Figure 3: The phantom prototype broken apart with nanoDots inserted.

1. Development and Implementation of a Remote Audit Tool for High Dose Rate (HDR) 192Ir Brachytherapy Using Optically Stimulated Luminescence Dosimetry Kevin Casey1, Paola Alvarez1, Ann Lawyer1, Stephen Kry1, Rebecca Howell1, Scott Davidson2, David Followill11Dept of Radiation Physics, UT MD Anderson Cancer Center, Houston, TX, 2The Methodist Hospital, Houston, TX Methods (continued) KB corrects for a number of factors which are unique to this project, such as OSLD overresponse at 192Ir spectrum energies, incomplete backscatter and lack of equivalence between polystyrene and water, and angular dependence of OSLD nanoDots. It was determined by irradiating dosimeters, correcting the reading for fading and linearity, and then dividing the TG-43 calculated dose (with a NIST-traceable source strength) at the point of measurement by the corrected reading. This isolates KB on one side of Equation 1. Results (continued) Materials (continued) Introduction The Radiological Physics Center’s (RPC) mission is to ensure consistency and comparability of radiation doses delivered at institutions participating in NCI-funded cooperative clinical trials. A major effort of the RPC to accomplish this mission is the mailable optically stimulated luminescence dosimeter (OSLD) program for remote audits of participating institutions’ external beam (EBRT) reference calibrations. This program has accuracy sufficient to establish a ±5% acceptance criterion for comparison between RPC-measured and institution-reported dose [1]. However, brachytherapy, which is the placement of radioactive sources in or near the tumor, is also used in clinical trials. Unfortunately, no program analogous to the RPC’s EBRT program exists for remote audits of high dose-rate (HDR) brachytherapy sources. Current RPC HDR activities consist only of plan checks, questionnaires, and infrequent site visits. This project aims to create a mailable, OSLD-compatible tool capable of remote audits of HDR brachytherapy sources with accuracy suitable for RPC monitoring of clinical trial sites. Table 2: Uncertainty budget for dose measurements. To date, remote audits have been performed at 8 institutions using the system described here (Table 3). For each audit, the OSLD-measured dose was compared to the dose reported by the institution’s treatment planning computer at the point of measurement. The average ratio is 1.000 and the standard deviation is 0.011. Results Figure 3: The phantom prototype broken apart with nanoDots inserted. Methods The equation used to calculate dose from an OSLD reading is as follows: (1) Materials Landauer’s nanoDot OSL dosimeters (Figure 1) were chosen for their near-planar geometry and proven accuracy when used as the basis for a mailed dosimeter program. The RPC has considerable experience with and infrastructure in place for using nanoDots, with over 10,000 currently in circulation as part of the external beam audit program. Where: reading raw, uncorrected OSLD reading ECF element-specific correction factor Sensitivity system sensitivity at time of reading [dose/counts] KFfading correction factor KLlinearity correction factor KBblock/energy correction factor It was found that KL=-9.433×10-5 × Dose + 1.0094 where Dose is the nominal dose in cGy. KL is shown along with its 95% confidence interval in Figure 4. KB was determined separately for two 192Ir HDR sources, the Nucletron microSelectron v2 and the Varian VariSource VS2000. KB results are summarized in Table 1. ECF corrects for differences in sensitivity between individual dosimeters and the overall batch average and has already been determined by the RPC for each dosimeter currently in use. Sensitivity is determined anew for each OSLD reading session through the reading of special “standards” dosimeters. These standards are dosimeters irradiated to 100 cGy under carefully controlled conditions using a 60Co beam. Reading a standard before and after each individual OSLD reading session allows for the establishment of a dose-per-OSLD-counts conversion (aka Sensitivity). KF corrects for OSLD signal fading over time after irradiation and has been previously established by the RPC. KL corrects for the linearity of OSLD response with dose. It was determined by irradiating 78 dosimeters to doses between 50 and 400 cGy. For each dosimeter, the nominal dose per OSLD reading (“dose response”) was plotted against the nominal dose. A linear fit was applied and normalized to the value at 100 cGy nominal dose. Thus, KL 1.000 at 100 cGy. Conclusion The estimated 2σ uncertainty of 2.4% or 2.5% is sufficient to establish a ±5% acceptance criteria on RPC-to-institution dose ratios [3]. Furthermore, preliminary remote audit results compare favorably to a sample of 193 well-chamber measurements performed by the RPC on site visits from 1994 to 2011. Average RPC-to-institution ratio for the well-chamber visits was 1.009 with standard deviation of 0.014. This is compared to 1.000 and 0.011, respectively, measured with this project. The tool established in this work is durable, simple, and most importantly accurate enough for RPC audits of HDR brachytherapy sources at institutions participating in NCI-funded clinical trials. This will greatly help the RPC in pursuit of its mission to ensure consistent and comparable radiation doses at these institutions as HDR brachytherapy becomes ever more prevalent in cooperative clinical trials. Figure 4: Linearity correction factor with 95% confidence interval. Table 3: Institution audit results from system feasibility study. An 8 x 8 x 10 cm3 phantom prototype was manufactured out of high-impact polystyrene (ρ=1.04 g/cm3) (Figure 2). The phantom has a single channel which admits a standard HDR endobronchial catheter. Two slots, one on either side of the channel, hold nanoDot dosimeters. The phantom breaks into two pieces for ease in loading and unloading dosimeters (Figure 3). Figure 1: nanoDot OSL Dosimeters Table 1: Block correction factors for two 192Ir HDR sources. The percent uncertainty in Dose measurements using the system was estimated by adding in quadrature the percent uncertainties of each term in Equation 1. Uncertainties for reading, ECF, sensitivity, and KF were provided by the previous work of Aguirre et al.[2]. Percent uncertainty in KL was 0.15% in the region of 90-110cGy (see Figure 4). Percent uncertainty in KB was the measured standard deviation for each source. 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, 3428 (2010). 2) Aguirre et al. “SU-E-T-126: Analysis of Uncertainties for the RPC Remote Dosimetry Using Optically Stimulated Light Dosimetry (OSLD)” Med Phys 38, 3515 (2011). 3) Kirby, et al. “Uncertainty analysis of absorbed dose calculations from thermoluminescence dosimeters” Med Phys 19, 1427-1433 (1992). Figure 2: Cross section of phantom prototype. All dimensions in mm. Support This investigation was supported by PHS grant CA10953 awarded by the NCI, DHHS.