ADC and SBRT for Recurrent/Metastatic Salivary Gland Carcinoma

This is a single-arm, single-center, exploratory clinical study. The study plans to enroll patients with recurrent or metastatic head and neck salivary gland carcinoma (HN-SGC) . The trial comprises two cohorts: Cohort 1 (adenoid cystic carcinoma, ACC) and Cohort 2 (non-ACC SGC). Patients in Cohort 1 will initially receive MRG003, an EGFR-targeted antibody-drug conjugate (ADC). Patients in Cohort 2 will initially receive either MRG003 (EGFR-ADC) or a TROP2-targeted ADC. The selection between these two ADC therapies for Cohort 2 will be determined by the investigator based on the expression levels of specific tumor surface receptors. Tumor response will be assessed by imaging every 6 weeks (±7 days). Subjects who are assessed as having stable disease (SD) on two consecutive evaluations or who develop oligometastatic progression will receive stereotactic body radiation therapy (SBRT). Following SBRT, maintenance therapy with the original ADC will be continued. Treatment discontinuation will be permitted due to disease progression, death, intolerable toxicity, withdrawal of consent, initiation of new anti-tumor therapy, or other protocol-specified reasons, whichever occurs first. After treatment completion, all subjects will enter a post-treatment phase for safety visits and survival follow-up. For subjects who discontinue treatment for reasons other than disease progression or death, tumor progression follow-up will also be conducted during the post-treatment period.