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: Strategies of extended - interval and conventional dosing have been utilized extensively in the general medical population ; however , data are lacking to support a dosing strategy in the hematology / oncology population .
### Output Text:
Strategies of extended - interval and conventional dosing have been utilized extensively in the general medical population ; however , data are lacking to support a dosing strategy in the hematology / oncology population . 