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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:
β
α
β
α
/
2 1
/
1
2
+
+
⋅ =
d
D LQD
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.