Tumor cell repopulation during conventional and accelerated radiotherapy in the in vitro megacolony culture

Abstract

Purpose: To analyze the repopulation rate of cancer cells in vitro during conventional and accelerated irradiation, using the megacolony culture. Methods and materials: Two cell lines—murine squamous cell carcinoma AT478 and human adenocarcinoma A549—were grown as epithelial megacolonies in vitro, and they were irradiated using Co-60 gamma source at the dose rate of 0.82 Gy/min. Single-dose irradiation, conventional fractionation, and continuous accelerated irradiation (CAIR) were applied to determine the dose–response relationship and to calculate the repopulation balancing dose. Radiosensitivity parameters and the rate of repopulation were calculated from the colony cure rates using direct maximum-likelihood regression and a linear-quadratic model. Cytogenetic radiation damage was measured as frequency of necrotic, apoptototic cells and cells with micronuclei. Mitotic index was used as a simple measure of cell proliferation kinetics. Results: When treatment time was increased, a significant drop in tumor control probability was detected. The loss of radiation dose calculated from LQ model parameters was equal to 0.8 Gy/day for both human and mouse cell lines. There was no evidence of a lag period for accelerated proliferation or altered proliferation during weekends. There were no significant differences in morphologic presentation of cellular radiation damage. Conclusion: In present in vitro experiments, we did not find any significant differences in repopulation or radiosensitivity between accelerated CAIR and conventional fractionation. Different mechanisms may be important for tumor cells repopulation in vitro and in vivo.