On the Transverse Diffusion of Beams of Photons in Radiation Therapy

Typical external radiotherapy treatments consist in emitting beams of energetic photons targeting the tumor cells. Those photons are transported through the medium and interact with it. Such interactions affect the motion of the photons but they are typically weakly deflected which is not well modeled by standard numerical methods. The present work deals with the transport of photons in water. The motion of those particles is modeled by an entropy-based moment model, i.e., the M₁ model. The main difficulty when constructing numerical approaches for photon beam modeling emerges from the significant difference of magnitude between the diffusion effects in the forward and transverse directions. A numerical method for the M₁ equations is proposed with a special focus on the numerical diffusion effects. (authors)