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: Severe polyneuropathy and motor loss after intrathecal thiotepa combination chemotherapy : description of two cases .
### Output Text:
Severe polyneuropathy and motor loss after intrathecal <span class="chemical">thiotepa</span> combination chemotherapy : description of two cases . 