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: BACKGROUND : Although the prevalence of nonsmall cell lung carcinoma ( NSCLC ) is high among elderly patients , few data are available regarding the efficacy and toxicity of chemotherapy in this group of patients .
### Output Text:
BACKGROUND : Although the prevalence of nonsmall cell lung carcinoma ( NSCLC ) is high among elderly patients , few data are available regarding the efficacy and toxicity of chemotherapy in this group of patients . 