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: CONCLUSION : The addition of thalidomide to docetaxel in the treatment of prostate cancer significantly increases the frequency of VTE .
### Output Text:
CONCLUSION : The addition of <span class="chemical">thalidomide</span> to <span class="chemical">docetaxel</span> in the treatment of prostate cancer significantly increases the frequency of VTE . 