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: The results obtained show a correlation with the anatomic area at risk similar to that obtained with thallium - 201 SPECT .
### Output Text:
The results obtained show a correlation with the anatomic area at risk similar to that obtained with <span class="chemical">thallium</span> - 201 SPECT . 