Is a tumor that grew from 10 to 25 cm in 5 mos. aggressive?

My aunt had a lung tumor that had metastasized from a primary uterine leiomyosarcoma and it grew from 10 cm to 25 cm in 5 months, while she was getting chemo.  They told her it was "low grade" and slow growing but that seems really fast to me.  Could she have misunderstood them?