Complex sentence: Ovarian cancer at its early stages (I/II) is difficult to diagnose until it spreads and advances to later stages (III/IV) s because most symptoms are nonspecific and thus of little use in diagnosis.
Split into simpler sentences: Ovarian cancer at its early stages (I/II) is difficult to diagnose until it spreads and advances to later stages (III/IV). This is because most symptoms are non-specific and thus of little use in diagnosis.