OBJECTIVES: Market access, clinical and commercial scenario analysis performed to assess the impact of indication expansion within a therapeutic area, requires an estimate of patient numbers in different lines of therapy. This can be challenging within rare and orphan diseases with limited available evidence and multiple subpopulations. Patient population size is typically based on incidence or prevalence data and physician judgement. Additional complexity comes when assessing commercial impact as the introduction of new treatment options in earlier lines may result in ‘cannibalisation’ due to loss of patients who progress to later lines. Moreover, from a market access perspective indication expansion often results in price erosion impacting both existing and new indications. A model was developed to dynamically estimate patient progression which can be used for revenue calculations and scenario analysis. METHODS: Patient progression was modelled using assumed PFS and OS data and following exponential distribution on the Kaplan-Meier Curve. Model inputs that could be used for scenario planning include clinical, commercial and market access dimensions. Variables that were used included patient progression, clinical utilization and dosing for the clinical dimension; regulatory approval timeline, loss of exclusivity and future competitor entry for the commercial dimension; market access delays, net price discounting and potential for multi-indication pricing for market access dimension. RESULTS: The model simulates patient flows to assess the impact on revenues of the introduction of new indications, modelled for the duration of the product’s life cycle through loss of exclusivity. The model defines specific patient segments within each line of therapy and for each patient segment there is a distribution of utilization among the appropriate therapies CONCLUSIONS: This patient flow modelling approach can be used for orphan and oncology conditions with limited data available, to better understand the impact of indication expansion considering different market access, commercial, clinical and competitor scenarios.