The results from a small, animal study challenge previously unsubstantiated published reports claiming that a five to 10mm “ice ball” margin beyond the tumour is necessary for effective cryoablation.
The new study, presented at the World Conference on Interventional Oncology 2014 (11–14 May, New York, USA), finds that an “ice ball” margin of 3mm seems adequate for effective cryoablation. “This is provided that both the tumour and the ice-ball are well visualised, otherwise a wider margin may be necessary,” researcher Christos Georgiades, Vascular and Interventional Radiology, American Medical Center, Nicosia, Cyprus, cautioned.
“Guidelines regarding the coverage of the ice ball beyond the renal tumour for effective cryoablation vary and are unsubstantiated. Our objective was to determine the distance between the visible ’ice ball’ and the lethal temperature isotherm for normal renal tissue during cryoablation,” he said explaining why the study was undertaken.
Georgiades and colleagues anaesthetised nine adult swine and placed a catheter in the renal artery of each under fluoroscopic guidance (in an MRI-angio hybrid machine). The animals were then transferred to the MRI section of the machine and renal cryoablation was performed under MR guidance. At the end of the cryoablation, with the ice ball at maximum size (frozen and not perfused), they infused the renal artery with intra-arterial stain. This infusion only stained the perfused tissue surrounding the “ice ball”. The animals were sacrificed, the probes thawed and removed, and the kidneys explanted. The researchers then determined under histological examination the distance from the “ice ball” (the stain boundary) to the periphery of the ablated zone (necrotic margin). From each slide they measured the maximum, minimum and an in-between distance from the stained to the tissue boundaries.
The researchers took 126 measurements of the margin (visible “ice ball” to lethal margin) from 29 slides. The mean width of the margin was 0.75±0.44mm and it was found to increase adjacent to large blood vessels (mean maximum was 1.15±0.51mm). To match the 98% efficacy of the established standard, resection, a 3mm margin is required, Georgiades stated. Additionally, the researchers noted a complete lack of cellular viability inside the ablated zone with cryoablation which confers an advantage over heat-based modalities.
