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: RESULTS : At a median follow - up of 14 . 1 months , the overall recurrence rate in the 51 patients was 3 . 9 % ( 2 / 51 ) .
### Output Text:
RESULTS : At a median follow - up of 14 . 1 months , the overall recurrence rate in the 51 patients was 3 . 9 % ( 2 / 51 ) . 