Repair of radiation induced damage in two human tumour cell lines grown as spheroids and monolayers.

: Growth curves of two human tumour cell lines grown as multicellular tumour spheroid (MTS) were used to obtain survival estimates by back-extrapolation after split and single dose irradiation. Neuroblastoma (NB-100) and squamous cell carcinoma (HN-1) single cells from monolayer culture were assessed for repair of sublethal and potentially lethal damage. The extent of repair was calculated on an iso-effect basis. When grown as spheroids squamous cell carcinoma cells showed a higher capacity to repair sublethal damage than neuroblastoma cells. Repair of potentially lethal damage did not contribute to this higher capacity of HN-1 cells, since this cell line was found to be deficient for this type of repair. Using the recovery ratio to estimate the beta-component of the survival curves, it was found that differences in repair capacity were determined by the alpha-component of the equation. Our results show the feasibility of back-extrapolating multicellular tumour spheroid growth curves to obtain survival estimates that can be applied to establish sublethal damage repair capacity.