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INTRODUCTION: Lacrimal sac squamous cell carcinoma (LSSCC) is the most common tumor of the rare disease subsite of the nasolacrimal system. Circulating tumor DNA (ctDNA) has emerged as a biomarker for non-invasive monitoring of tumor burden and treatment response across a variety of solid tumors.METHODS: In this case study, we report on two patients with histologically confirmed HPV + (HPV16 and HPV33) LSSCC managed with definitive chemoradiotherapy who underwent serial ctDNA monitoring prior to, during, and following treatment using an ultrasensitive multi-feature HPV whole genome sequencing liquid biopsy.RESULTS: In this case study, we report on two patients with histologically confirmed HPV + (HPV16 and HPV33) LSSCC managed with definitive chemoradiotherapy who underwent serial ctDNA monitoring prior to, during, and following treatment using an ultrasensitive multi-feature HPV whole genome sequencing liquid biopsy.CONCLUSIONS: This report suggests that ctDNA monitoring is a feasible approach for detection and monitoring of HPV + LSSCC. We advocate for continued p16 immunohistochemistry in all squamous cell carcinomas arising in the lacrimal sac. Confirmation with HPV ISH or PCR should be performed given the potential impact on management and surveillance.