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: This report describes unique contrast enhancement of the white matter on T1 - weighted magnetic resonance images of two patients with disseminated necrotizing leukoencephalopathy , which developed from acute lymphoblastic leukemia treated with high - dose methotrexate .
### Output Text:
This report describes unique contrast enhancement of the white matter on T1 - weighted magnetic resonance images of two patients with disseminated necrotizing leukoencephalopathy , which developed from acute lymphoblastic leukemia treated with high - dose <span class="chemical">methotrexate</span> . 