I'm sorry for your mum!
Topotecan is always considered the standard first-choice single agent for the treatment of relapsed SCLC (cancer that hasn't respond or came back after the first chemotherapy regimen)
Also, topotecan is an effective treatment in patients with poor daily functional status, patients that might otherwise not receive chemotherapy.
There are different schedules for Topotecan administration, but, the weekly one as in your mother's case is the most studied and is generally well tolerated in patients who have relapsed SCLC, with less blood toxicities, and other rare severe toxicities.
Best wishes!
Marija