Methods: A retrospective analysis on 35 patients with HER2-positive metastatic breast cancer was conducted, analyzing body composition parameters such as subcutaneous adipose tissue (SAT), visceral adipose tissue (VAT), skeletal muscle area (SMA), and skeletal muscle index (SMI)—assessed using CT scans at baseline (T0) and after a median follow-up of 4 months (T1)—and calculating ΔT0–T1% of each parameter. The gene discussed is ERBB2; the disease is breast carcinoma.