Time-resolved dose reconstruction by motion encoding of volumetric modulated arc therapy fields delivered with and without dynamic multi-leaf collimator tracking

Figures & data

Figure 1. A representative example of measured (M) and reconstructed (R) doses for the high modulation volumetric modulated arc therapy (VMAT) lung plan delivered with the high frequency lung motion trajectory. The dose in the central 6 × 6 cm of one of the biplanar arrays is displayed for the first three VMAT sub-arcs (i.e. first three tenths of the VMAT field) as well as accumulated for the entire VMAT field. The three columns show the dose of a static phantom (left), a moving phantom without tracking (middle), and a moving phantom with tracking (right). One diode was broken (marked with a black arrow) and invariably showed zero in the measured dose. The red arrow marks an example of dose redistribution due to interplay effects in this specific VMAT sub-arc.

Figure 2. Measured (curves) and reconstructed (symbols) cumulative doses in three sample diodes in the Delta4PT phantom: the diode in the center of the phantom (blue), a diode in a high dose gradient (green), and a diode in the periphery of the phantom (red). Doses are shown for the static phantom (full line) as well as the moving phantom without (dashed line) and with (dotted line) tracking. Top: high and low modulation lung VMAT plans with the high frequency lung tumor motion trajectory. Bottom: high and low modulation prostate VMAT plans with the Continuous drift prostate motion trajectory. The central and peripheral diodes were the same for all plots, whereas the high dose gradient diode differed between lung and prostate VMAT plans.

Table I. Results of dose reconstruction quantified by dose differences to the measured dose.

Plan	Static	Motion, no tracking	Motion, tracking	All
Dose for VMAT sub-arcs
Lung, low modulation	2.5%	2.3 [2.0,2.6]%	2.3 [2.2,2.5]%	2.4 [2.0,2.6]%
Lung, high modulation	2.6%	2.8 [2.4,3.2]%	2.5 [2.3,2.7]%	2.7 [2.3,3.2]%
Prostate, low modulation	1.5%	1.6 [1.5,1.7]%	1.6 [1.5,1.9]%	1.6 [1.5,1.9]%
Prostate, high modulation	1.7%	1.9 [1.7,2.0]%	1.8 [1.7,2.0]%	1.8 [1.7,2.0]%
Accumulated dose for complete arc field
Lung, low modulation	1.9%	1.7 [1.5,2.0]%	1.7 [1.5,1.8]%	1.7 [1.5,2.0]%
Lung, high modulation	2.1%	2.2 [1.9,2.3]%	2.0 [2.0,2.1]%	2.1 [1.9,2.3]%
Prostate, low modulation	0.8%	0.8 [0.8,0.9]%	0.8 [0.7,0.9]%	0.8 [0.7,0.9]%
Prostate, high modulation	1.6%	1.8 [1.7,2.0]%	1.9 [1.7,2.1]%	1.8 [1.6,2.1]%

Mean [and range] of root-mean-square dose differences between measured and reconstructed dose distributions in percentage of the maximum measured dose, averaged over the four trajectories investigated for each plan. Results are shown both for the dose delivered in each VMAT sub-arc (i.e. each tenth of the VMAT field, top four rows) and for the complete arc field (bottom four rows).

Table II. Results of dose reconstruction quantified by γ-tests using measured dose as reference.

Plan	Static	Motion, no tracking	Motion, tracking	All
Mean γ failure rates for VMAT sub-arcs
Lung, low modulation	4.6%	3.4 [2.8,3.9]%	3.6 [3.3,4.5]%	3.6 [2.8,4.6]%
Lung, high modulation	2.6%	3.1 [2.4,3.6]%	1.8 [1.5,2.2]%	2.5 [1.5,3.6]%
Prostate, low modulation	0.6%	0.7 [0.5,0.9]%	0.8 [0.5,1.1]%	0.8 [0.5,1.5]%
Prostate, high modulation	0.9%	1.5 [1.3,2.0]%	1.1 [0.9,1.4]%	1.1 [0.8,2.0]%
γ failure rates for complete arc field
Lung, low modulation	1.3%	0.5 [0.3,0.8]%	0.8 [0.3,1.0]%	0.7 [0.3,1.3]%
Lung, high modulation	2.3%	2.3 [1.4,2.7]%	1.5 [0.9,2.1]%	2.1 [0.9,2.7]%
Prostate, low modulation	0.0%	0.0 [0.0,0.0]%	0.0 [0.0,0.0]%	0.0 [0.0,0.0]%
Prostate, high modulation	0.1%	0.1 [0.0,0.6]%	0.1 [0.0,0.3]	0.1 [0.0,0.6]%

Mean [and range] of 3%/3 mm γ failure rates of reconstructed dose distributions compared with measurements, averaged over the four trajectories investigated for each plan. Results are shown averaged over the ten individual VMAT sub-arcs (top four rows) and for the complete arc field (bottom four rows).

Figure 3. 3%/3 mm γ failure rates in the final accumulated dose delivered to the moving target with (closed symbols) and without (open symbols) dynamic MLC tracking for lung (squares) and prostate (circles) VMAT plans. The reference dose for the γ-test was the dose to the static phantom. The scatter plot compares the γ failure rate in the measured doses with the γ failure rate in the reconstructed doses for the 32 combinations of VMAT treatment plans, motion trajectories, and tracking or no tracking. The line of unity is shown in black.