1. A computer-implemented method of modeling tumor growth for guiding medical intervention, comprising: (a) calculating ρ/D, an observed ratio of a malignant cell proliferation coefficient, ρ, to a diffusion coefficient, D, from one or more different measures of tumor volume from either a single or multiple images of a tumor, which calculating comprises mapping the following formula derived from an analytic solution of a linear tumor model to a non-linear tumor model: ( ρ D ) c = [ 4 ⁢ ⁢ ln ⁡ ( t 1 t 2 ) ( T ⁢ ⁢ 2 ⁢ r eqv ) 2 - ( T ⁢ ⁢ 1 ⁢ r eqv ) 2 ] ⁡ [ [ ⁢ T ⁢ ⁢ 1 ⁢ r eqv 2 ⁢ ln ⁡ ( t 1 t 2 ) ( T ⁢ ⁢ 2 ⁢ r eqv ) 2 - ( T ⁢ ⁢ 1 ⁢ ⁢ r eqv ) 2 ] + ln ⁢ ⁢ t 1 ⁢ k N 0 ⁡ [ ∏ ⁢ ( T ⁢ ⁢ 2 ⁢ r eqv 2 - T ⁢ ⁢ 1 ⁢ r eqv 2 ) ln ⁡ ( t 1 t 2 ) ] 3 / 2 ] where T1r eqv and T2r eqv are the radii of the equivalent, spherical tumor volumes corresponding to the one or more different measures of tumor volume, respectively; t 1 and t 2 are threshold tumor cell concentrations for two of said measures of tumor volume, k is maximum malignant-cell concentration; and N 0 is an initial malignant-cell concentration or number; (b) obtaining a value for D and a value for ρ; and (c) generating a computational model for growth of the tumor in a cancer patient using the values of D and ρ obtained in step (b); wherein steps (a)-(c) are performed using a computer system.