The patterns of systemic metastasis in major salivary gland cancer and therapeutic implications

This study aimed to investigate the patterns of metastatic spread of salivary gland cancer (SGC) and their therapeutic implications. Patterns of systemic metastasis, post-metastasis survival, and risk factors have not been defined clearly in SGC, and the validity of the concept of oligometastasis in SGC has never been explored.

This was a retrospective study of SGC patients with systemic metastasis (N = 199). The primary outcome was post-metastasis disease-specific survival, and multivariable survival analyses for risk factors were performed. The patients were divided into three pathology groups, adenoid cystic carcinomas (ACCs), non-ACC high-grade tumors, and non-ACC intermediate or low-grade tumors.

Median survival was 28.9 months after diagnosis of systemic metastasis. Only 30 % of patients had oligometastasis (≤ 3 metastatic lesions and ≤ 2 involved organs), and polymetastasis was observed in 70 % (33.7 % single organ polymetastasis and 36.2 % multi-organ polymetastasis). Poor patient performance status, shorter metastasis-free interval, non-ACC high-grade tumor, concurrent local disease, and multi-organ polymetastasis were identified as independent risk factors for post-distant metastasis survival. Of note, SGC patients with oligometastasis and single organ polymetastasis showed similar survival outcomes. Systemic chemotherapy did not improve post-metastasis survival of SGC patients significantly. However, HER-2 targeted therapy combined with chemotherapy extended post-metastasis survival in c-erb-B2 expressing tumors.

Systemic metastasis into multiple organs is a common pattern of SGC metastasis, and significantly affects patient survival. Tumor biology-based therapy should be further sought for improving post-metastasis outcomes of SGC patients.