Volumetric modulated arc therapy (VMAT) demands high level of precision and reliability from the linear accelerator (LINAC) control system because the gantry rotation is synchronized with multi-leaf collimator (MLC) movement for accurate dose delivery.1,2 Highly modulated dose distribution commonly requires MLC leaves of high speed moving along the whole arc.3–5 However, fast leaf motion during gantry rotation may be affected by interleaf friction or MLC motor problems that result in leaf position errors.6 Wijesooriya et al and Ling
Detecting dosimetric variation caused by MLC leaf errors is one important concern in patient-specific quality assurance (QA). Currently, several dose measuring systems are available for patient-specific QA.8–11 Fredh
Although previous studies have studied both systematic and random MLC leaf errors in VMAT plans, in actual beam delivery, if the difference between actual leaf position and planned one is larger than 2 mm, the LINAC will trigger an MLC interlock which invokes a “beam hold-off”.14 Therefore, the purpose of this study is to evaluate the dosimetric error of clinical VMAT plans caused by MLC leaf position errors that will not trigger MLC interlock (
We selected 11 VMAT plans (Table 1) on three types of targets: anus, brain and prostate. Leaf travel and modulation complexity score (LTMCS) was used to characterize the modulation complexity of each VMAT plan.15 LTMCS ranges from 0 to 1 and it approaches 0 for increasing degree of modulation and increasing total leaf travel distance. In this study, anal VMAT plans had low LTMCS while brain and prostate VMAT plans had moderate to high LTMCS (Table 1).
Volumetric modulated arc therapy (VMAT) plan parameters (dose, gantry speed, Leaf travel and modulation complexity score [LTMCS] and arcs), ranges of the maximal leaf speed, multi-leaf collimator (MLC) leaf position changes along 178 control points (CPs), and total modified leaves percentage of each VMAT arc
Case P: Prostate VMAT cases; B: Brain VMAT cases; A: Anal VMAT cases. | Dose Prescription (Gy/fx × fx) | Gantry speed (deg/s) | LTMCS Leaf travel and modulation complexity score (LTMCS) for a VMAT plan. LTMCS ranges from 0 to 1. Low LTMCS indicates high modulation complexity. | Arc | Maximal MLC leaf speed range along 178 CPs (cm/s) | Range of Leaf Position Changes along 178 CPs (mm) | Total Modified MLC leaves % |
---|---|---|---|---|---|---|---|
P1 | 2.0 × 33 | 4.8 | 0.164 | 1 | 1.87-1.92 | −1.90-1.88 | 1.5 |
P2 | 2.0 × 33 | 4.8 | 0.262 | 1 | 1.87-1.90 | −1.90-2.00 | 1.6 |
2 | 1.87-1.90 | −1.90-2.00 | 1.6 | ||||
P3 | 2.0 × 33 | 4.8 | 0.163 | 1 | 1.87-1.90 | −1.90-2.00 | 1.5 |
2 | 1.87-1.90 | −1.90-2.00 | 1.6 | ||||
B1 | 1.8 × 33 | 4.8 | 0.204 | 1 | 1.87-1.92 | −1.88-1.90 | 1.6 |
B2 | 1.8 × 33 | 4.8 | 0.199 | 1 | 1.87-1.92 | −1.88-1.90 | 1.5 |
2 | 1.87-1.92 | −1.88-1.90 | 1.7 | ||||
B3 | 1.8 × 33 | 4.8 | 0.217 | 1 | 1.85-1.90 | −1.88-2.00 | 1.5 |
2 | 1.85-1.90 | −1.88-2.00 | 1.6 | ||||
A1 | 1.8 × 33 | 4.8 | 0.081 | 1 | 1.90-1.92 | −1.80-1.80 | 1.7 |
2 | 1.90-1.92 | −1.80-1.80 | 1.7 | ||||
3 | 1.90-1.92 | −1.80-1.80 | 1.7 | ||||
4 | 1.90-1.92 | −1.80-1.80 | 1.8 | ||||
A2 | 1.8 × 33 | 4.8 | 0.083 | 1 | 1.92-1.95 | −1.87-1.87 | 1.5 |
2 | 1.92-1.95 | −1.87-1.87 | 1.6 | ||||
3 | 1.92-1.95 | −1.87-1.87 | 1.6 | ||||
4 | 1.92-1.95 | −1.87-1.87 | 1.5 | ||||
A3 | 1.8 × 33 | 4.8 | 0.105 | 1 | 1.92-1.95 | −1.87-1.87 | 1.7 |
2 | 1.92-1.95 | −1.87-1.87 | 1.7 | ||||
3 | 1.92-1.95 | −1.87-1.87 | 1.7 | ||||
A4 | 1.8 × 33 | 4.8 | 0.076 | 1 | 1.87-1.90 | −1.80-1.93 | 2.2 |
2 | 1.87-1.90 | −1.80-1.93 | 2.2 | ||||
3 | 1.87-1.90 | −1.80-1.93 | 2.3 | ||||
4 | 1.87-1.90 | −1.80-1.93 | 2.3 | ||||
A5 | 1.8 × 33 | 4.8 | 0.084 | 1 | 1.90-1.92 | −1.80-1.80 | 1.7 |
2 | 1.90-1.92 | −1.80-1.80 | 1.7 | ||||
3 | 1.90-1.92 | −1.80-1.80 | 1.8 |
LTMCS is derived by the product of leaf travel index (LTi) and modulation complexity score (MCSv).15,16 MCSv was derived by Masi et al to characterize the modulation degree of the MLC leaves of a VMAT plan. MCSv value of 1 indicates no modulation by MLC leaves (
All original treatment plan DICOM files were exported from the Eclipse™ (version 10.0, Varian Medical Systems, Inc., Palo Alto, USA) treatment planning system (TPS). The DICOM plans were modified through an in-house developed software. For each arc in the VMAT plan, in-field moving MLC leaves of 178 control points (CPs) on both banks were selected for leaf speed modifications. Speed of each moving MLC leaf per CP was calculated based on MLC leaf position, gantry rotation angle and gantry speed as shown in Equation [1].
