Osteidosteoma Thermoablation
Definition and Etiology
The Osteidosteoma is a benign tumor of the skeletal system and consists of a strongly vascularized “core” (Nidus), which is surrounded by reactive growth of sclerotic bone tissue. The OO is most commonly found in the long bones of the lower extremities and the spine. All other parts of the skeletal system may also be afflicted. The OO contributes to 14% of all bone tumors. The OO occurs in patients as early as 10 years of age and is rare in patients above the age of 30. OO cause symptoms such as localized pain, which typically worsens at night and responds well to Asprin and other non-steroidal pain medication (NSAID). If the OO is found in the spine, it can often induce scoliosis. In the proximity of joints or growth plates complications such as growth retardation or arthrosis can occur.
Osteidosteoma. Cross section of the fibula with a round tumor core. At the site of the cortical elevation, an oval, dark red nidus is found, which partially inflitrates the medulla.
Diagnostics
Other than the salicate test, the diagnosis of an Osteidosteoma is confirmed by radiographic imaging: x-Ray, bone szintigraphy, computer tomograohy (CT) and Magnetic Resonance Imaging (MRI), ideally with the administration of contrast agent, to show the enhancement of the vascularized nidus.
20 year old patient with pain, typical for an OO and typical radiographic image findings: a) x-Ray: Nidus: central cavity in the cortical bone of the right fibula, surrounded by reactive sclerosis.b) Image findings of the OO on MRI on a coronary image and on an axial image c). The green line indicates the planned access to the nidus under the consideration of nerves and blood vessels (circled).
Differential Diagnosis
In the differential diagnosis a Brodie Abscess should be primarily ruled out. However, the Brodie Abscess shows much slower up-take of the contrast agent in dynamic CT or MRI and only responds poorly, if at all to salicates (Asprin). Moreover, osteoblastomas might also resemble an OO. These are bigger in diameter (>15mm) and do not typically hurt at night and normally have no sclerotic border.
Therapeutic Options
Conservative treatment of OO consits of medical long-term therapy or NSAIDs. The disadvantage of this therapeutic regimen lies in the commonly known side effects and complications of these pharmaceutical products.
Conventional operative therapy is the full surgical resection of the tumor. The intraoperative localization of the tumor can be challenging and often results in expansive and disproportional resection of the nidus and the surrounding tissue. This can lead to infection or fractures, which then demand for internal stabilization with or without bone replacement therapy. The period of hospitalization amount so days and lengthly reconvlaescence periods.
Minimal-invasive therapy options for OO teatment are CT-guided nidus drilling, alcohol injection or thermoablation. CT-guided drilling of the tumor has lead to incomplete resection of the tumor in the past and ensuing regrowth of the tumor. However, the reoccurances could be reduced by alcohol injection. But the dispersion of the alcohol is difficult to confrol and thus often results in the damage of neighbouring structures.
Image-guided thermo-ablation has replaced open resection for the most part with success rates of 80-100%. In case of failiure of first line ablation, a second ablation can ensure pain relief in 99% of all cases. For thermoablation of osteidosteomas, radio frequency ablation and LASER ablation are used. In a comparative study of both techniques we were able to showthat there are no significant differences in terms of recurrences and complication rates. Serious complications have not been reported to date. As most research projects, we prefer the performance of thermoablation under general anesthesia, since the drilling of the tumor can be quite painful.
CT-guided thermoablation with RFA has been considered to be the”gold standard”. This method was successfully performed on 50 patients with symptomatic OOs during the last year. As a technical evolution, we performed the first LASER-therapy of an OO under open MRI guidance world wide. The procedure was successfully performed in the Charité’s open high-field MRI (1.0 T Panorama HFO, Philips Healthcare, Best, NL). Ever since, many other patients were treated with this method.
Case – 20 year old patient with an OO. Minimally invasive MR-guided Laser ablation in an open high-field MRI (1.0 T Panorama HFO, Philips Healthcare, Best, NL).
MR-guided LASER-Ablation
As opposed to the surgical removal of OO, MR-guided LASER-ablation allows the coagulation of the tumor with a small percutaneous puncture at the tumor site. The OO, which is responsible for the pain symptoms, can be imaged, localized and ablated under MR-guidance effectively and safely. Thermocoagulation is performed at 90°C for only a couple of minutes.
Case – Minimally invasive MRI-guided drilling of the tumor nidus at the right fibula.
The interventions in the open MRI without the exposure to ionizing radiation is especially appropriate for young patients. Further, the high-field open MRI offers good access to the patient and also enables modern physico-chemical imaging modalities such as temperature mapping (MR-thermometry). With these techniques, the heat distribution during the LASER procedure can be continuously monitored and visualized and excessive heat deposition to surrounding tissue can be prevented.
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Introduction of a thing <1mm LASER bare fiber into the drilled nidus and subsequent LASER-Ablation under continuous thermometric monitoring.
Patients can profit form the enhanced effectiveness of the procedure and the increased safety.
Final MRI-control images show the technically successful removal of the OO, due to the now diminished up-take of contrast agent.
The duration of the hospital stay is usually 24-48 hours post intervention. Most patients are pain free directly after the procedure. The patients can use the afflicted extremity without major precautions in adaption to pain 1-2 days after lasing. Depending on the localization extreme stress on the extremity should be avoided for the following 6-12 weeks.
Summary
With image-guided thermoablataion an effective and safe method for the minimally invasive therapy of OO is available and has mostly replaces the open surgical approach in most clinics. Compared to open surgery, this method reduced morbidity and reconvalescence considerably, due to its minimally invasive character. When compared to CT-guided RFA, the method shows similar results and bears certain technical advantages, for example MR-Thermometry, as well as the omission of ionizing radiation. Only in some cases, with very difficult tumor localizations, the open method is still indicated. Prolonged treatment with oral salicates or NSAR no longer seems justifiable under consideration of the associated complications.
Florian Streitparth
Felix V. Güttler (Ed.)
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