Management of three-dimensional intrafraction motion through real-time DMLC tracking

Abstract

Tumor tracking using a dynamic multileaf collimator (DMLC) represents a promising approach for intrafraction motion management in thoracic and abdominal cancer radiotherapy. In this work, we develop, empirically demonstrate, and characterize a novel 3D tracking algorithm for real-time, conformal, intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT)-based radiation delivery to targets moving in three dimensions. The algorithm obtains real-time information of target location from an independent position monitoring system and dynamically calculates MLC leaf positions to account for changes in target position. Initial studies were performed to evaluate the geometric accuracy of DMLC tracking of 3D target motion. In addition, dosimetric studies were performed on a clinical linac to evaluate the impact of real-time DMLC tracking for conformal, step-and-shoot (S-IMRT), dynamic (D-IMRT), and VMAT deliveries to a moving target. The efficiency of conformal and IMRT delivery in the presence of tracking was determined. Results show that submillimeter geometric accuracy in all three dimensions is achievable with DMLC tracking. Significant dosimetric improvements were observed in the presence of tracking for conformal and IMRT deliveries to moving targets. A gamma index evaluation with a 3%-3 mm criterion showed that deliveries without DMLC tracking exhibit between 1.7 (S-IMRT) and 4.8 (D-IMRT) times more dose points that fail the evaluation compared to corresponding deliveries with tracking. The efficiency of IMRT delivery, as measured in the lab, was observed to be significantly lower in case of tracking target motion perpendicular to MLC leaf travel compared to motion parallel to leaf travel. Nevertheless, these early results indicate that accurate, real-time DMLC tracking of 3D tumor motion is feasible and can potentially result in significant geometric and dosimetric advantages leading to more effective management of intrafraction motion.

Figure 1

Schematic illustration of the various types of target motion as seen in the beam’s view and the desired change in MLC configuration to account for each type of motion.

Figure 2

Logical flow of the real-time DMLC tracking algorithm.

Figure 3

Schematic illustration of the key steps in the tracking algorithm.

Figure 4

Schematic illustration of the leaf fitting operation (a) without and (b) with the use of subleaves.

Figure 5

Experimental arrangement for tracking studies using (a) the lab system and (b) a clinical system.

Figure 6

Image frames extracted from tracking movies acquired to determine the geometric accuracy of DMLC tracking. Two different patterns were separately mounted on the motion platforms to calculate tracking accuracy of (a) motion parallel and perpendicular to MLC leaf travel and (b) motion along the beam axis. In each case, the image frames were segmented in order to determine the peripheral bounds and the center of the MLC aperture (indicated by the outer and inner green circles, respectively) and the central point on the underlying geometric pattern (indicated by the magenta cross). The image frames of the geometric pattern used in (b) were also segmented to delineate the boundary of the white circle (indicated by the magenta outline).

Figure 7

Geometric tracking error in the parallel and perpendicular directions as a function of the number of virtual subleaves used in the DMLC tracking algorithm.

Figure 8

Geometric accuracy of DMLC tracking (using five subleaves) for target motion (a) parallel and (b) perpendicular to MLC leaf travel, and (c) along the beam axis. The horizontal shaded band denotes an error of 1 mm in (a) and (b) and an error of 0.5 mm in (c).

Figure 9

Tracking efficiency in directions (a) 2 cm parallel and (b) 0.5 cm perpendicular to leaf motion. Results are shown for a conformal (circular) and clinically derived D-IMRT and S-IMRT deliveries. Note the different y axes in (a) and (b). Beam holds were not asserted by the MLC controller during the VMAT delivery, and therefore the efficiency was 100%.

Figure 10

Isodose (dashed) lines for (a) conformal, (b) D-IMRT (c) S-IMRT, and (d) VMAT deliveries without tracking and corresponding curves [(e), (f), (g), and (h)] with tracking for a target moving 2 cm parallel and 0.5 cm perpendicular to the leaf motion direction. For comparison, isodose curves for delivery to a static object are shown in each figure by solid lines.