Catchment-time sensitivity for access modeling in capacity-constrained radiotherapy networks

Abstract Purpose: Radiotherapy (RT) access in the Philippines is severely limited by highly centralized infrastructure. To strategically site future centers, planners need models mapping the true spatial access landscape. The Enhanced Two-Step Floating Catchment Area (E2SFCA) model quantifies capacity-to-demand ratios across geographic boundaries. Evaluating its sensitivity under varying distance-decay parameters is critical in severely capacity-constrained environments. Methods: This is a cross-sectional geospatial analysis and simulation of radiotherapy access in Luzon, employing a capacity-demand framework via the E2SFCA model. Radiotherapy demand for each municipality was estimated using age-specific cancer incidence rates and population data, while capacity was capped at 450 patients/year/LINAC. Access scores were calculated with the catchment time systematically varied from 30 to 300 minutes. Facilities were grouped topologically as Core (metropolitan), Corridor (expressway-adjacent), and Peripheral (isolated) to analyze spatial decay dynamics. Results: The existing radiotherapy network operates at a massive capacity deficit; the theoretical demand (56,605) is more than twice the maximum system capacity (24,300). At a 30-minute catchment time, 62.3% of the population registers "No Access," while localized capacity allows 16.9% to achieve "Excellent" access (score > 1). Expanding t0 to 150 minutes mathematically expands geographic coverage while significantly decreasing access quality. Topological analysis reveals distinct E2SFCA ratio decay dynamics: core facilities remain saturated at t0=30 due to massive urban density. Corridor facilities exhibit steep exponential decay as expressways funnel immense regional demand. Peripheral facilities experience a prolonged drain, slowly exhausting artificially inflated capacity as boundaries expand to absorb surrounding isolated demand. Conclusion: Expanding travel boundaries in capacity-constrained systems creates a mathematical illusion of coverage by merely redistributing deficits. To prevent network collapse, future infrastructure must be strategically sited within isolated Peripheral bottlenecks to intercept localized demand, rather than simply adding to overwhelmed Core or Corridor networks.