Background and purpose: The impact of overall treatment time of fractionated irradiation on local control of slow growing human GL squamous cell carcinoma (SCC) was determined.
Materials and methods: Moderately well differentiated and keratinizing human GL SCC with a volume doubling time of 8 days were transplanted subcutaneously into the right hindleg of NMRI (nu/nu) mice and irradiated with 30 fractions under ambient conditions over 2, 3, 4.5, 6 and 10 weeks. Endpoint of the experiments was local tumor control at day 180 after end of treatment.
Results: The tumor control dose 50% (TCD50) increased from 40 to 57 Gy when the treatment time was extended from 2 to 10 weeks. The data can be well described by a linear increase in TCD50 with time. The recovered dose per day (D(r)) was 0.28 Gy (95% confidence interval 0.06; 0.48). The fit to the data was not significantly improved by assuming a biphasic (dog-leg) time course with constant TCD50 values in the initial part of treatment followed by a more rapid increase of TCD50 thereafter.
Conclusions: D(r) in GL SCC was significantly less than the value of 1.0 Gy (0.7; 1.3) previously reported for poorly differentiated, non-keratinizing and fast growing human FaDu SCC (Baumann M, Liertz C, Baisch H, Wiegel T, Lorenzen J, Arps H. Impact of overall treatment time of fractionated irradiation on local control of human FaDu squamous cell carcinoma in nude mice. Radiother. Oncol. 1994:32:137-143), indicating important heterogeneity of the time factor between different tumors of the same histological type.