Tumor cell radiosensitivity is a major determinant of tumor response to radiation.

Substantial evidence suggests that the radiosensitivity of the tumor cells is the primary determinant of tumor response to radiation. More recent studies suggest that tumor stroma radiosensitivity is the principle determinant of response. To assess the relationship between intrinsic tumor cell radiosensitivity and tumor response, we altered the intrinsic radiosensitivity of a cloned tumor cell line and analyzed the effect of this alteration on tumor response. A cloned tumor cell line derived from DNA double-strand break repair--deficient severe combined immunodeficient mice was transfected with the double-strand break repair gene DNA-PKcs. The intrinsic radiosensitivity of the transfected tumor line was decreased by a factor of approximately 1.5. The isogenic lines were used to initiate tumors in NCr-nu/nu mice. When transplanted in the same strain of mice and exposed to the same dose of radiation, the isogenic tumors may be expected to exhibit a similar response to radiation if radiation damage to host stroma is the principle determinant of response. This was not observed. Over the dose range of 20 Gy in four 5-Gy fractions to a single dose of 30 Gy, the 1.5-fold increase in intrinsic tumor cell radioresistance conferred by the introduction of DNA-PKcs caused a 1.5-fold decrease in tumor growth delay. The results show that the intrinsic radiosensitivity of tumor cells is a major determinant of tumor response to radiation.