187 Background: Controversy exists about progression free survival (PFS) as an endpoint in novel cancer therapy trials. PFS is attractive for practical and methodological reasons but is not always a surrogate for overall survival. It is debatable whether or not PFS has discernable clinical benefit for patients (pts), the main treatment (tmt) goal. Also overlooked are pt perceptions. Few trials directly address if disease stabilisation is worth tmt side effects (SEs) or include relevant patient reported outcome (PRO) measures. AVALPROFS examines longitudinally pts’ understanding of therapeutic intent, PFS and the value placed on drugs that control cancer but have SEs. We report the pilot study here. Methods: The primary pilot aim was to obtain feedback from pts having drugs offering only PFS or modest OS gains, about the acceptability and comprehensibility of PROs for use in a longitudinal study. These included validated quality of life and anxiety measures and 4 study specific interview schedules. In close collaboration with Independent Cancer Patients’ Voices (ICPV) and using an iterative process, draft interviews (pre-tmt, on-tmt, at withdrawal due to toxicity or progression) were developed. Pilot study pts’ clinic consultations were taped prior to home interviews by researchers. Results: 11/19 eligible pts with metastatic breast, lung and head and neck cancers at different treatment phases participated. No pt recalled the phrase PFS being used in consultations. Few knew their latest scan results. Some were confused about the therapeutic aims of further tmt, 4/10 thought it would extend survival. All had experienced or anticipated considerable toxicity. None found the interview schedules too upsetting; all provided comprehensive feedback about these and the trade-off questions. Conclusions: PFS is confusing and questions remain about its true value. Involvement of ICPV in potentially distressing research about study design and PROs at the outset was invaluable. Inclusion of feedback from pilot study pts permitted further refinements to the AVALPROFS study which is currently open and recruiting. Clinical trial information: 16342. [Table: see text]