Shares of Clovis Oncology (NASDAQ: CLVS) fell over 21% today after AstraZeneca (NYSE: AZN) and Merck (NYSE: MRK) reported promising data for olaparib as a treatment for BRCA-mutated ovarian cancer in patients who completed chemotherapy. The excitement is over a measure called progression-free survival (PFS), or how long a patient goes without seeing cancer worsen or reappear.In a study named SOLO-1, the median PFS for patients receiving placebo was 13.8 months.