International Journal of Radiation Oncology*Biology*Physics
Physics contributionA multileaf collimator phantom for the quality assurance of radiation therapy planning systems and CT simulators
Introduction
The goal of radiation therapy is to deliver a highly accurate dose to a well-defined target volume while avoiding surrounding healthy tissue. Through many technologic advances, radiation delivery is becoming more conformal to the target volume, lowering dose to the healthy tissues and critical organs. One of the most important advances affects beam shaping. Multileaf collimators (MLCs), which can form irregularly shaped fields by computer control, are now commonly used. They replace the previous rectangular fields produced by standard collimators and the tedious process of making customized cerrobend blocks (1). MLCs or micro-MLCs, which produce the smaller irregular fields generally used for stereotactic radiotherapy, are essential for intensity-modulated radiation therapy (IMRT). However, the use of MLCs in radiotherapy requires the use of computers to control leaf positions and speed of travel. With many components involved in the process of treatment planning and delivery, quality assurance (QA) practices are essential in ensuring that all components are working correctly and effectively (2). Phantoms have proven to be a useful tool in QA for both dosimetric and nondosimetric parameters (3, 4); however, no phantom exists to test specifically the handling and display of MLCs in treatment planning computers and computed tomography (CT) simulators.
Craig et al. (4) described a phantom designed to test primarily nondosimetric features of the radiation treatment planning system (RTPS). This phantom included both a rotatable component and a body component. They found that it was an effective QA tool, in relation to nondosimetric display features of the RTPS (4), but MLCs were not considered.
The aim of this study was to design a new phantom, based on the rotatable component of the QA phantom designed by Craig et al., to assess the display capabilities of RTPSs and CT simulators for irregularly shaped fields obtained with MLCs and micro-MLCs.
Section snippets
Phantom design criteria
In addition to other image display issues addressed previously the phantom should allow the testing of the following features: beam display and orientation of MLC and micro-MLC fields on CT multiplanar reconstructions and digitally reconstructed radiographs. Manufacturer's specifications for MLC (Varian, Palo Alto, CA, Elekta, Norcross, GA; and Siemens, Malvern, PA)and micro-MLC (Brainlab, Westchester, IL, Radionics, Burlington, MA) configurations that were incorporated into the phantom design
Phantom design
The phantom (as built by Modus Medical Devices, Inc. and now commercially available) measured within ±0.7 mm of the design in all dimensions.
QA application to radiation treatment planning systems
When testing the phantom's performance according to the procedure established (Table 2), using Theraplan Plus, all of the CT reconstructions and the DRR dimensions were accurate, with all of the measured dimensions within 1 mm using the distance measuring tool available in Theraplan Plus. The beam displays were examined for the various reconstructions and
Phantom design and construction
The design of the MLC phantom addresses all of the desired features and those of the original beam geometry phantom (4). The new design allows analysis of the RTPS's beam display capabilities for irregular fields produced by MLCs. Because of the nature of modern radiation therapy, with precise target volumes and the potential of dose escalation, small margins of error are tolerable at all points of the treatment process. Because MLCs are an integral part of most modern radiation therapy, such
Conclusion
Based on the commercially available beam geometry phantom, a new QA phantom has been designed to handle the irregular fields created by MLCs and micro-MLCs with the basic elements from the design based on a previous phantom. The phantom can be used at most institutions because it is designed to be compatible with the MLC and micro-MLCs from several manufacturers. A procedure has been developed to allow for the assessment of both RTPS and CT simulators, in regard to nondosimetric factors, using
Acknowledgments
Thank you to Tim Craig for his assistance with the project in regard to assistance with imaging and software use and advice. As well, thank you to John Miller and Modus Medical Devices, Inc., and Dennis Brochu and the LRCC machine shop for their assistance in the construction of the phantom and for their input in the feasibility of construction. The collaboration with Mostafa Heydarian at Princess Margaret Hospital is gratefully acknowledged.
References (14)
- et al.
A solid water pelvic and prostate phantom for imaging, volume rendering, treatment planning, and dosimetry for an RTOG multi-institutional, 3-D dose escalation study
Int J Radiat Oncol Biol Phys
(1998) - et al.
A quality assurance phantom for three-dimensional radiation treatment planning
Int J Radiat Oncol Biol Phys
(1999) - et al.
Commissioning and quality assurance for MLC-Based IMRT
Med Dosim
(2001) - et al.
Quality assurance procedures for the Peacock system
Med Dosim
(2001) Basic applications of multileaf collimators: Report of the AAPM radiation therapy task group 50
(2001)- et al.
Commissioning and quality assurance of treatment planning computers
Int J Radiat Oncol Biol Phys
(1993) - Elekta Product Specifications: MLC. Elekta Oncology System, Inc. Tender document, October 14,...
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Supported by the London Regional Cancer Centre, The University of Western Ontario, and the Ontario Research and Development Challenge Fund (The Ontario Consortium for Image Guided Therapy and Surgery).