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Thermal tumour ablation techniques such as radiofrequency (RF) ablation are applied for radical removal of local tumours as an easier, less invasive alternative to surgical resection. A serious drawback of thermal ablation, however, is that the ablation area cannot be accurately assessed during the procedure. To achieve real-time feedback and exact and safe ablation, a superfine thermocouple-needle system (TNS) comprising a 0.25-mm diameter thermocouple embedded in a 22-G, 15-cm-long needle was devised and efficacy was tested in vitro using porcine livers (n = 15) and in vivo using rabbit back muscles (n = 2) and livers (n = 3). A 17-gauge RF electrode with a 2 cm active tip was used for ablation. The TNS was inserted 1 cm from the active tip of the RF electrode and liver temperature around the electrode was measured concurrently. The RF current was cut off when the temperature reached 60°C or after 5 min at ≥50°C. Porcine livers and rabbit back muscles were then cut along a plane passing through the axes of the electrode and the TNS. In rabbit livers, contrast-enhanced CT was performed to evaluate ablation areas. Ablation areas in cut surfaces of porcine livers exhibited well-defined discoloured regions and the TNS tip precisely pinpointed the margin of the ablation area. Contrast-enhanced CT of rabbit livers showed the TNS tip accurately located at the margin of areas without contrast enhancement. These results indicate that the TNS can accurately show ablation margins and that placing the TNS tip at the intended ablation margin permits exact thermal ablation.
Thermal tumour ablation techniques, represented by the use of radiofrequency (RF) ablation, have been applied to treat tumours in various organs. These methods aim to completely remove local tumours more readily and less invasively compared with surgical resection . The serious problem associated with thermal ablation, however, is that the ablation margin cannot be accurately assessed during the procedure, often resulting in either insufficient or excessive operation. The ablation size of the same RF electrode differs by up to 30% because it varies depending on the water content and blood perfusion in both the tumour and the surrounding tissue . Various techniques to expand the ablation area, such as overlapping ablations, temporary block of blood flow and infusion of saline around the electrode have been used. In such cases, the ablation margin becomes even more unpredictable. Real-time feedback of the ablation margin is thus needed to perform exact and safe ablation. MR thermal imaging to allow monitoring of the advancing margin of ablation is under investigation and appears promising , but will only be applicable in restricted situations. In practical terms, RF ablation is performed under sonographic guidance in many cases. We have therefore devised and tested the efficacy of a superfine thermocouple-needle system (TNS) using fresh porcine livers in vitro and rabbit back muscles and livers in vivo.
The TNS is composed of a 0.25-mm diameter K-type thermocouple embedded in a 22-G, 15-cm-long needle with a pencil-tip (RFT; Hakko Medical, Tokyo, Japan). The thermocouple can be connected to a thermometer (Model HA, Anritsu Metre, Tokyo, Japan), allowing multipoint real-time temperature monitoring.
The organs used for RF ablation were excised fresh porcine livers (n = 15) purchased from a local butcher and in vivo rabbit back muscles (n = 2) and livers (n = 3). The rabbits were anaesthetised in accordance with the guidelines of our institutional animal care and use committee.
A 17-gauge internally cooled RF electrode with a 2-cm active tip (Cool-tip; Radionics, Burlington, MA) was used to ablate the organs. The single TNS was inserted in an arbitrary site 1 cm from the active tip of the RF electrode and the temperature of the liver around the RF electrode was measured concurrently. In rabbit livers, insertion of the RF electrode and TNS was performed under CT guidance.
Irreversible cellular damage is known to occur within 4–6 min at 50–55°C and instantaneously at 60–100°C . The RF current was thus cut off when the temperature reached 60°C or after 5 min at ≥50°C. After RF ablation, the porcine livers and rabbit back muscles were cut with a scalpel along a longitudinal plane passing through the axis of both the electrode shaft and the TNS. The RF electrode was removed in the rabbit livers but the TNS was left in place; contrast-enhanced CT was then performed.
The ablation areas in the cut surface of porcine livers and rabbit back muscles exhibited a well-defined discoloured region of left–right symmetry relative to the RF electrode. The TNS tip accurately pinpointed the margin of the ablation area in all cases (Figure 1).
Contrast-enhanced CT of the rabbit livers also showed the TNS tip precisely located at the margin of areas of no contrast enhancement (Figure 2). These results were identical between the point at which the RF current was stopped immediately after reaching a temperature of 60°C (n = 17) and after 5 min at ≥50°C but <60°C (n = 4). RF-induced formation of a discoloured area has been reported to correspond with histological coagulation necrosis . The region with no contrast enhancement after RF ablation on CT also reportedly corresponds to areas of histological coagulation necrosis with an error of ≤2 mm .
In cases of ablation of superficial lesions or intra-operative cases, multipoint temperature monitoring is quite simple. Multipoint monitoring, however, may be troublesome for deeply situated lesions. Nonetheless, the ablated areas usually spread concentrically around the electrode and are normally smaller in the direction perpendicular to the RF electrode axis than in the direction of the RF electrode axis (although exceptions are seen), typically owing to the conducting effect of blood vessels. We could thus estimate with relative accuracy whether a sufficient margin of ablation could be obtained by placing the single TNS tip at the intended tumour-free safety margin perpendicular to the RF electrode axis.
Our results indicate that the TNS accurately identified ablation margins and placement of the TNS tip at the intended ablation margin marks the defined end points and permits exact RF ablation and other thermal ablations, particularly in cases involving relatively large lesions, in which meticulous setting of ablation margins is required.