Aim: The apoptotic and necrotic dose-response of thyroid carcinoma cells following irradiation with I was evaluated.
Methods: In our in-vitro model, cells of well-differentiated papillary thyroid carcinoma (B-CPAP) were incubated with increasing activity concentrations of I for 2 days. Changes in cell viability and the extents of necrosis and apoptosis were evaluated both immediately and 2 days after irradiation.
Results: Viability of B-CPAP cells diminished with increasing I activity concentration. No apoptosis was detectable immediately after irradiation. Two days after irradiation significant apoptosis was found. The lowest I activity concentration at which apoptosis was detectable corresponds to about 1 MBq . ml. At higher activity concentrations a larger percentage of cells became apoptotic but the proportion decreased again at activity concentrations >10 MBq . ml. Likewise, necrosis was minimal at low activity concentrations and showed an exponential increase with rising I activity concentrations (>5-10 MBq . ml). Necrosis was already detectable immediately after irradiation and was the predominant form of cell death at high activity concentrations.
Conclusion: The data suggest that the nature of the cytotoxic effect of I and whether it leads to apoptotic or necrotic cell death is dose-dependent. High I doses seem to produce mainly necrotic phenomena, whereas at low I activity concentrations apoptotic phenomena prevail. The predominance of delayed apoptosis could explain why radioiodine therapy at lower doses is often linked to delayed onset and possible continuation of thyroid volume reduction over some months and even up to a year.