SU‐E‐T‐874: Investigation of the Interplay Effect Between MLC and Lung Tumor Motions Using 4DCT and RPM Profile Data

Abstract

Purpose: To evaluate the dosimetric impact of the interplay effect between the MLC motion and respiratory‐induced tumor motion in the context of 4D planning of small lung lesions treated with SBRT. Methods: IMRT plan was generated using the ITV from 10 respiratory‐correlated CT datasets on the reference (50% phase, CT50) image. The PTV included a 3 mm uniform expansion of ITV; the dose was calculated with Varian Eclipse AAA algorithm and 2.5 mm grid size. To account for the interplay effect, the time‐stamped signal from respiratory surrogate (RPM marker trace) was used to correlate the MLC control points to the individual CT phase. The dose to each phase was then computed using the actual MLC apertures and MU numbers for the specific phase. Doses from individual phases were mapped and accumulated to the CT50 dataset using a Demons‐based deformable registration algorithm. A “standard” 4D plan was also generated in which the entire treatment plan was applied to each phase with appropriate weighting of the time spent at each phase, but without accounting for the correlation of the MLC sequence with the respiratory phases. The method was applied to two lung SBRT cases with tumor motion 1.2 and 2.0 cm in amplitude sup/inf. Results: For mean PTV dose, the discrepancy induced by interplay effect was −5.7% to 33.4% (with mean −+ SD of 1.3 −+ 8.7%) for individual phases. For the accumulated dose it was 0.0% and 0.1% for the two cases respectively. The maximum point dose discrepancy within PTV was 0.4 out of 48 Gy. The interplay effect reduced the minimum PTV and ITV dose by about 0.5%. Conclusions: This initial study showed that the dosimetric impact of interplay effect dimmed out as 4D doses were accumulated. Further assessment for wide spectrum of plan and delivery scenarios should be performed.