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: We report on two fatal cases of accidental intrathecal vincristine instillation in a 5 - year old girl with recurrent acute lymphoblastic leucemia and a 57 - year old man with lymphoblastic lymphoma .
### Output Text:
We report on two fatal cases of accidental intrathecal <span class="chemical">vincristine</span> instillation in a 5 - year old girl with recurrent acute lymphoblastic leucemia and a 57 - year old man with lymphoblastic lymphoma . 