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: Survival for patients with advanced head and neck carcinoma and esophageal carcinoma is poor with radiotherapy and / or surgery .
### Output Text:
Survival for patients with advanced head and neck carcinoma and esophageal carcinoma is poor with radiotherapy and / or surgery . 