The Relative Significance of Repopulation and Hypoxic Clonogens in the Fractionated Radiotherapy of a Mouse Tumor

The aim of this study was to analyze the significance of the repopulation of tumor cell clonogens, reoxygenation and hypoxic cells for the outcome of fractionated radiotherapy. Our previous fractionation study using a murine tumor was used for the analysis (Nishimura and Urano, Int. J. Radiat. Oncol. Biol. Phys. 29, 141-148, 1994). In the previous study, single cell suspensions were prepared from the early-generation isotransplants of a spontaneous fibrosarcoma, FSa-II, in a C3Hf/Sed mouse and transplanted into the foot of C3Hf/Sed mice. Fractionation schedules consisted of equal graded daily doses (1-20 fractions) given in air or under hypoxic conditions with¹³⁷ Cs. Treatment was initiated when tumors reached an average diameter of 4 mm. A twice-a-day (b.i.d.) irradiation was tested for 20 doses. Tumor control was the end point, and the <tex-math>${\rm TCD}_{50}$</tex-math> (50% tumor control dose) was obtained. The surviving fractions resulting from these <tex-math>${\rm TCD}_{50}$</tex-math> doses were calculated using the α and β values for the FSa-II cells, assuming that these values were constant throughout the fractionation period. These surviving fractions were used to calculate the clonogen repopulation and the reoxygenation of the hypoxic cells. Tumor clonogens repopulated rapidly and the hypoxic cells appeared to reoxygenate substantially during 10-20 doses. In the fractionation regimens consisting of more than 5 doses, the repopulation became significant. This rapid repopulation was reflected as a small α/β ratio on the <tex-math>$F_{{\rm e}}$</tex-math> plot. During 20 daily doses given under hypoxic conditions, clonogens (including both oxygenated and hypoxic) repopulated by a factor of 1.9× 10⁶. The hypoxic clonogens (based on the <tex-math>${\rm TCD}_{50}$</tex-math> after doses in air) repopulated to a much lesser extent than did the total clonogens. Reoxygenation was not substantial up to 5 daily doses but appeared to become significant during 10 to 20 doses. The magnitude of reoxygenation appeared to be substantially greater during 20 daily doses than during 20 b.i.d. doses. The significance of the hypoxic cells appeared to be less with increasing fractionation for the tumor control, but still remained critical in 20 daily doses. This study also suggested that the inhibition of clonogen repopulation could improve the outcome of radiotherapy, but this inhibition should be accompanied by further reoxygenation as long as hypoxic clonogens remain critical for the tumor control.