A multileaf collimator phantom for the quality assurance of radiation therapy planning systems and CT simulators

doi:10.1016/j.ijrobp.2004.06.013

International Journal of Radiation OncologyBiologyPhysics

Volume 60, Issue 3, 1 November 2004, Pages 994-1001

Presented at 2003 AAPM Annual Meeting, San Diego, CA.

https://doi.org/10.1016/j.ijrobp.2004.06.013 Get rights and content

Purpose

The evolution of three-dimensional conformal radiation treatment has led to the use of multileaf collimators (MLCs) in intensity-modulated radiation therapy (IMRT) and other treatment techniques to increase the conformity of the dose distribution. A new quality assurance (QA) phantom has been designed to check the handling of MLC settings in treatment planning and delivery.

Methods and materials

The phantom consists of a Perspex block with stepped edges that can be rotated in all planes. The design allows for the assessment of several MLC and micro-MLC types from various manufacturers, and is therefore applicable to most radiation therapy institutions employing MLCs. The phantom is computed tomography (CT) scanned as is a patient, and QA assessments can be made of field edge display for a variety of shapes and orientations on both radiation treatment planning systems (RTPS) and computed tomography simulators.

Results

The dimensions of the phantom were verified to be physically correct within an uncertainty range of 0–0.7 mm. Errors in leaf position larger than 1 mm were easily identified by multiple observers.

Conclusions

The MLC geometry phantom is a useful tool in the QA of radiation therapy with application to RTPS, CT simulators, and virtual simulation packages with MLC display capabilities.

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.

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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).

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Physics contributionA multileaf collimator phantom for the quality assurance of radiation therapy planning systems and CT simulators

Purpose

Methods and materials

Results

Conclusions

Introduction

Section snippets

Phantom design criteria

Phantom design

QA application to radiation treatment planning systems

Phantom design and construction

Conclusion

Acknowledgments

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Med Dosim

Med Dosim

Basic applications of multileaf collimators: Report of the AAPM radiation therapy task group 50

Commissioning and quality assurance of treatment planning computers

Int J Radiat Oncol Biol Phys

Physics contribution
A multileaf collimator phantom for the quality assurance of radiation therapy planning systems and CT simulators