ESTRO 37 Abstract book

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ESTRO 37

Results Of the ‘reference’ tissues theoretically tested, this model leads to a mean error in SPR (for a 250 MeV proton) of 0.03% (<0.2% max error) and 0.2% (<0.3% max error) relative to a calculation by all elemental constituents using BAR for soft tissue and cortical bone, respectively. Model RMSE in soft tissue were 0.77% and 0.09% for mean ionization potential and SPR, respectively. Model RMSE in cortical bone were 1.8% and 0.2% for mean ionization potential and SPR, respectively. In the simple phantom, SPR was calculated to 1% using this multi-modal imaging method compared with theoretical SPR values. In comparison, SPR determined with CT by the stoichiometric method was found to be 3% different from theoretical SPR values. Conclusion We have proposed a novel method to model human tissue compositions for the purpose of accurate mean ionization potential and stopping power ratio calculation. For soft tissue, this model requires quantification of two parameters, percent water content by mass and percent content of hydrogen in organic molecules by mass, both of which are measurable using clinically available MR imaging techniques. In this work, we have demonstrated that this multi-modal imaging method can be used to accurately quantify proton SPR. PV-0139 Experimental verification of 4D Monte Carlo simulations of dose delivered to a deforming anatomy S. Gholampourkashi 1 , J.E. Cygler 1,2,3 , B. Lavigne 3 , E. Heath 1 1 Carleton University, Physics, Ottawa, Canada 2 University of Ottawa, Radiology, Ottawa, Canada 3 The Ottawa Hospital Cancer Center, Medical Physics, Ottawa, Canada Purpose or Objective To validate 4D Monte Carlo (MC) simulations of dose delivery to a deformable lung phantom. Material and Methods A deformable phantom (Fig.1) molded from flexible foam with lung density, holding a cylindrical plug containing a silicon rubber tumor with density of tissue was built. 30 Lucite beads were injected as landmarks throughout the phantom to help with the deformable registration and quantifying the phantom motion features. A piston at the inferior end of the phantom attached to a DC motor provided sinusoidal motion with 1.35 cm peak-to–peak respiratory motion at the tumor center.

PV-0138 A multi-modal MRI and CT imaging method for the accurate calculation of proton stopping power ratios A. Sudhyadhom 1 , J. Scholey 1 , T. Solberg 1 1 University of California UCSF, Radiation Oncology, San Francisco CA, USA Purpose or Objective The purpose is to report on a novel method to accurately calculate proton SPR using a combination of MRI and CT imaging. Material and Methods Proton SPR was calculated by the Bethe-Bloch equation. In this equation, the component of greatest uncertainty is the mean ionization potential (also known as the mean excitation energy), as electron density can be determined by CT. A method was devised to determine mean ionization potential using MRI and CT by categorizing biologically-based molecules into 3 major categories: water (H2O), organic (org), and hydroxyapatite (HA). In doing so, the Bragg additivity rule (BAR) to determine mean ionization potential by elemental constituents can be reordered as a sum over molecular compounds and reduced to: , where w is the mass content percentage for a particular type of molecule. An exponential relationship between organic molecule hydrogen density and its mean ionization potential was determined and was used to further simplify this equation to quantities measurable by MRI (water content and hydrogen density) and CT (HA/bone density): , where A and B are exponential fit constants and h is the hydrogen density within a voxel. We evaluated this model for determination of SPR in the 'reference’ tissues of ICRU 44 and in others available in the literature. A simple phantom was created of chemicals with known elemental compositions. Theoretical SPR was calculated by known elemental composition/density and used as a reference. MRI and CT scans were completed in this phantom to determine SPR using this multi-modal method, where the CT was used to determine mass/electron density. As a comparison against current clinical practice, CT was used to determine SPR directly using the stoichiometric method.

Measurements were performed on an Elekta Agility linac with the phantom in stationary and moving (3.3 s period) states. Dose within the tumour was measured using calibrated Gafchromic EBT3 film and the RADPOS 4D dosimetry system 1 . To measure the dose inside the lung, another RADPOS detector was mounted on the top