) Therefore, there is potential for both IMRT and VMAT to cause differences between the calculated dose distribution and the actually delivered dose distribution to a patient. 3) This can result in unintended treatment which is detrimental to patients, which should be avoided.…”

Dosimetric characteristics of a reusable 3D radiochromic dosimetry material

“…Recently, several studies questioned the clinical relevance of 2D gamma passing rates [6–8]. Nelms et al .…”

“…Park et al . also demonstrated that 2D gamma evaluation likely does not provide sufficient information for detecting small delivery errors in VMAT [8]. Kim et al .…”