IMRT

All NTCP-model parameters are generally derived for standard 2Gy/day fractionation. If NTCP is calculated from inhomogeneous irradiation of normal tissue and inhomogeneous dose distribution, the dose-per-fraction is inhomogeneous. Physical doses should be normalised and converted into equivalent doses at 2 Gy/fr ( LQD2 ) using the LQ model formalism:

d

1 + ⋅ = D LQD

β / 2 1 / β

α

2

+

α

where D is the total dose, d is dose-per-fraction and α and β are the LQ parameters for the organ under consideration (if physical total doses are used the contribution from damage to the low dose-volume is overestimated, due to the non-linear nature of the relationship between dose and cell kill). If different treatment phases are present, different doses-per-fraction and consequently different LQD2 are to be considered.