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: After two to seven cycles of chemotherapy , nine patients showed a decrease in tumor size and surrounding edema on contrast - enhanced computerized tomography scans .
### Output Text:
After two to seven cycles of chemotherapy , nine patients showed a decrease in tumor size and surrounding edema on contrast - enhanced computerized tomography scans . 