# Prognostic significance of baseline skeletal muscle index and its dynamics in patients with metastatic breast cancer undergoing eribulin treatment

## TL;DR

This study shows that higher baseline skeletal muscle index and its improvement during treatment are linked to better survival in metastatic breast cancer patients receiving eribulin.

## Contribution

The study identifies that on-treatment skeletal muscle index gain, but not PNI increase, improves survival in metastatic breast cancer patients.

## Key findings

- High baseline skeletal muscle index and PNI are associated with improved overall survival.
- Patients with skeletal muscle index gain during treatment had significantly longer survival.
- PNI increase during treatment was not significantly linked to better survival.

## Abstract

In breast cancer, a low skeletal muscle index (SMI) and prognostic nutritional index (PNI) negatively affect patient outcomes. However, the prognostic implications of changes in these values in patients with metastatic breast cancer (MBC) remain unclear. We evaluated the association between baseline levels and changes in SMI and PNI during eribulin treatment and patient outcomes.

We retrospectively analyzed 67 patients with MBC treated with eribulin. SMI and PNI were assessed at baseline (pre-SMI, and pre-PNI) and at disease progression; changes from baseline were calculated. Patient outcomes were compared according to baseline status and direction of change.

SMI and PNI were not significantly correlated (p = 0.26, R = 0.02). High pre-SMI and high pre-PNI were associated with significantly improved overall survival (OS) (SMI; hazard ratio [HR] = 0.54, p = 0.04, PNI; HR = 0.33, p < 0.001). Patients with SMI gain during eribulin had longer OS than those with stable SMI or loss (HR = 0.48, p = 0.04), whereas PNI increase was not significantly associated with OS (HR = 0.74, p = 0.32).

Baseline SMI and PNI provide complementary prognostic information in patients with MBC receiving eribulin. Furthermore, on-treatment SMI gain, but not PNI increase, was associated with improved survival. Monitoring and enhancing skeletal muscle mass may improve outcomes, highlighting the importance of integrating supportive care strategies during chemotherapy.

The online version contains supplementary material available at 10.1007/s10549-025-07827-y.