Ablation dynamics of subsequent thermal doses delivered to previously heat-damaged tissue during magnetic resonance-guided laser-induced thermal therapy

OBJECTIVE Intraoperative dynamics of magnetic resonance-guided laser-induced thermal therapy (MRgLITT) have been previously characterized for ablations of naive tissue. However, most treatment sessions require the delivery of multiple doses, and little is known about the ablation dynamics when additional doses are applied to heat-damaged tissue. This study investigated the differences in ablation dynamics between naive versus damaged tissue. METHODS The authors examined 168 ablations from 60 patients across various surgical indications. All ablations were performed using the Visualase MRI guided laser ablation system (Medtronic), which employs a 980-nm diffusing tip diode laser. Cases with multiple topographically overlapping doses with constant power were selected for this study. Single-dose intraoperative thermal damage was used to calculate ablation rate based on the thermal damage estimate (TDE) of the maximum area of ablation achieved (TDEmax) and the total duration of ablation (t(max)). We compared ablation rates of naive undamaged tissue and damaged tissue exposed to subsequent thermal doses following an initial ablation. RESULTS TDEmax was significantly decreased in subsequent ablations compared to the preceding ablation (initial ablation 227.8 +/- 17.7 mm(2), second ablation 164.1 +/- 21.5 mm(2), third ablation 124.3 +/- 11.2 mm(2); p = < 0.001). The ablation rate of subsequent thermal doses delivered to previously damaged tissue was significantly decreased compared to the ablation rate of naive tissue (initial ablation 2.703 mm(2)/sec; second ablation 1.559 mm(2)/sec; third ablation 1.237 mm(2)/sec; fourth ablation 1.076 mm/sec; p = < 0.001). A negative correlation was found between TDEmax and percentage of overlap in a subsequent ablation with previously damaged tissue (r = -0.164; p < 0.02). CONCLUSIONS Ablation of previously ablated tissue results in a reduced ablation rate and reduced TDEmax. Additionally, each successive thermal dose in a series of sequential ablations results in a decreased ablation rate relative to that of the preceding ablation. In the absence of a change in power, operators should anticipate a possible reduction in TDE when ablating partially damaged tissue for a similar amount of time compared to the preceding ablation.