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: The median duration of survival in the 12 patients was 54 weeks ( range 21 to more than 156 weeks ) , with an 18 - month survival rate of 42 % .
### Output Text:
The median duration of survival in the 12 patients was 54 weeks ( range 21 to more than 156 weeks ) , with an 18 - month survival rate of 42 % . 