Here ∆t(n) is the gantry rotation time between two adjacent CPs, θ(n) is the gantry angle of CP ‘n’, u(n) is the gantry speed of CP ‘n’, Vleaf (m, n) is the speed of mth leaf of CP ‘n’ and LP(m, n) is the position of mth leaf of CP ‘n’. MLC leaves on bank ‘A’ were marked from 1 to 60 while those on bank ‘B’ were marked from 61 to 120.
For each CP of the arc, leaves on both banks (MLC leaf: 1-120) with the highest speed were set to move at 2.3 cm/s, resulting in a leaf position difference at a maximum of 2 mm (Table 1:
If one leaf was moving with the highest speed at two consecutive CPs, leaf motion direction of each CP was further considered as shown below:
If the motion directions of the next two CPs remained the same, then both leaf positions of corresponding CP were subject to modification (Figure 1A). If the motion directions of the next two CPs were different, we only modified the leaf position of the middle CP so that both leaf speed values would be increased (Figure 1B).
The
In this study, modified plans were considered as ‘standard’ plans where MLC leaves were allowed to move at the maximal speed (2.3 cm/s). The original plans were considered as ‘slowing MLC’ plans where the highest MLC speed was lower than 2.3 cm/s. There were no changes in monitor unit (MU) and gantry speed per CP in all modified VMAT plans.
Having increased the leaf speed of one CP to the maximal limit without triggering the MLC error interlock (
Demonstration of the impact on leaf speed of adjacent control points (CPs) due to leaf modifications
Scenario Scenario A: Leaf moved forward from LP1 to LP2, then moved forward from LP2 to LP3. | LP1 (cm) | LP2 (cm) | LP3 (cm) | LP1-2 Speed (cm/s) | LP 2-3 Speed (cm/s) |
---|---|---|---|---|---|
A (ori) | 4.6 | 5.4 | 5.9 | 1.8 | 1.1 |
A (mod) | 4.6 | 5.6 | 5.9 | ||
B (ori) | 4.6 | 5.4 | 4.9 | 1.8 | −1.1 |
B (mod) | 4.6 | 5.6 | 4.9 |
Scenario B: Leaf moved forward from LP1 to LP2, then moved backward from LP2 to LP3.
In the table,
Because of the high complexity (i.e. low LTMCS) of the anal VMAT plans, we further analyzed MLC leaf changes in these VMAT plans. The
Target dose differences between ‘standard’ and ‘slowing multi-leaf collimator (MLC)’ anal volumetric modulated arc therapy (VMAT) plans, total leave states, and average percentages of modified leaves and faster moving leaves of anal cases
Case | ∆ Negative sign means dose of ‘standard’ plan is lower than that of ‘slowing MLC’ plan CP = control point | ∆ Negative sign means dose of ‘standard’ plan is lower than that of ‘slowing MLC’ plan CP = control point | Total leave states per arc (leaves on both banks/CP)×(CP) | Average modified MLC leaves (%) | Average faster moving leaves (%) |
---|---|---|---|---|---|
A1 | −0.8 | −0.2 | 120 × 178 | 1.7 | 56.5 |
A2 | −0.9 | −0.3 | 120 × 178 | 1.6 | 53.6 |
A3 | −1.2 | −0.5 | 120 × 178 | 1.7 | 51.3 |
A4 | 120 × 178 | ||||
A5 | −1.1 | −0.3 | 120 × 178 | 1.7 | 52.7 |
In this study, we used MapCHECK®2 2D diode array system (Model 1177, Sun Nuclear Co., Melbourne, FL) for evaluating the effect of slowing MLC leaves on planar dose delivery accuracy. MapCHECK®2 along with its software have been widely used as the clinical implementation for patient-specific verification of VMAT plans due to its compact diode size (0.8 mm×0.8 mm), dose linearity, real-time measurement, reproducibility and sensitivity.17–21
All the VMAT plans (‘standard’ and ‘slowing MLC’ plans) were delivered using a Varian Trilogy® LINAC on the same day. Measurement of each arc was then compared with the corresponding calculated planar dose from the TPS with respect to absolute dose Van Dyk distance-to-agreement (DTA) comparison (dose difference is normalized to global maximum) using 3%/3 mm criteria.22 All measurements were repeated on two consecutive days. The uncertainty was then obtained by evaluating the variation in repeated measurements.
