Positron emission tomography (PET) has potential as a complementary technique to biomarker analysis, especially for human epidermal growth factor receptor 2 (HER2)-expressing tumors characterized by high heterogeneity. In this study, zirconium-89 (Zr) and copper-64 (Cu) labeled trastuzumab were employed to monitor varying levels of tumoral HER2 expression. Additionally, we studied the use of the cholesterol-depleting lovastatin as a pharmacological approach to enhance cell-surface HER2 expression in tumors with moderate to low HER2 levels, aiming to increase antibody accumulation in these tumor types. Both Zr- and Cu-labeled trastuzumab effectively monitor HER2 expression levels in xenografts exhibiting varying HER2 expression. No significant difference in tumor uptake was observed between Zr- or Cu-labeled trastuzumab, and tumor uptake for both radioimmunoconjugates positively correlated with HER2 protein levels. These findings underscore the potential of PET to monitor HER2 protein levels across heterogeneous tumors. Furthermore, our results suggest that further optimization of statin dosing and timing could offer a promising strategy to enhance trastuzumab accumulation in HER2-high, HER2-moderate, and HER2-low tumors.