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: Recurrence was studied in 82 evaluable patients after 1 year of follow - up and in 72 patients followed for 2 - 3 years ( mean 32 months ) .
### Output Text:
Recurrence was studied in 82 evaluable patients after 1 year of follow - up and in 72 patients followed for 2 - 3 years ( mean 32 months ) . 