According to current pre-treatment IMRT QA method for VMAT plans with MapCHECK®2, measurement of each arc in the ‘standard’ plan was compared with calculated planar dose of the ‘standard’ plan with respect to absolute dose Van Dyk DTA comparison using 3%/3 mm and 2%/2 mm criteria. Pass rate (
Each ‘slowing MLC’ plan was considered as a ‘standard’ plan with MLC leaf errors that would not trigger any MLC interlock to interrupt the beam delivery. In order to evaluate the sensitivity of the IMRT QA method for VMAT plans with non-beam-hold leaf errors, we delivered each ‘slowing MLC’ plan and compared the measurement with calcuated planar dose of the ‘standard’ plan with respect to absolute dose Van Dyk DTA comparison using 3%/3 mm and 2%/2 mm criteria to acquire the pass rate (
The correlations between the decreases in pass rates using 3%/3mm and 2%/2mm criteria and LTMCS were analyzed through Spearman’s correlation coefficient.23
Finally, the 3D dose distribution of each plan was calculated in the TPS and dose-volume histogram (DVH) for targets and organs-at-risk (OAR) were obtained. For clinical dosimetric evaluation, mean target dose (Dmean), dose that covers 95% (D95) of the planning target volume, and Normal Tissue Complication Probability (NTCP) using Lyman Kutcher Burman (LKB) model23 were calculated for all the plans. Clinical dosimetric parameters of ‘standard’ and ‘slowing MLC’ plans were compared using the Wilcoxon signed-rank test.24
Figure 2 demonstrated pass rate of each arc and variation of measurements based on repeated measurements on two consecutive days. Among all the arcs in both ‘standard’ and ‘slowing MLC’ plans, the maximal variation found was 0.3% with respect to the 91.5% pass rate.
For all three prostate cases,
For all three brain cases,
The LTMCS scores of Anal VMAT plans were smaller than both brain and prostate VMAT plans (Table 1:
The correlation between decreases in pass rates of VMAT arcs using 2%/2 mm criteria and LTMCS is moderate to strong (rs = 0.597, Figure 6A). When using 3%/3 mm criteria, the correlation is weak to moderate (rs = 0.453, Figure 6B).
By using lasers and front pointer for device positioning, the measurement setup was of high consistency. Absolute dose calibration for MapCHECK®2 was performed every day before dose measurement.25,26 Therefore, the source of the uncertainty is mainly due to variability of MLC leaf motion. The small error bars in Figure 2 indicate that the measurement variability is very small.
For anal case A4, since the MU of each control point remained unchanged, and ‘slowing MLC’ plan had more slowly moving MLC leaves, more area were being irradiated that resulted in higher dose. The
Moreover, all four arcs of anal case A4 have large fields (e.g. 14 cm ×30 cm, 14 cm ×29 cm, 30 cm ×14 cm, 30 cm ×14 cm for arc 1, 2, 3, 4 respectively). Wijesooriya et al reported the accuracy of RapidArc delivery holds for leaf velocities with small dosimetric uncertainties for 5 mm width MLC leaves which are in the central 20 cm of field.7 They found that three VMAT plans with large MLC leaves with 1cm width at high speed (2.1–2.4 cm/s) demonstrated higher leaf position inaccuracy. Therefore, large MLC leaves in the VMAT plan of anal case A4 have more effect on dose delivery inaccuracy.
When using 3%/3 mm criteria, all 11 cases including ‘standard’ and ‘slowing MLC’ plans passed the institutional 90% acceptance threshold of absolute dose DTA comparison. Dosimetric differences (e.g. ∆
Some arcs in ‘slowing MLC’ plans of anal cases A3 and A5 showed less than 90% pass rates using 2%/2mm criteria although differences in dosimetric parameters are small (e.g. ∆
For ten out of eleven cases, DVH comparisons between ‘standard’ and ‘slowing MLC’ VMAT plans demonstrated minimal dosimetric changes in targets and OAR. Pass rate threshold (90%) using 3%/3 mm criteria is not sensitive in detecting MLC leaf errors that will not trigger the MLC leaf inter-lock. However, the consequential effects on target and OAR are negligible, which supports the reliability of current IMRT QA method for VMAT plan verification.