Multi-Phase Interplay and Gradient Dose Effects on Moving Tumors Calculated Using IGOR
Abstract
Purpose
To report on a GUI-driven program “IGOR” (Interplay & Gradient Outcomes in Radiotherapy) which simulates MLC interplay and gradient effects accounting for the tumor’s phase of motion at the start of treatment, and the different tumor start-phases of motion between multiple treatment arcs.
Methods
IGOR was written in-house using the Matlab programming language to calculate interplay and gradient effects for VMAT and conformal arc treatments. For each patient, information from the 4D tumor motion (from 4DCT scan) and the breathing trace/BPM (from RGSC waveform) are used in the simulation to move the tumor while dose is calculated in a homogeneous phantom via a dose engine (DOI:10.1088/2057-1976/acf5f3). Based on the breathing trace, ten different tumor start-phases are used to define the start-of-simulation position for the tumor motion. For each arc ten simulations are performed, and for multiple arcs all start-phase amplitude combinations are simulated (100 simulations for two arcs).
Results
Good agreement was observed between relative 2D dose differences from a moving tumor (1cm, 2 cm, 3 cm) versus a stationary tumor between CIRS lung phantom film measurements and simulation. Interplay, gradient, and total dose differences at D95 compared to the planned DVH for a tumor moving 2 cm SI show large variations across the 100 phase combinations for the total effects (0.6% to 5.1%) and interplay effects (-3.5% to 1.3%), and only small variations (3.8% to 4.2%) for gradient effects. In general, gradient effects increase tumor dose, interplay increases dose heterogeneities. On a standard PC, IGOR takes 20 minutes to calculate on a 2-arc plan.
Conclusion
A multi-phase interplay and gradient effect analysis should be performed on moving targets to obtain the full magnitude of these effects. Measurements or simulations not considering the tumor’s phase of motion at the start of treatment may considerably underestimated interplay and gradient effects.