Exceptionally Durable Response to First-Line S-1 Monotherapy in Metastatic Pancreatic Adenocarcinoma: A Case Report

Pancreatic adenocarcinoma with distant metastases is associated with a poor prognosis, and exceptionally durable responses to systemic chemotherapy are uncommon.We report a rare case of pancreatic adenocarcinoma with radiologically suspected multiple liver metastases that achieved a prolonged response to first-line tegafur/gimeracil/oteracil (S-1) monotherapy.A 76-year-old man with a history of cerebral infarction and diabetes mellitus was referred to our department because of rapidly worsening diabetes.Imaging revealed an approximately 3-cm pancreatic head tumor and multiple hepatic lesions highly suspicious for metastases.Serum carbohydrate antigen 19-9 (CA19-9) was markedly elevated.Endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) of the pancreatic head tumor demonstrated adenocarcinoma on both cytological and histopathological examinations.Gemcitabine plus nanoparticle albumin-bound paclitaxel (nab-paclitaxel) was proposed; however, the patient preferred oral treatment.Considering his age, quality of life, and treatment preference, S-1 monotherapy was initiated at 100 mg/day on a 2-week-on/2-week-off schedule.Following initiation of S-1, the pancreatic and hepatic lesions progressively regressed, accompanied by rapid normalization of CA19-9.By approximately 14 months after treatment initiation, the previously identifiable pancreatic primary tumor and hepatic lesions were no longer clearly visualized on computed tomography (CT).S-1 monotherapy was continued with few interruptions, and no apparent radiological progression was documented for approximately 47 months.Approximately two and a half months after the last CT showing no apparent progression, the patient developed fever and dyspnea.CT demonstrated newly developed bilateral ground-glass and interstitial pulmonary opacities, along with new-onset ascites, without evidence of regrowth of the pancreatic or hepatic lesions.S-1-induced interstitial lung disease (ILD) was clinically suspected; however, occult malignant progression could not be definitively excluded.His respiratory condition deteriorated despite treatment, and he subsequently died.This case demonstrates that an exceptionally durable response may rarely occur with first-line S-1 monotherapy even in pancreatic adenocarcinoma with radiologically suspected metastatic disease.