Purpose: Proton Pencil Beam scanning delivery sequence is critical to the evaluation of the motion interplay effect. We proposed an experimental approach to build a precise machine-specific model for standard, volumetric, and layer repainting delivery based on a cyclotron accelerator system. Then we assessed the interplay effect using a 4D mobile lung target phantom compared to a generic delivery sequence model (WPE) using the same type of cyclotron accelerator proton system published previously.
Methods: Test fields and clinical treatment plans were used to drive each beam delivery parameters that impacted BDT: energy layer switching time, spot switching time, and spot drill time. To quantitatively evaluate the interplay effect, a series of digital thoracic 4DCT image sets were used. The interplay effect was assessed based on the 4D dynamic dose accumulation method. Target motion was simulated based on the periodic respiratory motion with an amplitude 5 mm and breathing cycle 4s. Two-field Single Field Uniform Dose plans were generated using PA and lateral field. Standard delivery, volumetric and layer repainting delivery were simulated based on the machine-specific delivery sequence model and WPE model. The total BDT is also calculated. D99 (Dose received by 99% of target volume) of the target is used to estimate the delivery accuracy.
Results: The results show that the interplay effect is very different between the two models even though the BDT is similar. More specifically, the WPE model leads to a very different result from the machine-specific model from this study in layer repainting technique. The delivery sequence comparison with the actual log files shows that the WPE model has a difference delivery sequence, resulting in the difference in the interplay effect evaluation.
Conclusion: A precise machine-specific delivery sequence is highly recommended to ensure an accurate estimation of mobile target treatment's interplay effect.
Funding Support, Disclosures, and Conflict of Interest: The senior author Xuanfeng Ding received personal fee from IBA speaker Bureau.