EVALUATION OF THE EFFECTIVENESS OF ANISOTROPIC ANALYTICAL ALGORITHM IN FLATTENED AND FLATTENING-FILTER-FREE BEAMS FOR HIGH ENERGY LUNG DOSE DELIVERY USING THE RADIOLOGICAL PHYSICS CENTER LUNG PHANTOM

The effectiveness of the Anisotropic Analytical Algorithm (AAA) implemented in the Eclipse treatment planning system (TPS) was evaluated using theRadiologicalPhysicsCenteranthropomorphic lung phantom using both flattened and flattening-filter-free high energy beams. Radiation treatment plans were developed following the Radiation Therapy Oncology Group and theRadiologicalPhysicsCenterguidelines for lung treatment using Stereotactic Radiation Body Therapy. The tumor was covered such that at least 95% of Planning Target Volume (PTV) received 100% of the prescribed dose while ensuring that normal tissue constraints were followed as well. Calculated doses were exported from the Eclipse TPS and compared with the experimental data as measured using thermoluminescence detectors (TLD) and radiochromic films that were placed inside the phantom. The results demonstrate that the AAA superposition-convolution algorithm is able to calculate SBRT treatment plans with all clinically used photon beams in the range from 6 MV to 18 MV. The measured dose distribution showed a good agreement with the calculated distribution using clinically acceptable criteria of ±5% dose or 3mm distance to agreement. These results show that in a heterogeneous environment a 3D pencil beam superposition-convolution algorithms with Monte Carlo pre-calculated scatter kernels, such as AAA, are able to reliably calculate dose, accounting for increased lateral scattering due to the loss of electronic equilibrium in low density medium. The data for high energy plans (15 MV and 18 MV) showed very good tumor coverage in contrast to findings by other investigators for less sophisticated dose calculation algorithms, which demonstrated less than expected tumor doses and generally worse tumor coverage for high energy plans compared to 6MV plans. This demonstrates that the modern superposition-convolution AAA algorithm is a significant improvement over previous algorithms and is able to calculate doses accurately for SBRT treatment plans in the highly heterogeneous environment of the thorax for both lower (≤12 MV) and higher (greater than 12 MV) beam energies.

Development and Implementation of the Use of Optically Stimulated Luminescent Detectors in the Radiological Physics Center Anthropomorphic Quality Assurance Phantoms

The Radiological Physics Center (RPC) uses both on-site and remote reviews to credential institutions for participation in clinical trials. Anthropomorphic quality assurance (QA) phantoms are one tool the RPC uses to remotely audit institutions, which include thermoluminescent dosimeters (TLDs) and radiochromic film. The RPC desires to switch from TLD as the absolute dosimeter in the phantoms, to optically stimulated luminescent dosimeters (OSLDs), but a problem lies in the angular dependence exhibited by the OSLD. The purpose of this study was to characterize the angular dependence of OSLD and establish a correction factor if necessary, to provide accurate dosimetric measurements as a replacement for TLD in the QA phantoms. A 10 cm diameter high-impact polystyrene spherical phantom was designed and constructed to hold an OSLD to study the angular response of the dosimeter under the simplest of circumstances for both coplanar and non-coplanar treatment deliveries. OSLD were irradiated in the spherical phantom, and the responses of the dosimeter from edge-on angles were normalized to the response when irradiated with the beam incident normally on the surface of the dosimeter. The average normalized response was used to establish an angular correction factor for 6 MV and 18 coplanar treatments, and for 6 MV non-coplanar treatments specific to CyberKnife. The RPC pelvic phantom dosimetry insert was modified to hold OSLD, in addition to the TLD, adjacent to the planes of film. Treatment plans of increasing angular beam delivery were developed, three in Pinnacle v9.0 (4-field box, IMRT, and VMAT) and one in Accuray’s MultiPlan v3.5.3 (CyberKnife). The plans were delivered to the pelvic phantom containing both TLD and OSLD in the target volume. The pelvic phantom was also sent to two institutions to be irradiated as trials, one delivering IMRT, and the other a CyberKnife treatment. For the IMRT deliveries and the two institution trials, the phantom also included film in the sagittal and coronal planes. The doses measured from the TLD and OSLD were calculated for each irradiation, and the angular correction factors established from the spherical phantom irradiations were applied to the OSLD dose. The ratio of the TLD dose to the angular corrected OSLD dose was calculated for each irradiation. The corrected OSLD dose was found to be within 1% of the TLD measured dose for all irradiations, with the exception of the in-house CyberKnife deliveries. The films were normalized to both TLD measured dose and the corrected OSLD dose. Dose profiles were obtained and gamma analysis was performed using a 7%/4 mm criteria, to compare the ability of the OSLD, when corrected for the angular dependence, to provide equivalent results to TLD. The results of this study indicate that the OSLD can effectively be used as a replacement for TLD in the RPC’s anthropomorphic QA phantoms for coplanar treatment deliveries when a correction is applied for the dosimeter’s angular dependence.

