# Three-dimensional dose uncertainty maps based on the fraction of field edge dose for volumetric modulated arc therapy plans

## TL;DR

This study introduces a 3D method to map dose uncertainties near field edges in radiotherapy, showing how plan complexity affects dose accuracy in different patient regions.

## Contribution

A novel 3D uncertainty map (3DUMFED) is developed to quantify field edge dose uncertainties in volumetric modulated arc therapy.

## Key findings

- High FED values were mainly observed just outside the PTV border.
- FEDmean in PTV was highest for plans with increased complexity.
- 3DUMFED analysis showed that plan complexity affects different patient regions differently.

## Abstract

Absorbed dose uncertainties in radiotherapy plans are generally larger near field edges compared to the center of the field. The aim of this study was to investigate dose uncertainties related to the field edge in 3D for plans of varying complexities.

A method was developed for calculation of the fraction of field edge dose (FED), that could be visualized as a 3D uncertainty map (3DUMFED). Twelve clinical treatment plans were included for four different treatment sites that were reoptimized to create one plan with reduced complexity and one of increased complexity. 3DUMFED was calculated for all 36 plans. The highest FED for a 2 cm3 volume (FED2 cm3) and average FED (FEDmean) were calculated for the planning target volumes (PTV) and organs at risk (OAR) and compared with the edge area metric (EAM).

High FED (above 20 %) were mainly found just outside the PTV border. FEDmean in PTV was highest for the plans of increased complexity. The FEDmean for PTVs and OARs, as well as the FED2 cm3 for PTVs, correlated with ρ ≥ 0.81 to EAM. The FED2 cm3 for OARs had a weaker correlation with EAM (ρ = 0.55). 3DUMFED analysis revealed that plan complexity affects different parts of the patient volume in different ways.

3DUMFED offers a way to estimate dose uncertainties related to the field edge in 3D. It also allows for separate evaluation in different regions of interest, unlike EAM, which mainly correlates with the dose uncertainty related to the PTV.