Background: Biosimilar filgrastim (Zarxio) provides a more cost-efficient alternative to reference and pegylated filgrastim for CIN/FN prophylaxis. Using biosimilar conversion rates, we determined savings achieved, number needed to convert (NNC) to purchase additional Zarxio, and the incremental number (n) of patients (pts) that could be provided Zarxio on a budget neutral basis using various filgrastim cycle/treatments scenarios. Prior real-world practice studies have shown variations in GCSF duration including shorter durations (Weycker 2006; Gascon 2016) relative to registration trials and guideline recommendations. Methods: Calculations were done for a panel of 20000 pts; chemotherapy of 1 or 6 cycles with 5, 7 or 11 filgrastim injections; cost of medicines using average selling price (Zarxio $245.08, Neupogen $283.87, Neulasta $3761.22) and subcutaneous administration per Current Procedural Terminology ($42.31 hospital outpt). NNC is the n to be converted to Zarxio to purchase 1 additional Zarxio dose or the full regimen. This yielded the incremental n that could receive any of the 6 prophylaxis scenarios budget neutrally. Results: See table below. Conclusions: A 20000 pt panel yields expanded access to Zarxio, on a budget-neutral basis, to 2699 additional pts if 100% are converted from Neupogen; and to 4063 to 32939 additional pts, depending on cycles/treatments, if converted from Neulasta. Weycker et al, Ann Pharmacother 2006;40:402-7. Gascon et al, Support Care Cancer 2016;24:911-25.