  {\bf Purpose}: To develop a first principle and multi-scale model for normal
tissue complication probability (NTCP) as a function of dose and LET for proton
and in general for particle therapy with a goal of incorporating nano-scale
radio-chemical to macro-scale cell biological pathways, spanning from initial
DNA damage to tissue late effects.
  {\bf Methods}: The method is combination of analytical and multi-scale
computational steps including (1) derivation of functional dependencies of NTCP
on DNA driven cell lethality in nanometer and mapping to dose and LET in
millimeter, and (2) 3D-surface fitting to Monte Carlo data set generated based
on post radiation image change and gathered for a cohort of 14 pediatric
patients treated by scanning beam of protons for ependymoma. We categorize
voxel-based dose and LET associated with development of necrosis in NTCP.
  {\bf Result}: Our model fits well the clinical data, generated for post
radiation tissue toxicity and necrosis. The fitting procedure results in
extraction of in-{\it vivo} radio-biological $\alpha$-$\beta$ indices and their
numerical values.
  {\bf Discussion and conclusion}: The NTCP model, explored in this work,
allows to correlate the tissue toxicities to DNA initial damage, cell lethality
and the properties and qualities of radiation, dose and LET.
