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: PATIENTS : Seventy men , aged 50 - 80 years , with advanced androgen - independent prostate cancer .
### Output Text:
PATIENTS : Seventy men , aged 50 - 80 years , with advanced androgen - independent prostate cancer . 