Her existing hagiography, written between about 450 and 500 AD, is of no historical value and the relations it attributes to Susanna are entirely fictitious. It is probable that a real martyr named Susanna lies behind the literary invention.
The existing account of Susanna's life, written around 450-500 AD, is not historically reliable because it fabricates her relationships.  It's probable that a real person who suffered martyrdom exists behind this fictionalized story.