Physics Contribution
Commissioning and Quality Assurance of RapidArc Radiotherapy Delivery System

https://doi.org/10.1016/j.ijrobp.2008.05.060Get rights and content

Purpose

The Varian RapidArc is a system for intensity-modulated radiotherapy (IMRT) treatment planning and delivery. RapidArc incorporates capabilities such as variable dose-rate, variable gantry speed, and accurate and fast dynamic multileaf collimators (DMLC), to optimize dose conformality, delivery efficiency, accuracy and reliability. We developed RapidArc system commissioning and quality assurance (QA) procedures.

Methods and Materials

Tests have been designed that evaluate RapidArc performance in a stepwise manner. First, the accuracy of DMLC position during gantry rotation is examined. Second, the ability to vary and control the dose-rate and gantry speed is evaluated. Third, the combined use of variable DMLC speed and dose-rate is studied.

Results

Adapting the picket fence test for RapidArc, we compared the patterns obtained with stationary gantry and in RapidArc mode, and showed that the effect of gantry rotation on leaf accuracy was minimal (≤0.2 mm). We then combine different dose-rates (111–600 MU/min), gantry speeds (5.5–4.3°/s), and gantry range (Δθ = 90–12.9°) to give the same dose to seven parts of a film. When normalized to a corresponding open field (to account for flatness and asymmetry), the dose of the seven portions show good agreement, with a mean deviation of 0.7%. In assessing DMLC speed (0.46, 0.92, 1.84, and 2.76 cm/s) during RapidArc, the analysis of designed radiation pattern indicates good agreement, with a mean deviation of 0.4%.

Conclusions

The results of these tests provide strong evidence that DMLC movement, variable dose-rates and gantry speeds can be precisely controlled during RapidArc.

Introduction

Intensity-modulated radiotherapy (IMRT) has been established as accurate, reliable, and efficient in delivering conformal radiotherapy 1, 2. By maximizing tumor dose while sparing normal tissues, IMRT can improve the efficacy of radiotherapy. Delivery of IMRT is possible using different methods: for example, in slice-by-slice rotational therapy using binary collimation 3, 4, or computer-controlled multileaf collimators (MLC) in static (SMLC for segmental MLC) or dynamic (DMLC or sliding window) mode 1, 5.

Another IMRT method was proposed by Yu, combining gantry rotation and aperture changes for intensity-modulated arc therapy (IMAT) (6). Clinical implementation of IMAT was initially hampered because the optimization algorithm generated plans difficult to deliver with conventional linear accelerators (linacs) and MLCs. Subsequent research led to direct aperture optimization, that takes into account the constraints of varying the aperture shape with SMLC 7, 8, 9. That development led researchers to evaluate the potential of applying direct aperture optimization to IMAT optimization, constraining the plans to be deliverable by the linac systems 10, 11. One IMAT approach involves several gantry rotations and thereby increases treatment time. Another approach, proposed by Otto as volumetric modulated arc therapy (VMAT), requires only one gantry rotation and produces dose distributions equivalent to or better than those of IMRT (12).

The IMAT or VMAT approach has a number of potential advantages 6, 12. In being able to deliver radiation from 360°, it may offer more conformal dose distributions relative to IMRT using only a limited number of fields and gantry directions. Plan optimization becomes simpler, as it obviates questions of beam number and direction. Compared with tomotherapy, the use of a cone beam improves delivery efficiency significantly. Specifically, by using a volumetric irradiation volume, VMAT is five to 15 times more efficient, in terms of treatment time and monitor units, than the slice-by-slice tomotherapy (12). Another important factor is that it can be implemented with a general-purpose linac for maximum clinical flexibility.

Many publications have documented that highly conformal dose distributions can be achieved with IMAT or VMAT 6, 10, 12. A recent publication directly compared the dose distributions achievable with IMAT and tomotherapy (13). The overall conclusions are that IMAT dose distributions are highly conformal and are equal to or superior to those generated by tomotherapy in the majority of cases (13).

Recently, the Varian RapidArc has become available for the treatment planning and delivery of arc-dynamic IMRT. The RapidArc planning algorithm is based on the direct MLC leaf position optimization method described by Otto (12). Briefly, both MLC position and monitor unit (MU) are included as optimization parameters, with a cost function based on dose–volume constraints of the target and normal tissues. During optimization, further constraints are imposed on MLC motion, dose-rate, and gantry speed such that these are within the capabilities of the Clinac. The optimization process begins with a small number of control points, gradually increasing them to a sufficient number to ensure dose calculation accuracy. As implemented in the initial release of the Eclipse planning system, ≤177 control points are used for each RapidArc treatment.

To maximize the benefits of the RapidArc approach, both the treatment planning and linac systems incorporate the following capabilities: variable dose-rate, variable gantry speed, and DMLC movement, with the expectation that these will optimize dose conformality, delivery efficiency, accuracy, and reliability. Thus, although RapidArc is an extension of the IMRT-DMLC concept, the delivery of RapidArc requires added functionality. Over the last year, testing of the various components of RapidArc has provided proof-of-principle demonstration of the potentials of this approach 14, 15. Recently, extensive and systematic evaluation of both RapidArc planning and delivery has been conducted at several institutions and will be reported in upcoming meetings (private communications from Stine Korenman, Luca Cozzi, PengPeng Zhang, Wilko Verbakel, and others).

The advanced technologies required for RapidArc delivery, i.e. variable dose-rate, variable gantry speed, rapid and bi-directional MLC movement within a single arc treatment field, are only now becoming available. Methods for system commissioning and for routine quality assurance (QA) have not been previously reported. The purpose of this study is to design and test such commissioning and QA protocols in advance of clinical implementation of RapidArc. As of May 9, 2008, RapidArc treatment has been clinically implemented in three institutions in the United States and Europe. Before that, the protocols described in this paper were tested and validated using both film and electronic portal imaging device (EPID) dosimetry systems.

Section snippets

Methods and Materials

As RapidArc is only beginning to be implemented, practical experience is lacking. Thus, at present the commissioning and routine QA procedures cannot be clearly separated, but will evolve and mature with the accumulation of experience and physics data 16, 17, 18. That said, it is of immediate importance that programs be developed, and tests designed that assay the critical elements of RapidArc.

Accuracy of DMLC position during RapidArc

A film, mounted on the IMF and repeatedly exposed to a slit radiation field (defined by the MLC) of different lengths (defined by the upper jaws), is shown in Fig. 2a. The top and bottom sections were singly exposed at θ = 0°. Relative to the top and bottom sections, the in-between sections (double exposure at θ of 0° and 90°) show a broader image, indicative of a change in the position of the slit field on the film. The central region, exposed three times at θ of 0°, 90°, and 270°, shows an

Discussion

RapidArc, in a sense, is an extension of the IMRT and IMAT concepts 2, 6. In being able to plan and deliver IMRT in dynamic-arc mode with volumetric radiation fields, it offers important advantages over competing approaches 6, 12. Relative to either SMLC or DMLC IMRT using a fixed and limited number of fields, RapidArc will likely produce more conformal dose distributions, and obviate the question of the number and the directions of beams 12, 13. In addition, with RapidArc there is a

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    Conflict of interest: Drs. C. Clifton Ling, Yves Archambault, and Jiri Bocanek are affiliated with Varian Medical Systems.

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