Case Report: Delayed appearance of the ablation margin after CT-guided microwave ablation of a solid pulmonary nodule

During CT-guided thermal ablation of lung tumors, guidelines define the intraprocedural endpoint as a circumferential margin of increased attenuation extending 5-10 mm beyond the tumor. This criterion does not require the strict radiologic texture of ground-glass opacity. The timely appearance of this margin is the main real-time marker of an adequate treatment volume, and its absence is conventionally read as underablation, prompting additional energy delivery. We report a metastatic solid nodule in the left lower lobe treated with microwave ablation (45 W, 8 min total) via a single antenna, without repositioning. No such margin formed after three ablation cycles or after needle withdrawal, on a scan obtained 21 min after the first cycle and beyond the recommended 5- to 10-minute window; the only intraprocedural change was slight, non-circumferential enlargement of the solid-attenuation area. A well-defined margin with dense consolidation appeared only at 30.5 h; by 32 days, most consolidation had resorbed, forming a cavity. Despite adequate energy delivery, the guideline-defined margin in this solid nodule remained unconfirmed well beyond the standard observation window, and its absence should not be treated as an automatic indication for further ablation.