British Journal of Radiology 75 (2002),56-58 © 2002 The British Institute of Radiology
Design of a multiblock phantom for radiotherapy dosimetry applications
A W Seaby, BSc, MSc
D W Thomas, MSc, PhD
S J S Ryde, MSc, PhD
G R Ley, BSc
and
D Holmes
Department of Medical Physics and Clinical Engineering, Singleton Hospital, Swansea NHS Trust, Swansea SA2 8QA, UK
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Abstract
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This note describes the design of a multiblock semi-anatomic phantom, which lends itself to a variety of radiotherapy dosimetry applications, in particular, the audit of external beam treatment planning and delivery. The basic building blocks of the phantom were formed from a variety of tissue substitute materials and could be assembled in many ways to model different cross-sections through the body.
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Introduction
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A national dosimetry intercomparison for megavoltage X-ray was carried out in the period 1987–1991 at the 64 radiotherapy centres in the UK [1]. The audit included measurements within a water-equivalent epoxy resin phantom for testing a number of three field planned treatments. The phantom, which is a relatively simple geometrical shape, allows the measurement of dose at six positions and can be made inhomogeneous by the insertion of a cylinder of lung-substitute material. Phantoms of the same design have been widely used for intercomparison within the UK by the seven regional audit groups coordinated by the Institute of Physics and Engineering in Medicine (IPEM) [2]. Allahverdi et al [3] have evaluated the properties of epoxy resin materials commonly used in audit phantoms. A number of anthropomorphic and semi-anatomic radiotherapy application phantoms have been described by the International Commission on Radiation Units and Measurements [4]. Huang and Reinstein [5] have presented a design for an acrylic cube phantom that allows the depth of the chamber to be changed by moving anumber of small domino shaped slabs that occupy the space above and below the chamber.
This note describes a phantom that can be assembled from blocks of tissue-substitute materials to model different sections through the body. These sections may range in size and shape from the neck to a large pelvis (see Figure 1
). An ionization chamber may be accurately positioned at many points within the phantom.
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Figure 1. Schematic representation of the multiblock phantom assembled as a number of different body sections.
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Locally, the phantom has been used for a variety of radiotherapy applications.- Commissioning measurements for a new treatment planning system. For example, the phantom could be easily configured to test the effects of inhomogeneity, size or a re-entrant surface.
- Confirmation of in vivo clinical dosimetry measurements. On some occasions, patient measurements with semiconductor or thermoluminescent detectors (TLDs) may yield an unexpected result owing to a large dose gradient, patient movement etc. A repeat measurement may be made. However, this is not always considered desirable because of the shadowing effect of the detectors within the patient. An alternative strategy is to assemble the multiblock phantom as an approximate cross-section of the patient at the treatment level and make measurements on the surface and within the phantom to confirm the prescribed dose. In this situation it will be necessary to prepare a separate plan for the phantom using the same field parameters, e.g. collimator settings, beam weightings, wedge fractions, machine angles etc., as the clinical plan. If the measured dose at the centre of the "target" volume agrees with the prescribed dose for the phantom, this suggests that the delivered dose, in the clinical situation, will be correct. A similar approach was used by Morgan [6] for quality assurance of conformal treatments.
- Audit of treatment planning and delivery. The department has participated in several rounds of interdepartmental megavoltage X-ray audit using an IPEM type phantom [1]. Locally, this work has been extended using the multiblock phantom to represent sections through the head, neck, thorax and pelvis and using planning techniques that reflect the clinical workload. Podgorsak et al [7] have shown that beam data measured in a large plotting tank may be less accurate when applied to the treatment planning of a lesion within a small body cross-section. Treatment plans for these sections were prepared using an outline of the phantom drawn on a standard template (Figure 2) and input to the planning computer via a film scanner, or from CT studies of the phantom.
- Dose distributions for coplanar and three-dimensional plans. The phantom can accommodate films or other sheet dosemeters loaded in a number of horizontal or vertical planes. For film, the isodensity distribution, corrected for dose response of the film, can be compared with the plan isodose for the corresponding plane. The angular response of the detector places a limitation on the beam directions that can be tested.
