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: Intra - arterial BCNU chemotherapy for treatment of malignant gliomas of the central nervous system .
### Output Text:
Intra - arterial <span class="chemical">BCNU</span> chemotherapy for treatment of malignant gliomas of the central nervous system . 