Film measurement and analytical approach for assessing treatment accuracy and latency in a magnetic resonance-guided radiotherapy system

Abstract

Purpose: We measure the dose distribution of gated delivery for different target motions and estimate the gating latency in a magnetic resonance-guided radiotherapy (MRgRT) system.

Method: The dose distribution accuracy of the gated MRgRT system (MRIdian, Viewray) was investigated using an in-house-developed phantom that was compatible with the magnetic field and gating method. This phantom contains a simulated tumor and a radiochromic film (EBT3, Ashland, Inc.). To investigate the effect of the number of beam switching and target velocity on the dose distribution, two types of target motions were applied. One is that the target was periodically moved at a constant velocity of 5 mm/s with different pause times (0, 1, 3, 10, and 20 s) between the motions. During different pause times, different numbers of beams were switched on/off. The other one is that the target was moved at velocities of 3, 5, 8, and 10 mm/s without any pause (i.e., continuous motion). The gated method was applied to these motions at MRIdian, and the dose distributions in each condition were measured using films. To investigate the relation between target motion and dose distribution in the gating method, we compared the results of the gamma analysis of the calculated and measured dose distributions. Moreover, we analytically estimated the gating latencies from the dose distributions measured using films and the gamma analysis results.

Results: The gamma pass rate linearly decreased with increasing beam switching and target velocity. The overall gating latencies of beam-hold and beam-on were 0.51 ± 0.17 and 0.35 ± 0.05 s, respectively.

Conclusions: Film measurements highlighted the factors affecting the treatment accuracy of the gated MRgRT system. Our analytical approach, employing gamma analysis on films, can be used to estimate the overall latency of the gated MRgRT system.

Keywords: gating method; latency; magnetic resonance-guided radiotherapy; quality assurance.

FIGURE 1

External appearance of the newly developed phantom. Shown are the phantom body, the rod phantom inserted into the phantom body, and a driving system (Viewray Dynamic Phantom). The phantom body comprises a water‐fillable acrylic shell and driving system attached to the rod phantom

FIGURE 2

In‐house developed rod phantom: (a) picture of the rod phantom. (b) Inner view of the rod phantom; the two spaces inside the rod phantom accommodating the simulated target (c), and the film is designed to measure the dose distributions. (c) Simulated target containing four cylinders with different diameters (0.5, 1, 2, and 3 cm). (d) Example of target tracking in cine MRI. The white circle is the tracked target, the exterior red line shows the autocontouring of the tracking target by the MRIdian system, and the outermost green line is the boundary (at 3 mm from the tracking target) that triggers switching between beam‐hold and ‐on

FIGURE 3

Example of the treatment plan. White parts are the water‐filled areas, and the black parts are the acrylic shell and rod phantom. In this treatment plan, the field size is 4.2 × 4.2 cm² and the prescribed dose to the film is 3.5 Gy

FIGURE 4

Example of a sawtooth wave applied to the target motion as the phantom was moved in the superior–inferior (SI) direction. The origin of the phantom's position is at 0 mm on the vertical axis and the lowest position is −10 mm, meaning that the phantom moved by 10 mm in the inferior direction. The pause time defines the interval between periodic motions of the target

FIGURE 5

Ranges of calculated dose distributions of a moving target at beam‐hold, where the target moves outside the boundary, and beam‐on, where the target moves within the boundary. y_1‐0: origin of motion (0 mm); y_1‐1: coordinate at which the target reaches the boundary (−3 mm); y_1‐2 coordinate of beam switching after the latency of beam‐hold passing; y_2‐0: coordinate at which the target touches the boundary (−3 mm); y_2‐1: coordinate of beam‐on switching after the latency of beam‐on passing; y_2‐2: origin of motion (0 mm). T _{Lat‐Hold, i} and T _{Lat‐On, j} denote the latencies of beam‐hold and beam‐on, respectively. Each latency was varied from 0 to 0.7 s at 0.01 s$\mathrm{intervals}(i=1,2,\cdots 71,j=1,2,\cdots 71)$. V _t is the velocity of the target motion. R_{Hold, i} and R _{On ,j} are the calculation ranges of the target movement, which depend on the latency input to the analytical program

FIGURE 6

Flowchart of the analytical approach for estimating the overall latency. The planned dose distribution was divided by the irradiation time at TPS to obtain the dose distribution per unit time (the dose distribution rate $\dot{d}$(x,y)), which moves along the target motion (V _t) at the specified velocity (3, 5, 8, or 10 mm/s) with no pause time (recreating the motion of the target during film measurements). The dose distribution rates were calculated within the ranges R _{Hold, i} and R _{On, j} (see Figure 5). Outside of these ranges, the dose distribution rates were regarded as those at beam‐hold time, were not calculated. T _{Lat‐Hold, i} and T _{Lat‐On, j} are the latencies of beam‐hold and beam‐on, respectively, which are input separately as variable parameters in the analytical program (0 to 0.7 s at 0.01 s$\mathrm{intervals})(I=1,2,\cdots 71,j=1,2,\cdots 71)$. The sampled dose distribution rates were accumulated over the time of film measurements and the dose distribution D _calc calculated by the program replicated the dose distribution by gated radiotherapy with a moving target. The dose distributions from the TPS (D _TPS) were compared with D _calc in the gamma analysis. These results of gamma analysis from D _calc and D _TPS and from film measurements and D _TPS were compared and we defined a pair of input latency as the overall latency when the difference of the results of gamma analysis mentioned above was the smallest

FIGURE 7

Representative dose distributions in the treatment plan and in experiments with stationary and moving targets. The values 0, 1, 3, 10, and 20 s are the pause times for the moving target. Along the horizontal axis, the positive and negative coordinates denote superior and inferior positions relative to the origin of the target position, respectively. Each dose distribution is normalized to 100% of the maximum dose

FIGURE 8

Relationship between duty cycle and gamma pass rates in each target motion, referenced to the dose distribution of the treatment planning. The duty cycle denotes the ratio of beam‐on time to the total treatment time. Error bars depict the standard errors (SEs)

FIGURE 9

Relationship between the number of beam switches (beam‐hold and beam‐on) and gamma pass rate for different target motions, referenced to the dose distribution in the treatment planning. r is the Pearson correlation coefficient between the number of beam switches and the gamma pass rate. Error bars depict the SEs

FIGURE 10

Representative dose distributions of targets moving at different velocities. Along the horizontal axis, the positive and negative coordinates denote superior and inferior positions relative to the origin of the target position, respectively. Each dose distribution was normalized to 100% of the maximum dose

FIGURE 11

Relationship between target velocity and gamma pass rate, referenced to the dose distribution from the treatment planning. Error bars depict the SEs