To the Editor:
In the case report, an unusual complication of radiofrequency ablation treatment of osteoid osteoma, Finstein et al4 describe a complication of therapy of osteoid osteoma with a high-energy deposition technique. The authors used an array probe and impedance control energy delivery technique to ablate a superficially placed osteoid osteoma after failure of surgical therapy on two occasions.
The energy delivery method used by these authors is beyond the routine therapy for an osteoid osteoma. Rosenthal et al8 described the successful clinical use of a 5-mm exposed single-tip probe and a 50-Watt generator for treating osteoid osteoma. Four minutes of therapy was delivered. Energy delivery was controlled manually at 90°C.8 Radiofrequency ablation with this equipment and energy delivery technique reportedly had success rates of 76% to 100%.2
The therapy of visceral organ metastatic deposits or primary tumors has led to the development of higher-energy techniques. Physicians have demanded the ability to treat larger volumes of tissue through a single-probe application. This has been achieved by advances in probe and radiofrequency generator design.
Applicator technology has advanced, with the availability of longer exposed single and array probes. Array probes similar to the probe used by Finstein et al,4 expand from the applicator tip to treat a larger volume of tissue. Longer active probe lengths have been used as the radiofrequency generators have increased in energy output.
Generator design has also been modified. The automated impedance method of controlling current delivery has been developed as a surrogate for temperature measurement and manual feedback since tissue impedance alters during heating. As tissue temperature approaches the boiling point of water, tissue impedance increases. Vaporization in the tissue adjacent to the probe leads to electrical insulation. Once impedance has reached a set percentage above baseline the radiofrequency current is interrupted. The minimum rest period of 15 seconds allows tissue cooling. Once impedance has fallen to a reference level, radiofrequency energy is again delivered. This technique has allowed the expansion of the zone of radiofrequency ablation.6 Impedance-controlled energy delivery allows up to 5 cm of a tumor or soft tissue to be ablated at one probe placement.
Tillotson et al treated normal bone with a 5-mm probe at 80°C for as much as 4 minutes.9 The zone of ablation produced was 9 to 13 mm.9 In subsequent research using MRI and histology to size the zone of ablation in bone, the zone of ablation has been measured up to 31 mm with a single 2-cm water-cooled probe and 12 minutes of impedance control energy delivery from a 200-Watt generator.3
In the therapy of large volumes of tumor tissue, the advances in energy delivery have been extremely helpful. When focal small volumes of tissue are to be treated, such as an osteoid osteoma, there is evidence that increasing energy delivery can lead to improved clinical success.7 The article by Finstein et al provides an excellent cautionary note to the use of large volume tissue ablation techniques in small tumor volumes.
For patients who have had previous surgery, we have found it can be difficult to identify the site of persistent or recurrent osteoid osteoma for radiofrequency ablation. Finstein et al did not describe the diagnostic steps involved in localizing the recurrent osteoid osteoma, and they did not state whether a biopsy was performed.4 It would be useful to know if the osteoid osteoma was resected even partially in the primary or secondary surgical specimen. They also do not mention the experience and training of the doctors since this is important.4
Numerous techniques have been used to avoid complications such as skin necrosis. Using a shorter active tip probe and drilling through the lateral cortex of the tibia and thus approaching the osteoid osteoma through a longer tract may avoid this complication. In ex vivo work, 5 mm of cortical bone is thermally protective when applying radiofrequency ablation.1 Therefore it would be advantageous to site the probe tip in the cortical bone, medial to the tibial skin.
In 2005, Goldberg et al updated standardized terminology and reporting criteria originally published in 2003 to facilitate communication about radiofrequency ablation and to allow comparison of different treatments.5 Finstein et al might clarify some issues based upon these updated guidelines to help the physician better tailor treatment for patients with osteoid osteoma.
Finstein et al also should clarify the design and manufacturer of the probe. The maximum extent of the array probe length was stated to be 3 cm4; we need to know what extent of the 3 cm was exposed in the bone. Unfortunately the images are not technically adequate to determine the distal extent of the probe. In our practice, we measure from the tip of the probe backward along the probe for the length of the active tip on a final CT scan so we can determine appropriate positioning. We attempt to place the proximal extent of the active tip at the nearest extent of the osteoid osteoma so that therapy is delivered only to the osteoid osteoma and the marrow.
Finstein et al should describe the maximum output of the generator and generator model. The term “slow heating” seems to be a trademark and should be explained. The initial impedance, mean current delivered, treatment time, and end point of therapy also should be stated. In our experience, a scratch on the protective surface of the probe led to arching of a metal guiding bone-biopsy tool which produced very low resistance (< 100 ohm). This led to substantial skin and soft tissue necrosis in the superficial tissues without therapy of bone or marrow.
We recommend using a 5- to 10-mm probe length, and a probe that is not actively involved in ablation of the osteoid osteoma should be placed in the marrow and not in the soft tissue. We believe high-energy delivery can be successful, but treatment periods need to be tailored to the size and site of the osteoid osteoma.
Colin P. Cantwell, MSc, MRCS, FFR, FRCR
Stephen Eustace, MSc, MRCP, FFR, FRCR
Department of Radiology, Mater Misericordiae University Hospital, Dublin, Ireland
1. Bitsch RG, Rupp R, Bernd L, Ludwig K. Osteoid osteoma in an ex vivo animal model: temperature changes in surrounding soft tissue during CT-guided radiofrequency ablation. Radiology
. 2006;238: 107-112.
2. Cantwell CP, O'Byrne J, Eustace S. Current trends in treatment of osteoid osteoma with an emphasis on radiofrequency ablation. Eur Radiol
3. Cantwell CP, Kerr J, O'Byrne J, Eustace S. MR imaging features after radiofrequency ablation of osteoid osteoma. AJR Am J Roentgenol
4. Finstein JL, Hosalkar HS, Ogilvie CM, Lackman RD. An unusual complication of radiofrequency ablation treatment of osteoid osteoma. Clin Orthop Relat Res
. 2006; Epub ahead of print.
5. Goldberg SN, Grassi CJ, Cardella JF, Charboneau JW, Dodd GD3rd
, Dupuy DE, Gervais D, Gillams AR, Kane RA, Lee FT Jr, Livraghi T, McGahan J. Phillips DA, Rhim H. Silverman SG; Society of Interventional Radiology Technology Assessment Committee. Image-guided tumor ablation: standardization of terminology and reporting criteria. J Vasc Interv Radiol
6. Goldberg SN, Stein M, Gazelle GS, Sheiman RG, Kruskal JB, Clouse ME. Percutaneous radiofrequency tissue ablation: optimization of pulsed-radiofrequency ablation technique to increase coagulation necrosis. J Vasc Interv Radiol
7. Martel J, Bueno A, Ortiz E. Percutaneous radiofrequency treatment of osteoid osteoma using cool-tip electrodes. Eur J Radiol
8. Rosenthal DI, Alexander A, Rosenberg AE, Springfield D. Ablation of osteoid osteomas with a percutaneously placed electrode: a new procedure. Radiology
9. Tillotson CL, Rosenberg AE, Rosenthal DI. Controlled thermal injury of bone: report of a percutaneous technique using radiofrequency electrode and generator. Invest Radiol