Volumetric modulated arc therapy: IMRT in a single gantry arc

Abstract

In this work a novel plan optimization platform is presented where treatment is delivered efficiently and accurately in a single dynamically modulated arc. Improvements in patient care achieved through image-guided positioning and plan adaptation have resulted in an increase in overall treatment times. Intensity-modulated radiation therapy (IMRT) has also increased treatment time by requiring a larger number of beam directions, increased monitor units (MU), and, in the case of tomotherapy, a slice-by-slice delivery. In order to maintain a similar level of patient throughput it will be necessary to increase the efficiency of treatment delivery. The solution proposed here is a novel aperture-based algorithm for treatment plan optimization where dose is delivered during a single gantry arc of up to 360 deg. The technique is similar to tomotherapy in that a full 360 deg of beam directions are available for optimization but is fundamentally different in that the entire dose volume is delivered in a single source rotation. The new technique is referred to as volumetric modulated arc therapy (VMAT). Multileaf collimator (MLC) leaf motion and number of MU per degree of gantry rotation is restricted during the optimization so that gantry rotation speed, leaf translation speed, and dose rate maxima do not excessively limit the delivery efficiency. During planning, investigators model continuous gantry motion by a coarse sampling of static gantry positions and fluence maps or MLC aperture shapes. The technique presented here is unique in that gantry and MLC position sampling is progressively increased throughout the optimization. Using the full gantry range will theoretically provide increased flexibility in generating highly conformal treatment plans. In practice, the additional flexibility is somewhat negated by the additional constraints placed on the amount of MLC leaf motion between gantry samples. A series of studies are performed that characterize the relationship between gantry and MLC sampling, dose modeling accuracy, and optimization time. Results show that gantry angle and MLC sample spacing as low as 1 deg and 0.5 cm, respectively, is desirable for accurate dose modeling. It is also shown that reducing the sample spacing dramatically reduces the ability of the optimization to arrive at a solution. The competing benefits of having small and large sample spacing are mutually realized using the progressive sampling technique described here. Preliminary results show that plans generated with VMAT optimization exhibit dose distributions equivalent or superior to static gantry IMRT. Timing studies have shown that the VMAT technique is well suited for on-line verification and adaptation with delivery times that are reduced to approximately 1.5-3 min for a 200 cGy fraction.

A novel and independent method for time‐resolved gantry angle quality assurance for VMAT

“…Volumetric‐modulated arc therapy (VMAT) is a modern radiation treatment technique that allows a precise three‐dimensional (3D) radiation dose to be delivered as the gantry rotates through one or more arcs 1, 2, 3. Compared to intensity‐modulated radiation therapy (IMRT), where the gantry is static during dose delivery, VMAT offers more precise target coverage with lower monitor units (MU) 3, 4.…”

“…Compared to intensity‐modulated radiation therapy (IMRT), where the gantry is static during dose delivery, VMAT offers more precise target coverage with lower monitor units (MU) 3, 4. Furthermore, the delivery times for VMAT treatments are shorter compared to IMRT treatments,5 potentially reducing the intra‐fractional patient movement during deliveries 6.…”

“…Furthermore, the delivery times for VMAT treatments are shorter compared to IMRT treatments,5 potentially reducing the intra‐fractional patient movement during deliveries 6. VMAT deliveries require a complex treatment plan, involving three key dynamic components; (a) gantry rotation, (b) dose rate modulation, and (c) multi‐leaf collimator (MLC) motion, which are all simultaneously varied during the delivery 2, 3, 4, 5, 6, 7…”

Time‐resolved beam symmetry measurement for VMAT commissioning and quality assurance

“…RapidArc is the Varian version (Varian Medical Systems, Palo Alto, CA) of VMAT, which was first proposed by Otto in 2008 (1) . VMAT is an extension of IMRT where, as well as dynamic MLC motion and dose rate modulation, the treatment is delivered as a rotating gantry arc with potential for gantry speed modulation.…”

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

“…The main goal of these techniques is to achieve dose distribution of increased conformity to the target volumes, while further reducing the doses to the healthy normal tissue (3) . VMAT consists of treating the patient by using one or more gantry arcs with continuously varying beam aperture shape, gantry speed, and dose rate 4 , 5 . The complexity of IMRT and VMAT in treatment delivery places new demands on ensuring the quality of the linear accelerator dose delivery in real time, as its controller must now determine in real time the gantry position (VMAT), the multileaf collimator (MLC) operation, and jaw speed, as well as the dose rate (VMAT & IMRT).…”