An experimental evaluation of three preoperative radiation regimens for resectable rectal cancer

Background We investigated the degree of tumor cell killing after radiotherapy regimens commonly used in clinical practice in comparison with an accelerated schedule. Methods Mtln3 mammary adenocarcinoma tumor cells were inoculated subcutaneously in the hind leg of syngeneic Fischer 344 rats. Tumors were irradiated with 5×5 Gy in 5 days, 10×3 Gy over 10 days, or 5×(2×3) Gy in 5 days. After excision of the irradiated tumors, the dye exclusion, a tetrazolium-based colorimetric and the clonogenic assays were used to determine tumor cell viability and surviving fractions. Results Estimated potential doubling time values indicate a rapid proliferation capacity, comparable with potential doubling time values in human rectal cancer. The dye exclusion and clonogenic assays revealed a significantly higher degree of cell killing after the hypofractionated and the accelerated regimens of, respectively, 5×5 Gy and 5×(2×3) Gy over 5 days compared with 10×3 Gy over 10 days. Conclusions A shorter treatment time offered the best therapeutic efficacy. The schedule involving two daily fractions of 3 Gy over 5 days should be less toxic than 5×5 Gy and may therefore provide a therapeutic advantage.