INFLUENCE OF EXPERIMENTAL FACTORS ON INTRINSIC RADIOSENSITIVITY ASSAYS AT LOW-DOSES OF RADIATION - CELL MULTIPLICITY

Evaluation of the probability of biological phenomena per cell by colony formation, requires knowledge not only of the number of cells at risk but also of their microdistribution. In the present study, the influence of cellular multiplicity (number of cells per potential colony-forming unit) on the determination of radiation sensitivity is evaluated for a range of multiplicity distributions. Cell surviving fraction was calculated using no multiplicity correction, an average multiplicity correction or the fractional distribution of multiplicities of the control and irradiated population. The results obtained show that: (a) multiplicity corrections are required when the number of cells per potential colony-forming unit is greater than 1.00 either immediately after plating or at the time of irradiation; (b) both the control and irradiated populations must be corrected for multiplicity; (c) multiplicity errors are most pronounced in the low-dose range, e.g. in the survival range with 2 Gy; and (d) the error introduced by using an average vs fractional distribution of multiplicities increases with the multiplicity dispersion. Seemingly small errors due to uncorrected multiplicity effects lead to markedly different predicted isoeffect doses when amplified through multiple (e.g. 30) fractions.