Assessing the Interplay Effect Based on a Precise Machine-Specific Delivery Sequence and Time for Cyclotron Accelerator Proton Therapy System

Abstract

Proton Pencil Beam Scanning (PBS) delivery sequence is critical in the evaluation of the motion interplay effect. We proposed an experimental approach to build a precise machine-specific model for standard delivery, volumetric repainting delivery, and layer repainting delivery based on a cyclotron accelerator system and assessed the interplay effect quantitatively.Test fields and clinical treatment plans were used to derive each beam delivery parameters experimentally. The machine delivery log files were retrospectively analyzed to model beam delivery parameters, such as energy layer switching time (ELST), spot switching time, and spot drill time, for standard, volumetric and layer repainting delivery techniques. To quantitatively evaluate the interplay effect, a series of digital thoracic 4DCT image sets were used through 4D dynamic dose accumulation method. A small tumor in radius 5mm (labeled by R5) and a large tumor in radius 2cm (R20) phantom were created to simulate the mobile target with different sizes. Target motion was simulated based on the periodic respiratory motion with an amplitude 5 mm. The breathing cycle is 4s. Two-field Single Field Uniform Dose (SFUD) plans were generated using PA and lateral field. Different delivery techniques such as standard delivery, volumetric repainting delivery (n = 2,3,4) and layer repainting delivery (n = 2,3,5,25) were simulated based on the machine-specific delivery sequence model and a simplified model published by West German Proton Therapy Essen (WPE). D99 (Dose received by 99% of target volume) of the target is used to estimate the delivery accuracy.The results showed that the WPE model's spot delivery sequence deviated from the log file significantly compared to the machine-specific model, which results the difference in the interplay effect evaluation. More specifically, for a lung treatment plan with target size (65 mm³) and layer repainting 25 times, the difference is about 21.01% due to a coarse spot scanning speed modeling and an oversimplified assumption of a constant ELST from WPE model. Such a difference also resulted in a very different interplay effects estimation between the two models even though both institutions used the same proton system from IBA and calculated using the same 4DCT imaging set.A precise machine-specific delivery sequence model is highly recommended to ensure an accurate estimation of mobile target treatment's interplay effect for each institution.