EPC contribution to tumor growth. Tumor growth curves for the case in which both ECs and EPCs contribute to angiogenesis and that in which the EPC contribution is neglected. Model predictions for the case in which both ECs and EPCs are present were generated by comparing with experimental tumor growth data¹⁰  for tumors of comparable size. When the EPC contribution is ablated, the tumor exhibits a growth delay of 4.1 d or 14%. (Growth delay is defined here as the difference in time required for 2 tumors growing at different rates to reach a given size, R_t ≈ 1.5 mm. The percentage indicates the fraction of the total time attributable to some intervention that retards growth.) Experimental data are given as means ± SD.