‘Relationship between thermal dose and cell death for “rapid” ablative and “slow” hyperthermic heating’

<b>Aim:</b> Thermal isoeffective dose (TID) has not been convincingly validated for application to predict biological effects from rapid thermal ablation (e.g., using &gt;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 <i>in vitro,</i> after either ‘rapid’ ablative or ‘slow’ hyperthermic exposures. <b>Methods:</b> MTT assay viability data was obtained from two human colon cancer cell lines, (HCT116, HT29), subjected to a range of TIDs (120–720 CEM₄₃) using a thermal cycler for hyperthermic (&gt;2 minutes, &lt;50 °C) treatments, or a novel pre-heated water bath based technique for ablative exposures (&lt;10 seconds, &gt;55 °C). TID was initially estimated using a constant R_CEM&gt;43°_C=0.5, and subsequently using R_CEM(T), derived from temperature dependent cell survival (injury rate) Arrhenius analysis. <b>Results:</b> ‘Slow’ and ‘rapid’ exposures resulted in cell survival and significant regrowth (both cell lines) 10 days post-treatment for 240 CEM₄₃ (R_CEM&gt;43°_C=0.5), while 340-550 CEM₄₃ (R_CEM&gt;43°_C =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 × 10⁵ J mole⁻¹, frequency factor <i>A</i> = 3.27 ± 11 × 10⁹¹ sec⁻¹) yielded R_CEM=0.42 * e^0.0041*T which better-predicted cell survival than using R _{CEM&gt; 43}°_C=0.5. <b>Conclusions:</b> TID calculated using an R_CEM(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 CEM₄₃ using this revised measure.