Modulation indices for volumetric modulated arc therapy

Abstract

The aim of this study is to present a modulation index (MI) for volumetric modulated arc therapy (VMAT) based on the speed and acceleration analysis of modulating-parameters such as multi-leaf collimator (MLC) movements, gantry rotation and dose-rate, comprehensively. The performance of the presented MI (MIt) was evaluated with correlation analyses to the pre-treatment quality assurance (QA) results, differences in modulating-parameters between VMAT plans versus dynamic log files, and differences in dose-volumetric parameters between VMAT plans versus reconstructed plans using dynamic log files. For comparison, the same correlation analyses were performed for the previously suggested modulation complexity score (MCS(v)), leaf travel modulation complexity score (LTMCS) and MI by Li and Xing (MI Li&Xing). In the two-tailed unpaired parameter condition, p values were acquired. The Spearman's rho (r(s)) values of MIt, MCSv, LTMCS and MI Li&Xing to the local gamma passing rate with 2%/2 mm criterion were -0.658 (p < 0.001), 0.186 (p = 0.251), 0.312 (p = 0.05) and -0.455 (p = 0.003), respectively. The values of rs to the modulating-parameter (MLC positions) differences were 0.917, -0.635, -0.857 and 0.795, respectively (p < 0.001). For dose-volumetric parameters, MIt showed higher statistically significant correlations than the conventional MIs. The MIt showed good performance for the evaluation of the modulation-degree of VMAT plans.

The effect of beam interruption during VMAT delivery on the delivered dose distribution

“…Therefore, H&N VMAT plans were highly modulated than did the prostate VMAT plans in this study [10]. Tables 1 and 2 show the global and local gamma passing rates of 0 times, 4 times and 8 times of VMAT plans with various gamma criteria, respectively.…”

“…The prescription dose to target 1, target 2 and target 3 were 67.5 Gy (daily 2.25 Gy), 54 Gy (daily 1.8 Gy), and 48 Gy (daily 1.6 Gy), respectively. To evaluate modulation degree of prostate and H&N VMAT plans, modulation index was calculated for each VMAT plan [10].…”

Textural feature calculated from segmental fluences as a modulation index for VMAT

“…Intensity modulated radiation therapy modulates photon beam intensities by modulating multi-leaf collimator (MLC) positions, while VMAT modulates photon intensities by modulating MLC positions, gantry rotation speed and dose-rate, simultaneously [1,4,7]. Although modulation of photon beam intensity is essential for both IMRT and VMAT, excessive modulation results in differences between calculated dose, which is the dose intended to be delivered to the patient, and the dose which is actually delivered to the patient [8][9][10][11][12][13][14][15][16][17][18][19]. Excessive modulation increases uncertainties in the mechanical operation of linac components, as well as increases the use of small or irregular MLC apertures of which dose calculation with commercial treatment planning system (TPS) is currently inaccurate as compared to that of large or regular MLC apertures [14,16,17].…”

“…Although modulation of photon beam intensity is essential for both IMRT and VMAT, excessive modulation results in differences between calculated dose, which is the dose intended to be delivered to the patient, and the dose which is actually delivered to the patient [8][9][10][11][12][13][14][15][16][17][18][19]. Excessive modulation increases uncertainties in the mechanical operation of linac components, as well as increases the use of small or irregular MLC apertures of which dose calculation with commercial treatment planning system (TPS) is currently inaccurate as compared to that of large or regular MLC apertures [14,16,17]. Therefore, pre-treatment quality assurance (QA) for each patient being treated with IMRT or VMAT is routinely performed in the clinic to detect clinically unacceptable discrepancies in dose distributions between calculation and delivery due to excessive modulation [20][21][22][23][24][25][26][27].…”

“…Besides the gamma-index method and log file analysis, several modulation indices have been suggested as alternative QA methods for both IMRT and VMAT [8,9,[11][12][13][14][15][16][17][18][19]. One of the benefits of using a modulation index is the reduction in the use of clinical resources, as the index could be calculated at the planning stage.…”