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Experimentally Validated Pencil Beam Scanning Source Model in TOPAS

L Lin

L Lin*, M Kang , T Solberg , C Ainsley , J McDonough , Univ Pennsylvania, Philadelphia, PA


SU-E-T-363 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Monte Carlo method can provide the most accurate dose calculation for pencil beam scanning (PBS) proton therapy if the proton sources and particle interaction mechanisms are correct.

TOPAS 1.8, a simulation tool based on Geant4.9.6, was utilized to simulate proton spot profiles. Proton sources, placed at the phantom surface, were modelled with three two-dimensional Gaussian functions to fit measured in-air spot profiles up to 100 mm radius. Simulations were compared with profiles measured using EBT3 film in Solidwater phantoms at various depths from the surface to the end of range for 100, 115, 150, 180, 210 and 225 MeV proton beams with phantom surface locations at 270 mm upstream and at isocentre.

Simulation can reproduce one-dimensional integral dose versus radius within 1 mm/1% and two-dimensional profiles within 1 mm out to 0.1% of the central spot dose for all the studied depths of all energies. For two-dimensional 0.01% isodose, simulation can reproduce all 210 MeV proton beam measurements, but cannot predict the diamond-shaped isodose distributions of the 115 MeV beam. When the proton spots are scanned 100 mm off axis, no profile difference can be found between the central axis and the off axis locations for the 210 MeV beam. For the 115 MeV beam, differences of up to 2 mm are observed at the ends of the diamond-shaped 0.01% isodose contour, implying there are lower energy components in the energy spectrum which are less than 4% of the primary energy. Subsequent simulation demonstrates that adopting such an energy spectrum within the source model can improve the distance agreement by 5-6 mm for isodoses below 0.1% near the end of range of the 115 MeV proton beam.

Excellent agreement between simulation and measurement validates our PBS source model and the particle interaction mechanism embedded in TOPAS.

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