An anthropomorphic head phantom with a BANG polymer gel insert for dosimetric evaluation of IMRT treatment delivery

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1 An anthropomorphic head phantom with a BANG polymer gel insert for dosimetric evaluation of IMRT treatment delivery G. Ibbott a, M. Beach a, M. Maryanski b a M.D. Anderson Cancer Center, Houston, Texas, U.S.A. b MGS Research, Inc., Guilford, Connecticut, U.S.A. Abstract. Radiation therapy has seen remarkable advances in the ability to plan complex 3-D treatments to tumour target volumes. Intensity-modulated radiation therapy (IMRT) is one such method used to conform dose distributions to irregularly shaped tumour volumes while minimizing the dose to nearby critical structures. IMRT offers the possibility of high dose gradients, and it is therefore possible to deliver high doses to target volumes while maintaining low doses to nearby critical normal structures to a much greater extent than is the case with conventional radiation therapy. Comprehensive QA procedures are critical if IMRT is to be delivered consistently. The Radiological Physics Center (RPC) has developed a mailable anthropomorphic head phantom to assist in the evaluation of IMRT head and neck treatment delivery. To improve the ability of the phantom to provide 3-dimensional dose information, modifications have been made to accommodate a polymer gel dosimeter. The gel dosimeter insert made of Barex plastic has been designed and constructed. The insert can be replaced with a similar insert containing structures of water equivalent plastics as well as conventional dosimeters. A preliminary evaluation of the gel dosimeter phantom insert has been conducted. 1. Introduction 1.1 IMRT and requirements for QA Radiation therapy has seen remarkable advances in the ability to plan complex 3-D treatments to tumour target volumes. Intensity-modulated radiation therapy (IMRT) is one such method used in cancer treatments to conform doses to irregularly shaped tumour volumes while minimizing the dose to nearby critical structures. [1-4] IMRT offers the possibility of high dose gradients, and it is therefore possible to deliver high doses to target volumes while maintaining low doses to nearby critical normal structures to a much greater extent than is the case with conventional radiation therapy. At the same time, the high gradients achievable with IMRT mean that localization of the dose distribution is critical. Small errors in positioning of the patient can mean that a target volume is missed, or that a sensitive normal structure is irradiated to a higher dose than intended, and perhaps higher than can be tolerated. Consequently, comprehensive QA procedures are critical if IMRT is to be delivered consistently. [5] 1.2 The Radiological Physics Center The Radiological Physics Center (RPC) is funded by the U. S. National Cancer Institute to assure the cooperative study groups that conduct multi-institutional clinical trials that institutions participating in the trials have adequate QA procedures and that no major systematic dosimetry discrepancies exist. Remote monitoring procedures include the use of mailed thermoluminescent dosimeters (TLDs) to verify machine output, comparison of an institution s dosimetry data with RPC standard data to identify potential discrepancies, 1

2 evaluation of reference or actual patient calculations to verify the treatment planning algorithms and manual calculations, review of the institution s written QA procedures and records to verify adherence to published standards, and mailed anthropomorphic phantoms to verify tumour dose delivery for special treatment techniques. In some cases, the RPC participates in the credentialing of institutions wishing to participate in specific clinical trials. These institutions are required to submit the details of their treatment planning capabilities, representative treatment plans for benchmark cases or actual patient treatments, and in some cases, actual measured data. Recently, several study groups began to require, or encourage, the use of IMRT for treatment of patients submitted to some clinical trials. The Radiological Physics Center (RPC) has developed a mailable anthropomorphic head phantom to assist in the evaluation of IMRT head and neck treatment delivery. [6] The phantom currently has a dosimetry insert that uses film and thermoluminescent dosimeters (TLD) to evaluate the dose distributions. While these conventional dosimeters have well known characteristics, they can only provide 1-D (TLD) and 2-D (film) evaluation of treatment delivery. However, it is desirable to use a volumetric dosimeter in the evaluation of the complex distributions offered by IMRT. 1.3 Polymer Gel Dosimeter The BANG polymer gel dosimeter (MGS Research, Inc., Guilford, CT) was chosen for this project. BANG polymer gel dosimetry has been shown to be a promising alternative to film and TLD because it allows for 3-D evaluation of treatment planning. Previous investigations have demonstrated that complex dose distributions can be measured and displayed accurately with the BANG gel [7-12]. This project was undertaken to design and construct a BANG gel insert for the RPC head and neck IMRT phantom to provide for volumetric dosimetry evaluations. 2. Methods and Materials 2.1 Phantom Design and Construction A cylindrical insert container made of Barex plastic was designed and constructed, to accommodate the polymer gel dosimeter. Barex was chosen because it has low permeability to oxygen and can be thermomolded according to design parameters. The insert was designed to fit within the RPC head and neck phantom to encompass a simulated treatment region. (Figure 1.) A second imaging insert was constructed containing TLD and film dosimeters, to mimic the original imaging/dosimetry insert designed for this phantom. This was done to facilitate a direct comparison between conventional dosimeters and the gel. The imaging insert was designed with a primary target volume, a secondary target volume and a critical structure volume that were of similar material and geometry to that of the original block-style insert. (Figure 2.) Because of the change in insert dimensions, the phantom itself was redesigned to accommodate the inserts. The cylinders had a radius of 5 cm and a depth of 8.5 cm. An optical computed tomography (CT) scanner was used to determine the optical density (OD) throughout the gel and relate the OD to dose. A straightforward relationship between OD and dose has been demonstrated previously. [13] The cylindrical design of the gel insert was chosen to facilitate optical CT densitometry. 2

