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: Twenty - four patients with recurrent Grade I to IV astrocytomas , whose resection and irradiation therapy had failed , received two to eight courses of intra - arterial BCNU therapy .
### Output Text:
Twenty - four patients with recurrent Grade I to IV astrocytomas , whose resection and irradiation therapy had failed , received two to eight courses of intra - arterial <span class="chemical">BCNU</span> therapy . 