Sustained HBsAg seroclearance after pembrolizumab-containing neoadjuvant therapy in a patient with synchronous esophageal and gastric cancer: a case report

Hepatitis B surface antigen (HBsAg) seroclearance during immune checkpoint inhibitor (ICI) therapy has been described in hepatocellular carcinoma (HCC), but evidence outside HCC remains limited and requires further validation. We report a patient with synchronous early-stage esophageal cancer and locally advanced gastric cancer who achieved HBsAg seroclearance after three cycles of pembrolizumab containing neoadjuvant therapy. Before treatment, qHBsAg was detectable at a low level (0.41 IU/mL), whereas HBV DNA was below the assay lower limit of quantification (LLOQ) (<30 IU/mL). HBsAg first became nonreactive on July 20, 2022 and remained below the assay cutoff on subsequent measurements. At the most recent follow-up on September 10, 2026, qHBsAg remained below the cutoff (<0.05 IU/mL) and HBV DNA remained below the LLOQ, with normal liver biochemistry. Notably, the patient did not receive antiviral therapy during treatment and remained free from both tumor recurrence and HBV reactivation for more than four years after surgery. This case raises the possibility that ICIs may confer concurrent antitumor and assisting antiviral effects in selected HBsAg-positive patients with cancer. However, given the single-case design, the extremely low baseline HBsAg level, and concomitant multimodal anticancer therapy, a causal relationship between pembrolizumab and HBsAg loss cannot be established. Further studies are warranted to clarify whether ICIs can contribute to HBV immune control, particularly when integrated with current antiviral strategies.