Transfer of fractionated treatment plans from ring-gantry to L-shaped linacs using fluence map convolution

Transferring treatment plans from a ring-gantry Ethos system to a standard L-shaped linear accelerator (linac), such as TrueBeam, is often a time-consuming and labor-intensive process, requiring repeated optimization steps. This study proposes a novel method for converting Ethos IMRT plans to TrueBeam IMRT plans using Fluence Map Generation (FMG) based on a fluence map convolution approach, intended to minimize treatment delays in the event of Ethos downtime. The FMG method comprises three key steps: determining an optimal convolution kernel, generating fluence maps from Ethos plans, and applying 2D convolution to simulate corresponding TrueBeam fluence maps. Forty original Ethos IMRT plans were used to validate the FMG approach, and the number of transferable fractions for each converted plan was evaluated. The FMG method successfully generated a moderate number of transferable fractions, ranging from one to six. The FMG-generated plans showed comparable dose distributions between the cumulative dose (FMG + Original) and the original Ethos plans (Original). This FMG-based conversion strategy provides a practical solution for converting Ethos IMRT plans to TrueBeam IMRT plans, helping mitigate treatment interruptions and reduce the clinical workload associated with plan re-optimization.