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: Thirty - six patients were treated with BCNU every 6 to 8 weeks , either by transfemoral catheterization of the internal carotid or vertebral artery or through a fully implantable intracarotid drug delivery system , beginning with a dose of 200 mg / sq m body surface area .
### Output Text:
Thirty - six patients were treated with <span class="chemical">BCNU</span> every 6 to 8 weeks , either by transfemoral catheterization of the internal carotid or vertebral artery or through a fully implantable intracarotid drug delivery system , beginning with a dose of 200 mg / sq m body surface area . 