Reaction of a tumour spheroid to irradiation.

<p>Panel <b>A</b> shows the cell phase distribution during growth of a tumour spheroid and in response to irradiation with 4 Gy. <b>B</b> Redistribution in response to 4 Gy and subsequent dynamics in the fraction of viable cells. <b>C</b> Lateral cut through a tumour spheroid during different phases of growth and irradiation. An initial small number of seeder cells will form a solid tumour spheroid, where cells in high-density regions go into quiescence. Nutrient deprivation and subsequent dissolution of necrotic cells lead to the formation of a hollow core. After irradiation with 4 Gy a majority of cells will be apoptotic, which leads to a reactivation of quiescent cells. Consequently a fast regrowth and the re-establishment of the necrotic core are observed. Cells are visualised as spheres but are handled as polyhedra while in contact within the 3D Delaunay triangulation used in the model <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003295#pcbi.1003295-Kempf1" target="_blank">[25]</a>. Cells in G1, S or G2-phase in cyan, mitotic cells in red, quiescent cells in grey, necrotic cells in brown and apoptotic cells in green.