(Saturday, 4/7/2018)
Room: Foyer
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 density of the implant and surrounding tissues). The use of full VMAT arcs versus VMAT arcs avoiding the hip implants (i.e. skip arcs) was also studied.
Methods: Six prostate patients with hip prostheses were studied. 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, copies were made and the doses were recalculated with the densities of the prostheses and surrounding tissues overridden (5 g/cc and 1 g/cc, respectively). The plans were then exported to the Monaco TPS and recalculated using a Monte Carlo dose calculation algorithm. The changes in dose to PTVs and surrounding Organs at Risk (OAR) were evaluated in VelocityAI.
Results: Plans calculated with CCCS with correct density information showed reasonable agreement with MC calculations. Doses to OAR were significantly decreased when full arc VMAT plans were used instead of skip arc VMAT plans. For full arc plans, there was some difference in Pinnacle³ when plans were recalculated using correct density information. Plans in Pinnacle3 showed good agreement with Monaco when correct density information was used.
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, while maintaining adequate PTV coverage, when using a model-based or MC dose calculation.
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