CHARACTERIZATION OF THE COUNT RATE PERFORMANCE AND EVALUATION OF THE EFFECTS OF HIGH COUNT RATES ON MODERN GAMMA CAMERAS

CHARACTERIZATION OF THE COUNT RATE PERFORMANCE AND EVALUATION OF THE EFFECTS OF HIGH COUNT RATES ON MODERN GAMMA CAMERAS Michael Stephen Silosky, B.S. Supervisory Professor: S. Cheenu Kappadath, Ph.D. Evaluation of count rate performance (CRP) is an integral component of gamma camera quality assurance and measurement of system dead time (τ) is important for quantitative SPECT. The CRP of three modern gamma cameras was characterized using established methods (Decay and Dual Source) under a variety of experimental conditions. For the Decay method, input count rate was plotted against observed count rate and fit to the paralyzable detector model (PDM) to estimate τ (Rates method). A novel expression for observed counts as a function of measurement time interval was derived and the observed counts were fit to this expression to estimate τ (Counts method). Correlation and Bland-Altman analysis were performed to assess agreement in estimates of τ between methods. The dependencies of τ on energy window definition and incident energy spectrum were characterized. The Dual Source method was also used to estimate τ and its agreement with the Decay method under identical conditions and the effects of total activity and the ratio of source activities were investigated. Additionally, the effects of count rate on several performance metrics were evaluated. The CRP curves for each system agreed with the PDM at low count rates but deviated substantially at high count rates. Estimates of τ for the paralyzable portion of the CRP curves using the Rates and Counts methods were highly correlated (r=0.999) but with a small (~6%) difference. No significant difference was observed between the highly correlated estimates of τ using the Decay or Dual Source methods under identical experimental conditions (r=0.996). Estimates of τ increased as a power-law function with decreasing ratio of counts in the photopeak to the total counts and linearly with decreasing spectral effective energy. Dual Source method estimates of τ varied as a quadratic with the ratio of the single source to combined source activities and linearly with total activity used across a large range. Image uniformity, spatial resolution, and energy resolution degraded linearly with count rate and image distorting effects were observed. Guidelines for CRP testing and a possible method for the correction of count rate losses for clinical images have been proposed.

The performance of human papillomavirus high-risk DNA testing in the screening and diagnostic settings.

OBJECTIVE: We sought to evaluate the performance of the human papillomavirus high-risk DNA test in patients 30 years and older. MATERIALS AND METHODS: Screening (n=835) and diagnosis (n=518) groups were defined based on prior Papanicolaou smear results as part of a clinical trial for cervical cancer detection. We compared the Hybrid Capture II (HCII) test result with the worst histologic report. We used cervical intraepithelial neoplasia (CIN) 2/3 or worse as the reference of disease. We calculated sensitivities, specificities, positive and negative likelihood ratios (LR+ and LR-), receiver operating characteristic (ROC) curves, and areas under the ROC curves for the HCII test. We also considered alternative strategies, including Papanicolaou smear, a combination of Papanicolaou smear and the HCII test, a sequence of Papanicolaou smear followed by the HCII test, and a sequence of the HCII test followed by Papanicolaou smear. RESULTS: For the screening group, the sensitivity was 0.69 and the specificity was 0.93; the area under the ROC curve was 0.81. The LR+ and LR- were 10.24 and 0.34, respectively. For the diagnosis group, the sensitivity was 0.88 and the specificity was 0.78; the area under the ROC curve was 0.83. The LR+ and LR- were 4.06 and 0.14, respectively. Sequential testing showed little or no improvement over the combination testing. CONCLUSIONS: The HCII test in the screening group had a greater LR+ for the detection of CIN 2/3 or worse. HCII testing may be an additional screening tool for cervical cancer in women 30 years and older.