3 FIG 1. The RPC s redesigned IMRT head and neck phantom with imaging insert installed. The Barex BANG gel insert is shown in the foreground. FIG 2. The BANG gel and imaging/dosimetry inserts. Note the PTVs, critical structure and film/tld provisions in the disassembled imaging insert. Both the gel and imaging/conventional dosimeter inserts were fitted with a registration pin. The pin served several purposes: 1) It ensured that the inserts were registered within the phantom in a reproducible orientation, 2) it served as an alignment marker for the x-ray CT image taken of the phantom with the imaging insert (which was used for treatment planning) and 3) they served as alignment markers for OCT measurement and dosimetry image reconstruction of the gel insert. The registration pin also provided a reference for alignment of isodose distributions (based upon x-ray CT images of the imaging insert) with the measured distributions from the gel dosimeter. 3. Results: 3.1 Simple treatment evaluation An initial test was conducted to confirm correct functioning of the components of the phantom and the dosimetry procedure. A uniform dose was delivered to the gel insert while the insert was submerged in a water tank. This allowed measured gel dose points to be compared to ion chamber measurements as well as calculated doses. The calibration of the treatment machine, a Clinac-2100C/D (Varian Medical Systems, Palo Alto, Calif.), was first verified through dose output measurements made using the AAPM TG- 51 formalism. Test tubes filled with the same batch of BANG gel were irradiated to doses of 2, 6 and 10 Gy and used to determine an optical density (OD) versus dose response curve. One of the gel insert canisters was also irradiated to 6 Gy. This gel provided a correction factor accounting for volumetric dependencies affecting the OD readings. The water tank containing the gel insert was irradiated with equally weighted parallel-opposed 6 MV beams to a dose of 7 Gy at isocentre. Ion chamber measurements were taken along the three principal axes of the water tank with the origin acting as the common isocentre for the ion chamber measurements, treatment planning calculations, and gel measurements. The relative dose comparisons proved to be promising, considering this was the first analysis using our recently commissioned OCT scanner. Representative plots along the X (transverse) axis and the Z (longitudinal) axis demonstrated general agreement with ion chamber measurements and the treatment planning calculations (figures 3 & 4). As can be seen, the 3

4 gel measurement process is perturbed by the reflections of the laser along the sides of the canister walls. As a result, only the central 70% of the canister provides reliable data. X-Axis Relative Doses Gel1 Z-Axis Relative Doses Gel Gel I.C I.C TPS TPS Off-Axis Distance [cm] Off-Axis Distance [cm] FIG 3. Relative dose measured from the gel dosimeter in the X axis. FIG 4. Relative dose measured from the gel dosimeter in the Y axis. We intend to further investigate the gel insert using a non-anthropomorphic environment and conventional treatment methods. However, for demonstration purposes the anthropomorphic head phantom was taken through the normal IMRT head and neck treatment process. The phantom with imaging insert installed was scanned on an AcQsim simulation CT scanner. The imaging study was then used to assemble 9-field IMRT plan using our Corvus treatment planning computer. The RTOG H 0022 protocol dose requirements were adhered to for the target volumes and the critical structure. The phantom was then treated on a Clinac- 2100C/D linear accelerator with the conventional dosimeter insert installed. The dosimeter results were consistent with those achieved with the original RPC phantom. [1] 4. Conclusions: The phantom with gel dosimetry insert is proposed as an evaluation tool for IMRT, and to credential institutions participating in national clinical trials. Dose distributions measured with the gel dosimeter in the simulated target volumes and critical structure region should agree with the calculated distributions to within ±5%. Spatial localization is expected to be within 5 mm. ACKNOWLEDGEMENTS This investigation was supported by PHS grants CA10953 and CA81647 awarded by the NCI, DHHS. 4

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