# Effective spot size parameters for Acuros dose calculation algorithm using enhanced leaf modelling: Estimation based on small rectangular MLC fields

## TL;DR

This study optimizes dose calculation accuracy for small MLC-shaped fields in radiotherapy using the Acuros algorithm with enhanced leaf modeling.

## Contribution

A method to determine effective spot size parameters for accurate dose calculations in elongated small MLC fields is presented.

## Key findings

- Optimized ESS parameters reduced FOF discrepancies between calculated and measured doses for elongated fields.
- Dose calculations matched measurements within 0.2% ± 0.4% with a maximum difference of 1.1%.

## Abstract

Small field dosimetry plays a critical role in stereotactic radiotherapy, particularly for elongated fields shaped by multileaf collimators (MLCs). This study aims to optimize the Effective Spot Size (ESS) parameters in the Acuros dose calculation algorithm (Eclipse v18.0, with the enhanced leaf modelling ELM for MLC modelling) for accurate dose calculations of small, rectangular MLC‐shaped fields. Measurements were based on the TRS‐483 protocol and a recently proposed equivalent square field size determination method.

Field output factors (FOFs) were measured using a Varian TrueBeam linear accelerator with HD‐MLC for 6 MV and 10 MV beams (both flattened and unflattened). Symmetric square and rectangular fields (0.5–4 cm sides) were shaped by the MLC, with jaws fixed at 4.4 × 4.4 cm2 to minimize scatter variation. Measurements were performed at isocenter, 10 cm depth using three detectors: microDiamond, PinPoint3D, and DiodeE. The microDiamond detector results were used for ESS optimization due to its complete output correction factor availability. FOFs were calculated in Eclipse v18.0 using Acuros with varying ESS values (0–1.5 mm). The optimal ESS was determined by minimizing FOF differences between calculated and measured values for extreme elongated field sizes in the studied range (0.5 × 4 cm2 and 4 × 0.5 cm2).

The optimized ESS parameters minimized FOF discrepancies between calculated and measured doses for elongated fields, improving calculation accuracy compared to vendor‐recommended settings (ESSx = 0.5 mm, ESSy = 0.7 mm). Profiles and penumbra analyses supported these findings.

ELM in Eclipse v.18 improves MLC modelling by representing rounded leaf ends. Only ESS parameters remain user‐tunable. A method to determine the ESS for the Acuros algorithm is presented, resulting in accurate algorithm configuration for elongated small fields shaped by an HD‐MLC. Results are presented using 6 MV and 10 MV beams, both flattened and unflattened for an HD‐MLC Varian. ESS affects output factors and penumbra, with dose calculations matching measurements within 0.2% ± 0.4% and a maximum difference of 1.1%.