Room: Karl Dean Ballroom A2
Purpose: To assess proton pencil beam scanning (PBS) treatment quality for lung cancer patients who exist both large tumor motion and tissue heterogeneity using 4D Monte Carlo simulation based on open-source fast Monte Carlo (MC) algorithm, MCsquare.
Methods: Twelve lung patients with different motion amplitudes previously treated using double scattering proton therapy were replanned with PBS. The motion magnitude is obtained from the deformable registration field between end exhalation and end inhalation CTs. Discrepancy between recalculated dose (MC static dose) using MCsquare and TPS was used to assess the impact of inaccuracies of analytical dose calculation (ADC) in TPS. 4D dose was calculated by accumulating the spot dose on the corresponding respiration phase to the reference phase by deformable image registration based on spot timing and patient breathing phase to assess interplay effect.
Results: The twelve patients had a range of motion from 3.1mm to 12.3mm. The difference in D95 between ADC and MC static dose ranged from 2.1% to 8.2%; between MC interplay average and MC static dose: -0.2% to 2.3%; between maximum and minimum interplay dose: 0.3% to 4.1%. The motion amplitude correlates with the range of the interplay dose, and the D95 difference between ADC and MC can be significantly higher than between MC and interplay dose. The other indices follow the same trend as D95.
Conclusion: We demonstrate a 4D fast Monte Carlo simulation to assess both the impact of heterogeneity and motion for lung patients treated with PBS. Implementation of MC in clinic can better quantify the impact a patientâ€™s motion on the dose coverage.