Repopulation in mouse oral mucosa: treatment splits.

The efficacy of repopulation during treatment splits in conventional radiotherapy (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 irradiation. Following 5 fractions, compensation of about 3 dose fractions was assessed after the first weekend and original tissue tolerance was restored after a split of 4-10 days. About 4.6 dose fractions were repopulated during the second treatment week, followed by a further 1.5 fractions during the first 3 split days; restoration of the initial tolerance required 3-8 days. These results indicate that repopulation was more efficient during fractionated radiotherapy than during a subsequent treatment split. Latent times to complete denudation after the top-up treatment decreased to a dose-dependent minimum after 5 fractions and remained at the decreased level when a second treatment week was added. Original values were restored within 5-8 days after 5 fractions and 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 counts were then restored after 5 days. Cellularity remained at 60-70% during the second treatment week and subsequently reached normal values within 4 days. In conclusion, reconstitution of epithelial cellularity precedes restoration of radiation tolerance during treatment splits.