Evaluating Monaco 6.2.2 in complex radiotherapy across matched LINACs: improved MLC modelling and dose accuracy with virtual source model 2.0

Abstract

This study assesses the updated Monaco TPS virtual source model (VSM) 2.0, which removes multileaf collimator (MLC) and jaw characterization as editable factors from the MLC geometry section within Monaco. The focus is on the impact of changes to stereotactic radiotherapy (SRT) cases for spinal and intracranial treatments for two beam matched linear accelerators. A validated custom VSM 1.6 model optimized for SRT was compared with the Elekta Accelerated Go Live 6 MV flattening filter-free (FFF) and VSM 2.0. Evaluations included measured MLC characteristics with a high-resolution detector, measured output factors (OPF), ion chamber fields in the thorax phantom, and recalculations of clinically relevant SRT cases. VSM 2.0 improves MLC modelling. Ion chamber measurements for IAEA TD1583 measurements were found to be within expected tolerances. Gamma pass rates for two matched LINACs evidenced improvement at 1%, 1 mm and 10% threshold for single and multi-SRS brain and SABR Spine treatments. VSM 2.0 represents a meaningful advancement in beam modelling within a Monte Carlo-based TPS environment, offering improved dosimetric performance and operational simplicity. Commercially available detectors were used to demonstrate that VSM 2.0 enhances agility MLC modelling, supporting more precise SRT and SABR delivery for matched LINACs. Removing configurable dependencies from the beam model will result in more consistent high quality beam models, an improves workflows for commissioning of the Monaco TPS.

Keywords: Characterisation; MLC; Modelling; SIMT; SRT; Spine.

Fig. 1

Clinical validation workflow for VSM 2.0 model without geometry factors

Fig. 2

Four-L sequence, where progressive deliveries expose an L shape nested in the previous delivery running from shape A to D

Fig. 3

3 Segment abutting sequence, where progressive deliveries expose a 2.0 cm strip across the detector from shape A to C. Figure D is the jaw only characterization field that collimates with Y1 and Y2 jaw only

Fig. 4

SRS MapCHECK and custom 3D printed holder and Perspex inserts

Fig. 5

Output factors for measured and calculated effective field size. Measured OPF were completed with Measured, VSM 2.0 and VSM 1.6

Fig. 6

Dose profile across the in-plane direction through the leaf groove region as detailed by the black line on the inset image for measured versus VSM 1.6 and VSM 2.0

Fig. 7

7SegA picket fence field measured versus VSM 1.6 and VSM 2.0 calculated

Fig. 8

Y Jaw only field for comparison between measured and calculated for VSM 1.6 and VSM 2.0

Fig. 9

CIRS thorax phantom and measurement positions in tissue, lung and bonelike structures

Fig. 10

This figure illustrates boxplots that compare gamma pass rates for single-target brain cases, calculated using VSM 2.0 and VSM 1.6, as assessed with the SRS MapCHECK on LINAC 1 and LINAC 2. The results encompass cases delivered on the central axis, 30 mm off-axis in the cross-plane, and 60 mm off-axis in the in-plane

Fig. 11

The figure illustrates boxplots depicting the gamma pass rates for measurements conducted at 3%, 1 mm; 2%, 1 mm; and 1%, 1 mm, with a 10% threshold. The results pertain to the five measured targets within a single stereotactic SIMT plan, evaluated for a case calculated using VSM 2.0 and VSM 1.6 for LINAC 1 and LINAC 2

Fig. 12

The boxplots illustrate the gamma pass rates for a cohort of stereotactic spine plans, comparing optimized and original beam models as measured with SRS MapCHECK. These boxplots present results for 3%, 1 mm; 2%, 1 mm; and 1%, 1 mm, calculated using VSM 2.0 and VSM 1.6 for LINAC 1 and LINAC 2