# Consideration of Optimal Evaluation Metrics for Internal Gross Tumor Dose Relevant to Tumor Response in Multi-fraction Stereotactic Radiosurgery of Brain Metastasis

## TL;DR

This study examines how to best evaluate internal tumor doses in brain metastasis radiosurgery to improve treatment outcomes.

## Contribution

The study introduces DeIIV as a better metric for evaluating internal doses in VMA-based SRS compared to traditional metrics.

## Key findings

- GTV - 2 mm and GTV - 4 mm coverage with DeIIV showed significant variability across planning systems.
- DeIIV was significantly higher than GTV D50% for GTV - 4 mm, but not for GTV - 2 mm.
- Doses 2-4 mm inside the GTV decrease with larger tumors, reducing exposure to surrounding brain tissue.

## Abstract

Introduction

In stereotactic radiosurgery (SRS) for brain metastasis (BM), the target dose inhomogeneity remains highly variable among modalities, irradiation techniques, and facilities, which can affect tumor response during and after multi-fraction SRS. Volumetric-modulated arcs (VMAs) can provide a concentrically-layered steep dose increase inside a gross tumor volume (GTV) boundary compared to dynamic conformal arcs. This study was conducted to review the optimal evaluation method for the internal GTV doses relevant to maximal response and local control, specifically to examine the significance of the doses 2 mm and 4 mm inside the GTV boundary in VMA-based SRS.

Materials and methods

This was a planning study for the clinical scenario of a single BM and targeted 25 GTVs of >0.50 cc, including eight spherical models with diameters of 10-45 mm and 17 clinical BMs (GTV: 0.72-44.33 cc). SRS plans were generated for each GTV using VMA with a 5-mm leaf-width multileaf collimator and the optimization that prioritized the steepness of the dose gradient outside the GTV boundary without any internal dose constraints. The dose prescription and evaluation were based on the GTV DV-0.01 cc, a minimum dose of GTV minus 0.01 cc. Two planning systems were compared for the GTV - 2 mm and GTV - 4 mm structures that were generated by equally reducing 2 mm and 4 mm from the GTV surface. The DeIIVs, a minimum dose of the irradiated isodose volume equivalent to the GTV - 2 mm and GTV - 4 mm, were compared to other common metrics.

Results

The GTV - 2 mm and GTV - 4 mm volumes differed significantly between the systems. In the spherical GTVs, the irradiated isodose surfaces of GTV D80% and D50% corresponded to 0.4-1.6 mm (<2 mm) and 1.0-4.6 mm inside the GTV boundary, respectively. In the 25 GTVs, the GTV - 2 mm coverage with the DeIIV varied from 83.7% to 98.2% (95-98% in 68% of the cases), while the GTV coverage with the GTV - 2 mm DeIIV was 20.2-75.9%. In the 23 GTVs of ≥1.26 cc, the GTV coverage with the GTV - 4 mm DeIIV varied from 1.9% to 55.6% (<50% in 87% of the cases). No significant difference was observed between the GTV D50% and the GTV - 2 mm DeIIV, while the GTV - 4 mm DeIIV was significantly higher than the GTV D50%. No significant correlations were observed between the GTV D50% and the DeIIVs of the GTV - 2 mm and GTV - 4 mm.

Conclusions

The doses 2 mm and 4 mm inside a GTV have low correlations with the GTV D50% and may be more relevant to maximal response and local control for SRS of BM. The DeIIV instead of the minimum dose of a fixed % coverage (e.g. D98%) is suitable for reporting the doses 2 mm and 4 mm inside the GTV boundary in terms of avoiding the over- or under-coverage, with consideration to substantial variability in minus margin addition functions among planning systems. In VMA-based SRS with a steep dose gradient, the doses 2-4 mm inside a GTV decrease significantly as the GTV increases, which can attenuate the excessive dose exposure to the surrounding brain in a large BM due to the GTV shrinkage during multi-fraction SRS.