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: A total of 167 courses at doses of 15 to 70 mg / m2 were evaluable .
### Output Text:
A total of 167 courses at doses of 15 to 70 mg / m2 were evaluable . 