Modeling dose modulation in VMAT – Northwest Medical Physics Center

M. R. Zaini,^1,2 T. A. Blackwell^1,2
¹Northwest Medical Physics Center, Lynnwood, WA; ²Skagit Valley Regional Health Center, Mount Vernon, WA

ASTRO Annual Scientific Meeting, Chicago, IL (2009).

Purpose/Objective(s): Volumetric Modulated Arc Therapy (VMAT) is a new treatment modality in radiation therapy marketed by Elekta. VMAT uses both temporal and spatial modulation in delivering large arcs of radiation to patients. The aim is to quantify dose modulation levels based on the VMAT treatment parameters.

Materials/Methods: Temporal aspect of VMAT is modeled first, and then the spatial considerations are added. This model reveals the compromises one must make in order to arrive at an acceptable dose pattern. Temporal Point Spread Function (TPSF) or impulse response of VMAT is obtained using two different techniques. One method for determining the TPSF uses a single dose point as a delta function input to the VMAT system. The second method uses of Edge Response Function (ERF) and is employed to confirm the simple delta function scenario. The sharp edge of a semicircularly shaped dose function is the intended input to the system. Differentiating the ERF after performing the Radon transform of the semicircle in the fan-beam geometry gives the line spread function (LSF). According to the Central Slice Theorem, the abscissa of the Fourier Transform (FT) of the LSF is the integral of the FT of the system impulse response; TPSF is then simply the inverse FT of the result. Since VMAT is circularly symmetric, these computations were performed for energy angle of gantry rotation. In order to incorporate the spatial extent of the treatment target into the model, two spatially separated dose points were used as the input to the VMAT system. The ratio of the gantry speed to that of the MLCs is the chief parameter of the spatial extent of the model.

Results: The two methods of computing the VMAT TPSF yield similar results. The TPSF derived from the single delta function method is smoother and has less ringing artifact than that from the ERF method. In order to deliver a single dose point, the maximum gantry speed of 1 minute per rotation suffices. However, it takes 13 minutes to deliver the semicircularly shaped dose map that has a radius of 13 centimeters. For the two delta function case of incorporating the spatial dimensions into the model, the distance between them is varied between 2 and 20 millimeters. The treatment times for a single arc then vary from 1 to 4.6 minutes, respectively. The shorter the treatment time the less desirable the dose coverage becomes. The compromise between the dose shapes, treatment time, and using multiple arcs determine the design of a VMAT treatment.

Conclusions: Both the temporal and spatial aspects of VMAT need to be considered in designing acceptable treatment plans. The interplay between the MLC and gantry speed limits affects the dose coverage of the treatment target. For larger extent tumors, treatment times more than 10 minutes are required upon using a single arc. Multiple arcs can help reduce the treatment times, if the target can be considered disjoint units.