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Stephen F. Kry Ph.D. 1 , Paola Alvarez M.S. 1 , Andrea Molineu M.S. 1 ,

Algorithms used in heterogeneous dose calculations show systematic error as measured with the Radiological Physics Center’s anthropomorphic thorax phantom used for RTOG credentialing. Stephen F. Kry Ph.D. 1 , Paola Alvarez M.S. 1 , Andrea Molineu M.S. 1 ,

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Stephen F. Kry Ph.D. 1 , Paola Alvarez M.S. 1 , Andrea Molineu M.S. 1 ,

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  1. Algorithms used in heterogeneous dose calculations show systematic error as measured with the Radiological Physics Center’s anthropomorphic thorax phantom used for RTOG credentialing Stephen F. Kry Ph.D.1, Paola Alvarez M.S.1, Andrea Molineu M.S.1, Carrie Amador B.S.1, James Galvin Ph.D.2, and David Followill Ph.D.1 1Radiological Physics Center, The University of Texas MD Anderson Cancer Center, Houston, TX 2Radiation Therapy Oncology Group, Philadelphia, PA Wednesday, October 31, 2012 ASTRO Annual Meeting Boston, MA

  2. Introduction • Dose calculation in thorax is challenging because of heterogeneous environment. • Homogeneous and low quality heterogeneous dose calculations (e.g., Batho-corrected pencil beam) are highly inaccurate • These algorithms are no longer allowed in NCI-sponsored clinical trials

  3. Introduction • Dose calculation in thorax is challenging because of heterogeneous environment. • Homogeneous and low quality heterogeneous dose calculations (e.g., Batho-corrected pencil beam) are highly inaccurate • These algorithms are no longer allowed in NCI-sponsored clinical trials

  4. Introduction • Dose calculation in thorax is challenging because of heterogeneous environment. • Homogeneous and low quality heterogeneous dose calculations (e.g., Batho-corrected pencil beam) are highly inaccurate • These algorithms are no longer allowed in NCI-sponsored clinical trials • Convolution-Superposition/AAA algorithms are generally considered accurate

  5. Verify dose delivery: • RPC phantoms (treated like a patient)

  6. Verify dose delivery: • RPC phantoms (treated like a patient)

  7. RPC phantoms: • Long history of use • Homogeneous phantoms: • Average agreement of TLD and TPS: <1% • (Ibbott G et al. Technol Ca Res Treat 2006;5:481) • Thoracic phantom • 2 TLD in center of lung target (3 cm x 5 cm) • Film in 3 planes

  8. Irradiations • In this study: • 304 irradiations • 6 MV irradiations • IMRT or 3D CRT • Moving or static • Various algorithms • All used heterogeneity corrections • Evaluate • TLD dose (vs TPS) • Planar agreement • DTA or gamma

  9. TLD Measurement vs TPS calculation

  10. TLD Measurement vs TPS calculation

  11. TLD Measurement vs TPS calculation

  12. Measured doses systematically lower than calculated doses for C/S AAA algorithms (p<0.0001) No significant difference between C/S AAA algorithms For C/S AAA algorithms: No significant difference between IMRT (mean=0.963) and 3D CRT (mean=0.964) irradiations (p=0.7) No significant difference between moving (mean=0.961) and static (mean=0.964) irradiations (p=0.5) No significant trend versus irradiation date (p=0.2) TLD Dose Findings

  13. Planar film results • Pencil Beam algorithms showed worst DTA (p=0.03) • No significant difference between MC and CS/AAA Post-2010 analysis Pre-2010 analysis

  14. Discussion/Conclusions • Overestimation of dose with C/S AAA (3.7%) • Dose to center of target • Agreement distribution not bimodal • Why difference with MC? • Radiation transport • Dose reporting • CT calibration • Combination of these • Issue for dose calculation accuracy (TG-65 goal: 1-2%) • Potentially issue for dose reporting/prescribing

  15. Thank You! • This work was supported by grants CA010953, CA081647, CA21661

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