The efficacy of repopulation during treatment splits in conventional r
adiotherapy (5 fractions/week, 2.5, 3.5, 4.5 Gy/fraction) was studied
by delayed top-up treatment of mouse tongue epithelium. Splits of 6 h
to 13 days were introduced after 1 or 2 weeks of fractionated irradiat
ion. Following 5 fractions, compensation of about 3 dose fractions was
assessed after the first weekend and original tissue tolerance was re
stored after a split of 4-10 days. About 4.6 dose fractions were repop
ulated during the second treatment week, followed by a further 1.5 fra
ctions during the first 3 split days; restoration of the initial toler
ance required 3-8 days. These results indicate that repopulation was m
ore efficient during fractionated radiotherapy than during a subsequen
t treatment split. Latent times to complete denudation after the top-u
p treatment decreased to a dose-dependent minimum after 5 fractions an
d remained at the decreased level when a second treatment week was add
ed. Original values were restored within 5-8 days after 5 fractions an
d 6 days after 10 fractions. Epithelial cell density during treatment
with 3 or 4 Gy/fraction decreased to a dose-dependent nadir of 63% and
52% of the original number after 5 fractions, and original cell count
s were then restored after 5 days. Cellularity remained at 60-70% duri
ng the second treatment week and subsequently reached normal values wi
thin 4 days. In conclusion, reconstitution of epithelial cellularity p
recedes restoration of radiation tolerance during treatment splits.