Introduction

1. Development and Implementation of the use of Optically Stimulated Luminescent Detectors in the Radiological Physics Center Anthropomorphic Quality Assurance Phantoms 1Jennelle Bergene, 1Stephen Kry, 1Andrea Molineu, 2David Bellezza, 1Laurence Court, 1Valen Johnson, 1David Followill 1Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center, Houston, TX 2St. Luke’s Episcopal Hospital, Houston, TX Introduction The Radiological Physics Center (RPC) anthropomorphic quality assurance (QA) phantom program is one tool the RPC uses to remotely audit institutions participating in clinical trials. The phantoms contain thermoluminescence dosimeters (TLDs) as the absolute dosimeter in the phantoms, and a switch to optically stimulated luminescent dosimeters (OSLDs) is desired. OSLD have been well studied by the RPC under reference conditions, and have been shown to agree well with TLD and ion chamber measurements [1], however, the use of the OSLD within the anthropomorphic phantoms has not been studied. The problem with implementing the OSLD in the anthropomorphic phantoms lies in the angular dependence exhibited by the dosimeters. This study aims to characterize the angular dependence of the OSLD in the RPC pelvic phantom for effectively utilizing the dosimeters as a replacement for TLD in the RPC’s anthropomorphic QA phantoms. The color scale distribution maps of the pixels passing the 7%/4 mm gamma criteria were compared for the coronal film normalized to TLD doses (Figure 5) and corrected OSLD doses (Figure 6) from an IMRT irradiation. The areas of pixels passing the criteria are essentially the same for both the TLD normalized film and corrected OSLD normalized film, and confirm that the corrected OSLD dose can be used to normalize film for the purpose of credentialing with the RPC’s anthropomorphic QA phantoms. The angular dependence correction factors for the 6 MV and 18 MV coplanar, and 6 MV non-coplanar irradiations of the spherical phantom were determined and are summarized in Table 2. Table 2. Angular correction factors for OSLD from spherical phantom irradiations Figure 3. Positioning of OSLD and TLD (red circles) in dosimetry insert at the center of the target The spherical phantom results reveal an under-response of the OSLD of approximately 4% at 6 MV and 2% at 18 MV. These results are in agreement with the data published by Kerns et al. [2], which demonstrated an under-response of the OSLD of 4% and 3% for 6 MV and 18 MV photon beams, respectively, for beams incident parallel to the surface of the dosimeter. The doses measured by the OSLD from the pelvic phantom irradiations were corrected with the correction factors in Table 2. The ratios of the measured TLD doses to the corrected OSLD doses can be seen in Table 3. The angular correction factors effectively corrected the OSLD dose to within 1% of the TLD dose for both the coplanar and institution trial irradiations, but not for the three CyberKnife irradiations. Treatment plans of increasing angular beam delivery were developed for the pelvic phantom. Three coplanar plans were developed in Pinnacle, and one non-coplanar plan was developed in Accuray’s MultiPlan. Each plan was delivered three times to the phantom loaded with TLD-100 capsules and nanoDot OSLD. The IMRT treatments included the same dosimeters, in addition to radiochromic film in the coronal and sagittal planes. The pelvic phantom was also sent to two institutions to be irradiated, one delivering IMRT and the other CyberKnife. The doses measured from the TLD and OSLD were calculated for each irradiation, applying the correction factor to the OSLD dose. The ratio of TLD measured dose to angular corrected OSLD dose was determined for each irradiation. The films from the IMRT deliveries and institution trials were normalized to the TLD and corrected OSLD doses. Dose profiles were taken and gamma analysis was performed using a 7%/4 mm criteria, for both TLD and corrected OSLD normalized films, and the results were compared. Figure 5. IMRT color scale gamma results for coronal film normalized to TLD dose Methods and Materials A 10-cm diameter, high-impact polystyrene spherical phantom (Figure 1 left) was constructed to hold a nanoDot™ OSLD (Figure 1 right) from Landauer, to study the angular response of the dosimeter under the simplest of circumstances. The OSLD were irradiated to 100 cGy in a coplanar geometry for 6 MV and 18 MV photon beams, and in a non-coplanar geometry for a 6 MV photon beam. The responses of the dosimeters were normalized to the response when the beam was incident normally on the face of the dosimeter (‘face-on’). The inverse of the average normalized responses, not including the ‘face-on’ response, was calculated and used as the angular dependence correction factor. Figure 6. IMRT color scale gamma results for coronal film normalized to corrected OSLD dose Table 3. Ratios of TLD to corrected OSLD doses from pelvic phantom irradiations Conclusion For all irradiations, with the exception of the three CyberKnife irradiations performed, the angular dependence correction factors established from the spherical phantom irradiations effectively corrected the OSLD measured dose to within 1% of the TLD measured dose. Based on the results of the study, OSLD can effectively be used as the absolute dosimeter in the RPC’s anthropomorphic QA phantoms for coplanar treatment deliveries when a correction factor is applied for the angular dependence exhibited by the dosimeters. The angular correction factor determined for non-coplanar treatment deliveries is not recommended for use, due to considerable differences in the resulting TLD to OSLD dose ratios from the CyberKnife irradiations. References 1. Aguirre et al. "WE-D-BRB-08: Validation of the commissioning of an optically stimulated luminescence (OSL) system for remote dosimetry audits," Med Phys 37 (6), 3428 (2010). 2. Kerns et al. "Characteristics of optically stimulated luminescence dosimeters in the spread-out Bragg peak region of clinical proton beams," Med Phys 39 (4), 1854-1863 (2012). Support This investigation was supported by PHS grant CA10953 awarded by the NCI, DHHS. The films included in the pelvic phantom irradiations were normalized to the TLD measured doses and the OSLD doses, to validate that the dosimeters can provide equivalent dose profile and gamma analysis results when the angular dependence of the OSLD have been corrected. The lateral dose profiles from the coronal films, normalized to TLD doses and angular corrected OSLD doses were compared. The two dose profiles appear almost identical, and confirm that the corrected OSLD dose can be used to normalize film for the purpose of credentialing with the RPC’s anthropomorphic QA phantoms. The percentage of pixels passing the 7%/4 mm gamma criteria are shown in Table 4 for the coronal and sagittal films for the two institution trials, as well as the average IMRT pixel passing rate. Figure 4. High-impact polystyrene slab phantom Figure 1. Spherical phantom showing base and insert holding OSLD (left), nanoDot OSLD from Landauer (right) Energy correction factors for OSLD in full phantom conditions were determined for 6 MV and 18 MV using a high-impact polystyrene slab phantom (Figure 4), with an OSLD placed at a depth of 10-cm. The response of the dosimeter at the investigated energy was compared to the response of a dosimeter irradiated in a cobalt-60 beam, and this was used to calculate energy correction factors. Results The OSLD full phantom energy correction factors for 6 MV and 18 MV photon beams are shown in Table 1. These energy correction factors were used to calculate the dose to OSLD for the pelvic phantom irradiations. The RPC’s pelvic phantom (Figure 2) was used for this study to investigate the angular response of the OSLD in the anthropomorphic QA phantoms. The dosimetry insert of the pelvic phantom was modified to contain two OSLD in the axial plane, in addition to the two TLD within the target volume (Figure 3). Figure 2. RPC pelvic phantom shell (left), dosimetry insert (middle), and imaging insert (right) Table 4. Percent of pixels passing gamma criteria of 7%/4 mm for TLD and corrected OSLD normalized films Table 1. Full phantom OSLD energy correction factors