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

Background and purpose

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.

Materials and methods

A method was developed for calculation of the fraction of field edge dose (FED), that could be visualized as a 3D uncertainty map (3DUM_FED). 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. 3DUM_FED was calculated for all 36 plans. The highest FED for a 2 cm³ volume (FED_{2 cm}³) and average FED (FED_mean) were calculated for the planning target volumes (PTV) and organs at risk (OAR) and compared with the edge area metric (EAM).

Results

High FED (above 20 %) were mainly found just outside the PTV border. FED_mean in PTV was highest for the plans of increased complexity. The FED_mean for PTVs and OARs, as well as the FED_{2 cm}³ for PTVs, correlated with ρ ≥ 0.81 to EAM. The FED_{2 cm}³ for OARs had a weaker correlation with EAM (ρ = 0.55). 3DUM_FED analysis revealed that plan complexity affects different parts of the patient volume in different ways.

Conclusions

3DUM_FED 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.