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Radiobiology
▪
SBRT involves the application of high fractional doses in a range not studied in
prior decades
▪
Conversion of physical dose to biologically equivalent dose (e.g. in 2-Gy
fractions)
▪
Derived from linear-quadratic model which may not describe all tissue
effects
▪
Uncertainty in
a
/
b
parameter:
▪
Prostate: 4 x 9.5 Gy (
a
/
b
= 2
±
1 Gy) => 109 (95 – 133) Gy
▪
Uncertainty in normal tissue tolerance (small volumes; high doses)
▪
Wide variation on fraction duration, overall treatment time, prescription
isodose line:
50-80%
(high dose regions inside tumor)
Stereotactic body radiation therapy: The report of AAPM Task Group 101