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: Thirty - one patients , age 10 - 45 years ( median age 17 . 8 years ) were evaluated 2 . 3 - 14 . 1 years ( median 8 . 9 years ) following completion of treatment .
### Output Text:
Thirty - one patients , age 10 - 45 years ( median age 17 . 8 years ) were evaluated 2 . 3 - 14 . 1 years ( median 8 . 9 years ) following completion of treatment . 