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 present study assessed the combination of radiotherapy ( 60 Gy / 30 fractions / 40 days ) and topotecan ( 0 . 9 mg / m ( 2 ) / day on days 1 - 5 on weeks 1 , 3 and 5 ) in 50 adults with histologically proven and untreated GBM .
### Output Text:
The present study assessed the combination of radiotherapy ( 60 Gy / 30 fractions / 40 days ) and <span class="chemical">topotecan</span> ( 0 . 9 mg / m ( 2 ) / day on days 1 - 5 on weeks 1 , 3 and 5 ) in 50 adults with histologically proven and untreated GBM . 