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 toxicity was correlated with the higher cumulative dose of 60 g / m2 of ifosfamide , a younger age ( less than 2 1 / 2 years old ) , and a predominance of vesicoprostatic tumor involvement .
### Output Text:
Severe toxicity was correlated with the higher cumulative dose of 60 g / m2 of <span class="chemical">ifosfamide</span> , a younger age ( less than 2 1 / 2 years old ) , and a predominance of vesicoprostatic tumor involvement . 