Unraveling Tumor-to-Tumor Metastasis: Insights into Pathogenesis, Diagnostic Challenges, and Treatment Modalities.

Tumor-to-tumor metastasis (TTM) is defined as the metastasis of one distinct malignancy to another independent tumor without directly extending into the substance of a histologically distinct and separate tumor. TTM is an extremely rare phenomenon that constitutes a very small percentage of all tumor metastases. The detailed histogenic mechanisms of TTM remain unclear. TTM is easily confused with composite tumors and synchronous tumors. Due to the rarity and complexity of the disease, it presents significant challenges in providing accurate diagnoses and appropriate treatment options. The exploration of TTM not only provides an in-depth understanding of the metastasis process, but also has significant implications for the management and treatment of patients with multiple primary malignant tumors, underscoring the necessity of comprehensive diagnostic and treatment strategies. The purpose of this review is to increase awareness of tumor-to-tumor metastasis, with a focus on pathogenesis, diagnosis, and treatment.