Differentiation status of human squamous cell carcinoma xenografts does not appear to correlate with the repopulation capacity of clonogenic tumour cells during fractionated irradiation

Abstract

Purpose: To investigate the magnitude and kinetics of repopulation in a moderately well differentiated UT‐SCC‐14 human squamous cell carcinoma [hSCC] in nude mice. This question is of interest because clinical data indicate a higher repopulation capacity in those SCC that have preserved characteristics of differentiation, which appears to be in contrast to results on FaDu and GL hSCC previously reported from this laboratory.

Methods and Materials: UT‐SCC‐14 tumours were transplanted subcutaneously into the right hind leg of NMRI nu/nu mice. Fractionated radiation treatments were delivered, either under clamped hypoxia at 5.4 Gy/fraction or under ambient conditions (consistent with an OER of 2.7). Tumours were irradiated every day, every 2nd day, or every 3rd day with 6, 12 or 18 fractions. 1, 2 or 3 days after the last fraction, graded top‐up‐doses under clamped conditions were given for the purpose of estimating the 50% tumour control dose (TCD₅₀). A total of 22 TCD₅₀ assays were performed and analysed using maximum likelihood techniques.

Results: The data demonstrate a slow but significant repopulation of clonogenic cells during fractionated irradiation of UT‐SCC‐14 hSCC. The results under hypoxic conditions are consistent with a constant repopulation rate, with a clonogenic doubling time (T_clon) of 15.6 days (95% CI: 9.7, 21.4). This contrasts with ambient conditions where T_clon was 68.5 days (95% CI: 124, 161). Both T_clon values are longer than the 6‐day volume doubling time of untreated tumours.

Conclusions: Less pronounced repopulation for irradiation under ambient compared to clamped hypoxic conditions might be explained by preferential survival of hypoxic and therefore non‐proliferating clonogenic cells. Taken together with previous studies on poorly differentiated FaDu and moderately well differentiated GL hSCC, the results are consistent with considerable variability in the magnitude and kinetics of repopulation in different experimental squamous cell carcinomas, and with a relationship between reoxygenation and repopulation during fractionated irradiation. The differentiation status of hSCC growing in nude mice does not to appear to correlate with the proliferative capacity of clonogenic tumour cells during treatment. The results do not support the hypothesis gained from clinical data of higher repopulation in well‐differentiated tumours.