Engineering

A thermal ablation paper reports an ablation zone or a completely ablated fraction, and to get that number from a temperature history one must choose a criterion. The literature contains several, all defensible and all in current use: CEM43 >= 240 minutes, CEM43 at 60 or 120, a lethal isotherm at 50, 55 or 60 C, or a sustained-temperature rule. These are different functionals of the same history, and how much the choice matters appears not to have been asked. We solve an axisymmetric Pennes bioheat problem for a cooled RF needle ONCE per configuration and feed ten published criteria from that identical temperature field, so any difference between them is purely the convention. At 20 W, 15 min and a 1.5 cm tumour, coverage of the tumour-plus-5mm margin ranges from 0.330 (Tmax >= 60 C) to 1.000 (CEM43 >= 60), a span of 0.670. For calibration, sweeping delivered power across the entire clinically plausible 10-90 W range in a companion analysis of the same solver moves coverage by 0.682. The reporting convention is worth as much as a nine-fold change in delivered power. The span exceeds 0.53 in every configuration at 1.5-2.0 cm and collapses to 0.147 at 1.0 cm, so the conventions diverge exactly where the clinical question is live. We also report a negative result: replacing the conventional R = 0.25 below 43 C with a flat R = 0.5 reproduces the standard criterion to three decimals in all five configurations, because ablated tissue crosses the 37-43 C band too fast for that branch to integrate to anything. Moving the breakpoint from 43 to 43.5 C does matter. Of the two modelling details, the one that is argued about is the irrelevant one. Grid refinement leaves the span stable to +-0.012 and the criterion rank ordering identical at all three resolutions tested. No clinical recommendation is made or implied.