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Rizzoli Orthopedic Institute
Bologna 40136, Italy
We have read the article "Technical Considerations in CT-Guided Radiofrequency Thermal Ablation of Osteoid Osteoma: Tricks of the Trade" [1] with great interest. It reports the authors' experience in thermoablation by radiofrequency clearly and efficaciously. We particularly appreciated the detailed description of the procedure.
Between June 2001 and November 2003, we performed thermoablation of osteoid osteoma in 110 patients (four osteomas situated in the spine, 106 in other sites) using the same generator (RFG 3C RF Lesion Generator System, Radionics), the same kit (SMKTC 15, Radionics), and the same insertion and biopsy set (Bonopty, Radi Medical Systems) as the authors.
At first, we performed thermoablation on the nidus of the osteoid osteoma
for 4 min at approximately 90°C, but, having had several patients with
recurrences (
40%), we used longer and longer times until we reached 18
min without severe local or regional complications. For more than a year, we
have been pausing the supply for 2 min at 60°C.
It is indispensable to check the final position of the needle with 1-mm-thick contiguous slices, extended a few millimeters beyond the nidus, to obtain the exact position of the needle and decide the supply time of the radiofrequency and the position of the dispersive electrodes.
Since September 2002, we have been using two dispersive electrodes, similar to the ones described by the authors, that we set up at the end of the procedure in an appropriate area to correct any minor errors of needle placement; usually both are placed caudally or cranially to the lesion to channel more radiofrequency energy in these directions.
With the said supply parameters, precise spatial assessment of the needle, and final placing of the two dispersive electrodes, our patients have had no complications or recurrences in the last 14 months.
Therefore, we are satisfied with the equipment used, especially the hot monopolar needle, the reduced caliber of which enables a minimally invasive percutaneous approach (skin hole, < 1 mm) and a good forecast of the different volume of necrosis in relation to the active tip used, times and ways of supplying the radiofrequency, and so on. These characteristics and the possibility to partially condition the spatial diffusion of the radiofrequency and placing the dispersive electrodes have also enabled us to treat four patients with osteoid osteoma in spinal locations (two in the body and two in the arch).
Therefore, we agree with the authors that radiofrequency thermoablation is a safe and effective technique in the treatment of osteoid osteoma, provided the procedure is performed accurately; in particular, the way of supplying the radiofrequency must comply with the characteristics of the generator and needle used, after assessing precisely the "morphovolumetry" of the nidus and the position of the needle.
References
Tauranga Public Hospital
Tauranga, New Zealand
Leiden University Medical Center
Leiden NL-2300 RC, The
Netherlands
We thank Dr. Albisinni and colleagues for their letter concerning our article on CT-guided radiofrequency thermal ablation of osteoid osteoma [1]. It was interesting to learn of the authors' own considerable experience and clear success in this field.
It is difficult to comment on their initially high recurrence rate (
40%). However, we believe that longer times (
18 min) for ablation are
probably not necessary. As we mentioned in our article, we ablate for 4 min at
90°C, with a success rate of 92%. Rosenthal and his Boston group
[2] prefer 6 min at the same
temperature also and have high success. In fact, Tillotson et al.
[3] performed original
experiments in healthy bone in dogs that clearly showed the threshold for
osteocyte necrosis to be 50°C for 30 sec. We note that Albisinni and
colleagues pause the supply for 2 min at 60°C. Although we believe this to
be an unnecessary step, it is important, as we mentioned, to be careful to not
introduce too much current or increase it too rapidly because heating may be
so intense that charring and solidification may limit current flow. We slowly
and smoothly increase current for 30-60 sec until the desired temperature is
reached.
We also note that they use two dispersive electrodes both caudal and cranial to the lesion at the end of the procedure, presumably to ensure entire lesion destruction. However, although an argument could be made for the usefulness of this extra step in long or tubular bones where lesions tend to be more elongated and overlapping ablations are potentially necessary, this precaution may not be applicable in other areas. In other bones, the dimension may be 10 mm or slightly more in every plane, so for those lesions the precise 3D size (which Albisinni and colleagues call "morphovolumetry") is the critical issue. As we mentioned in our article and stress again, individualized planning is required for each lesion with respect to the need for one or more needle ablation positions. A fairly reliable rule of thumb with the equipment detailed in our study is that approximately a 1-cm sphere will be ablated per needle probe position.
References
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