'Relationship between thermal dose and cell death for "rapid" ablative and "slow" hyperthermic heating'.

Aim: Thermal isoeffective dose (TID) has not been convincingly validated for application to predict biological effects from rapid thermal ablation (e.g., using >55 °C). This study compares the classical method of quantifying TID (derived from hyperthermia data) with a temperature-adjusted method based on the Arrhenius model for predicting cell survival in vitro, after either 'rapid' ablative or 'slow' hyperthermic exposures.

Methods: MTT assay viability data was obtained from two human colon cancer cell lines, (HCT116, HT29), subjected to a range of TIDs (120-720 CEM) using a thermal cycler for hyperthermic (>2 minutes, <50 °C) treatments, or a novel pre-heated water bath based technique for ablative exposures (<10 seconds, >55 °C). TID was initially estimated using a constant R°=0.5, and subsequently using R(T), derived from temperature dependent cell survival (injury rate) Arrhenius analysis.

Results: 'Slow' and 'rapid' exposures resulted in cell survival and significant regrowth (both cell lines) 10 days post-treatment for 240 CEM (R°=0.5), while 340-550 CEM (R° =0.5) delivered using 'rapid' exposures showed 12 ± 6% viability and 'slow' exposures resulted in undetectable viability. Arrhenius analysis of experimental data (activation energy ΔE = 5.78 ± 0.04 × 10 J mole, frequency factor A = 3.27 ± 11 × 10 sec) yielded R=0.42 * e which better-predicted cell survival than using R °=0.5.

Conclusions: TID calculated using an R(T) informed by Arrhenius kinetic parameters provided a more consistent, heating strategy independent, predictor of cell viability, improving dosimetry of ablative thermal exposures. Cell viability was only undetectable above 305 ± 10 CEM using this revised measure.