In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: In patients that were free of recurrence during the first year , 80 % remained tumor - free during the 2 - to 3 - year follow - up period .
### Output Text:
In patients that were free of recurrence during the first year , 80 % remained tumor - free during the 2 - to 3 - year follow - up period . 