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Figure 2. A standard template for outlining of the multiblock phantom. Diagonal=48.2 cm, Block=4.0 cm x 4.0 cm x 30.0 cm.
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Materials and methods
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The phantom blocks were 30 cm in length with square or right angle triangle cross-sections of side 4 cm. The blocks were made of an epoxy resin based tissue-equivalent material [8] to represent water (WT1), lung (LN10) and bone (IB7). The phantom materials were obtained from St Bartholomew's Hospital (London, UK) in the form of 4 cm thick slabs and were cut and machined locally to the required size.
A square section block of WT1 was drilled to accommodate a 0.6 cm3 graphite ionization chamber (NE2571). The centre of the chamber was 15 cm from the end of the block and displaced 1 cm laterally from the long axis. A square block of WT1, drilled with a 2 cm wide hole, was used to represent an internal air space such as the trachea.
Phantoms were assembled by placing blocks within a simple adjustable wooden frame. The ends of the frame were joined by a number of 5 mm diameter wooden dowel rods. Figure 3 shows the assembly of a thorax phantom in the frame. Triangular spacers, made from expanded polystyrene, were used to support oblique phantom surfaces.
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Figure 3. Multiblock phantom partially assembled, within an adjustable wooden frame, to represent a thorax.
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The blocks were stored on sliding shelves in a large wheeled container (Figure 4) that allowed a phantom to be quickly assembled or disassembled. In addition, the storage container ensured that individual blocks were not subjected to forces that might cause distortion.
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Discussion
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In addition to variable geometry and composition, the multiblock phantom has other beneficial features.- In a disassembled state, individual blocks will reach thermal equilibrium with their surroundings more quickly than a large solid phantom.
- A person lifting individual blocks weighing less than half a kilogram can assemble a very large phantom, weighing 30 kg or more, in a few minutes.
- A large number of potential measurement points are available. These points are indicated by crosses on Figure 2
. By using a sheet of laminated card to support a column of blocks, it is possible to remove the measurement block and insert it in another position.
- The phantom can accommodate sheet dosemeters and a wide variety of ionization chambers and other detectors, subject to the preparation of a block with the appropriate cavity.
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Conclusions
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The multiblock phantom has proved to be a very useful and flexible tool for dosimetry audit and has been effective in testing limitations in the treatment planning system. The wide range of phantom shapes and sizes that can be prepared and the large variety of inhomogeneities and measurement points that can be accommodated in a phantom make it very versatile. The phantom can be assembled quickly and easily and, being based on solid materials, is more convenient to use than liquid phantoms. In this centre it is used routinely for the in vitro dose verification of complex or unusual treatment plans. Other centres within the South-West Radiotherapy Physics Audit Group have used the phantom. Currently, a breast/chest wall phantom based on a similar design is being constructed in collaboration with the Medical Physics Department of Velindre Hospital (Cardiff, UK) for use by the audit group.
Received for publication November 6, 2000.
Revision received July 5, 2001.
Accepted for publication August 13, 2001.
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References
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Allahverdi M, Nisbet A, Thwaites DI. An evaluation of epoxy resin phantom materials for megavoltage photon dosimetry. Phys Med Biol 1999;44:1125–32.[Medline]
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International Commission on Radiation Units and Measurements, Inc. Phantoms and computational models in therapy, diagnosis and protection, ICRU Report 48. Bethesda, MD: ICRU, 1992.
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Huang JC, Reinstein LE. Poster abstract: a new QA phantom for expedient energy and output constancy measurements. Med Phys 1995;22:1010.
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Morgan HM. Quality assurance of computer controlled radiotherapy treatments. Br J Radiol 1992;65:409–16.[Abstract/Free Full Text]
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Podgorsak EB, Pla C, Evans MDC, Pla M. The influence of phantom size on output, peak scatter factor and percentage depth dose in large field photon irradiation. Med Phys 1985;12:639–45.[Medline]
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White DR, Martin RJ, Darlison R. Epoxy resin based tissue substitutes. Br J Radiol 1977;50:814–21.[Abstract/Free Full Text]
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Abstract
Introduction
