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

Abstract 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 CEM43) 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 RCEM>43°C=0.5, and subsequently using RCEM(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 CEM43 (RCEM>43°C=0.5), while 340-550 CEM43 (RCEM>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 A？=？3.27？±？11？×？1091？sec？1) yielded RCEM=0.42？*？e0.0041*T which better-predicted cell survival than using R CEM> 43°C=0.5. Conclusions: TID calculated using an RCEM(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