Cell kinetics and repopulation mechanisms during multifraction irradiation of spheroids

Background and purpose: The objective of the present study was to evaluate the predictive potential of cell kinetic parameters and repopulation rates determined by flow cytometry during multifraction irradiation of spheroids, a system in which the fate of all cells can be determined with high precision. Ultimately, similar analytical techniques should provide a reproducible and prognostically significant clinical predictive assay. Materials and methods: Multicellular spheroids of Chinese hamster V79 lung cells were irradiated with 2.5 Gy of 250 kVp X-rays twice daily to a total dose of 25 Gy. Repopulation parameters and cell kinetic parameters were followed throughout the irradiation period and for 5 days after completion of exposure. Results: (1) Regrowth (RG) took place early during multifraction irradiation. (2) Potential doubling time (T-pot) decreased steadily from the early part of treatment, remaining of short duration until the spheroids almost attained the pre-treatment number of clonogenic cells. (3) Accelerated repopulation was mainly due to a decreased cell loss factor (Phi) and increased growth fraction (GF), although a modest decrease in cell cycle time (t(c)) was suggested. (4) Phi decreased during exponential RG. (5) Other parameters such as observed doubling time (t(d)) and labelling index (LI) paralleled these findings. Conclusions: Clonogen repopulation that began early in the irradiation scheme and accelerated rapidly is not consistent with the prevailing view that accelerated repopulation begins several weeks into clinical protocols. Also, pre-treatment T-pot did not adequately estimate the repopulation speed in the spheroids. Equivalent studies in animal tumour systems, and then in the clinic, are consequently indicated and of some urgency. (C) 1998 Elsevier Science Ireland Ltd.