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VMAT Optimization and Dose Calculation in the Presence of Metallic Hip Prostheses

H Parenica

H Parenica1*, R George1 , E Bowers1 , P Mavroidis2 , Z Shi1 , Y Li1 , W Jones3 , C Ha1 , N Papanikolaou1 , S Stathakis1 , (1) University of Texas HSC at San Antonio, San Antonio, Texas, (2) Univ North Carolina, Chapel Hill, NC, (3) South Texas Veterans Health Care System, San Antonio, Texas


SU-I-GPD-T-327 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To quantify and compare the effect of hip prostheses on dose distributions calculated using Collapsed Cone Convolution Superposition (CCCS) and Monte Carlo (MC) (with and without correcting for the densities of the implant and surrounding tissues). The use of full VMAT arcs versus VMAT arcs avoiding the hip implants was also studied.

Methods: Seven prostate patients with hip prostheses were included in this study. All were prescribed a dose of 7800 cGy over 39 fractions. The hip prostheses and the streaking artifacts on the CT images were contoured by a single physician. Two plans were created in the Pinnacle³ TPS: one using full VMAT arcs and one using VMAT arcs that avoided going through the prostheses. From both of those plans, two more plans were optimized in which the densities of the prostheses and surrounding tissues were overridden (5 g/cc and 1 g/cc, respectively). The plans were then exported to the Monaco TPS and recalculated using a MC dose calculation algorithm. The changes in dose to PTVs and surrounding Organs at Risk (OAR) were evaluated in Velocity.

Results: Plans calculated with CCCS with correct density information showed reasonable agreement (within +/- 1.5%) with MC calculations. Full arc VMAT plans calculated with CCCS showed the largest discrepancy (3.84% decrease to PTV) with MC calculations when correct density information was not used. Doses to OAR (particularly the bladder) were significantly decreased when full arc VMAT plans were used instead of skip arc VMAT plans.

Conclusion: When planning for prostate patients with hip prostheses, correct density information for implants and surrounding tissues should be used to optimize the plan and ensure optimal accuracy. Full arcs could be used to spare dose to OAR (i.e. bladder, rectum), while maintaining adequate PTV coverage, when using a model-based or MC dose calculation.

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