Comparing 4D Dynamic and Static Monte Carlo Recalculations of Treatment Plans to Evaluate Interplay Effects In Proton Lattice Therapy
Abstract
Purpose
To investigate the role of interplay effects between respiratory motion and proton scanning beam interactions on free-breathing proton lattice radiotherapy (PLT) plans relative to dose distributions from static calculations.
Methods
A cohort of 6 proton therapy patients, consisting of 4 thoracic and 2 abdominal cases, were replanned retrospectively using the Varian Eclipse proton treatment planning system with single-field optimization for individual vertices. In each treatment plan, 3–17 targets (average diameter 1.6 cm) were distributed with 2.5-3.5 cm inter-vertex spacing. 18 Gy was prescribed to each vertex, whereas the valley received 3 Gy. Simulations, including a “static” calculation of each plan on a 4DCT average, “respiratory motion” average of plan doses calculated on individual 4DCT phases deformably registered to average images, and “interplay” simulations using spot delivery and respiratory timing to distribute dose onto different phases, utilized the open-source platform OpenReggui and Monte Carlo dose engine MCsquare. Dosimetric evaluations of plans use the following metrics: GTV Dmean, D95%, Vertex D90%, generalized equivalent uniform dose (gEUD a=-10), and three peak-to-valley dose ratios (PVDR1, PVDR2, PVDR3) defined as DRx/Dmean(95%-100%), D10%/D90%, and D2%/D50% respectively.
Results
Evaluating PVDR invariance among static, respiratory motion, and interplay dose simulations, PVDR3 demonstrated the smallest deviation when factoring in the interplay effect with a -1.9% difference, compared to 5.5% and -4.4% for PVDR1 and PVDR2 respectively. Dosimetric parameters for the GTV between static and 4D scenarios showed no significant differences. A decrease of 25.7% for Vertex D90% between static calculations and the average of interplay simulations for each subject was statistically significant (p<0.05).
Conclusion
Dose differences between static calculations and 4D analyses are dominated by respiratory motion, however these differences are not statistically significant. When factoring in the interplay effect, only differences between Vertex D90% are significant, which are driven primarily by vertex position shift and dose smearing.