# A Case of Corneal Thinning During S-1 Oral Administration

## TL;DR

A 69-year-old man developed corneal thinning while taking the cancer drug S-1, which improved after stopping the medication.

## Contribution

Reports a new potential ocular side effect of S-1: corneal thinning.

## Key findings

- Bilateral corneal thinning exceeding 100 µm occurred during S-1 treatment.
- Visual acuity and corneal thickness returned to normal after discontinuing S-1.
- Corneal thinning is a previously unreported side effect of S-1.

## Abstract

We experienced a case of bilateral corneal thinning during the oral taking of S-1, a combination anti-cancer drug of tegafur, gimeracil, and oteracil-potassium. A 69-year-old man was prescribed oral S-1 for the treatment of duodenal papilla adenocarcinoma and intraductal papillary mucinous neoplasm. However, he developed a decrease in visual acuity in both eyes after three cycles of S-1 oral taking, and ophthalmic examination revealed corneal thinning exceeding 100 µm and an increase in high-order irregularity of cornea in both eyes. After one month after discontinuation of S-1, his visual acuity and corneal thickness returned to its previous levels. Besides corneal ulcers and perforations, corneal thinning can be recognized as a potential corneal side effect necessitating monitoring during S-1 treatment.