Tumor Interstitial Pressure — 2D Radial Field Model

This is the 2D field-based counterpart of the particle-based 3D tumor-transport simulator. Instead of evaluating Baxter and Jain's spherical closed-form pressure profile directly, this engine treats the tumor as a flat disk cross-section and solves the identical governing PDE — ∇²P − α²P = −α²P0 — by over-relaxed Gauss–Seidel (SOR) on a 96×96 finite-difference grid, the numerical route to a solution that a true disk geometry actually needs (its closed form is a modified Bessel function I0, not the sphere's sinh/r). The resulting pressure field's real 2D gradient drives Darcy convection on nanoparticles moving in the (x, y) plane, combined with independent-axis Stokes–Einstein Brownian diffusion, so the same interstitial-pressure "penetration barrier" phenomenon — particles stalling in a perivascular shell because the interior pressure gradient is nearly flat — emerges from a distinct numerical method on a distinct 2D geometry.