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Osteoid osteoma is a benign tumor with characteristic clinical and radiological findings. The major differential diagnoses are Brodie's abscess and, occasionally, stress fractures. Many techniques have been proposed to treat osteoid osteoma, including surgical excision, percutaneous extraction, alcoholization, radio-frequency ablation, and ILP.

Surgical or percutaneous excision of the nidus can provide rapid relief of the patient's symptoms (3-9). In open surgical resection, the osteoid osteoma is often difficult to identify intraoperatively, and an excessive amount of bone is often resected to ensure removal of the nidus. This weakens the cortical bone that is important for weight bearing, increases the risk of fracture, and imposes limitation of activity for up to three months after surgery in these patients. In addition, osteoid osteoma situated in a deep-seated, intra-articular or epiphyseal site may require wide excision or arthrotomy for its excision. Compared to open surgical resection, percutaneous extraction of nidus under CT guidance allows precise localization of tumor by CT with removal of less amount of bone and, hence, offers less risk than open surgery. However,

FIGURE 2 Osteoid osteoma of the posterior end of the rib. (A) Bone scan with hyperfixation of the rib. (B,C) The nidus is localized with computed tomography (CT) scan. Note the subpleural thickening. (D) Procedure performed under general anesthesia under CT guidance. (E) Control CT scan one year after the procedure.

FIGURE 2 Osteoid osteoma of the posterior end of the rib. (A) Bone scan with hyperfixation of the rib. (B,C) The nidus is localized with computed tomography (CT) scan. Note the subpleural thickening. (D) Procedure performed under general anesthesia under CT guidance. (E) Control CT scan one year after the procedure.

FIGURE 3 Osteoid osteoma of the femoral bone. (A) Typical subperiostal nidus. (B) Interstitial laser photocoagulation under computed tomography control.

the trephine needle used is often large, ranging from 7 to 10 mm internal diameter (8,9). The large size of the instrument may also incur risk of neurologic and vascular injury in some anatomic regions, particularly in children. Although smaller diameter drills and needles (3-4 mm) have been used, the small size of the core increases the risk of incomplete nidus removal and recurrences, and multiple passes are often required to complete the resection of the nidus (3-5). Although immediate weight bearing is possible when small drill or needle has been used (5), weight bearing is not allowed for four to six weeks to prevent risk of fracture when large-size needle or drill is used (8,9). Reported complications with percutaneous resection include skin burns at the entry site of instrument, soft-tissue hematoma, dysesthesia, superficial and deep infection including osteomyelitis, and fracture (6,8,9).

CT-guided percutaneous drilling with subsequent alcoholization has also been used (18), but this technique does not have precise control over the size and morphology of the tissue damage. In this respect, thermal ablation by radio-frequency ablation or ILP has the advantage of precise control over the size of the tissue damage achieved, with an excellent dose-response characteristic (Fig. 3) (11,34,38).

Since June 1993, ILP has been performed on patients with presumed osteoid osteomas in our institution. ILP was only performed in cases where the clinical and radiological features typical of osteoid osteomas existed in the patients. Because this tumor is completely benign, with no case of reported malignant transformation (1), and the size of the tumor is within the range of thermal coagulation by ILP, it is amenable to curable treatment by ILP. ILP has proven to be effective in 83 of 84 patients, with pain relief occurring within one week of the procedure in most patients. Although we had six recurrences in our series, with an average follow-up of 47.1 months (range 4-97 months), these recurrences were successfully treated with a second ILP. In four of the recurrences, the largest diameter of the nidus ranged from 10 to 24 mm, and the large tumor size may account for the recurrence (12). Two recurrences had occurred at six weeks and at four months after the initial ILP, and these were due to imprecise needle positioning within the tumor nidus leading to incomplete tumor coagulation. The other four recurrences occurred 12 to 27 months from the initial ILP. Two of these recurrences (children under 10-years-old), in fact, had previous surgical recurrences despite histological demonstrations of complete resection margins, and both were located within the tibial shafts. We postulate that these cases of recurrences with a pain-free interval of 12 months or more after the initial treatment may have triggered the regeneration of a new tumor at the same site. This may explain the two cases of repeated recurrences, one occurring after complete surgical resection and one occurring after ILP treatment, in similar locations.

Three out of the six recurrences occurred in an intra-articular location, two within the hip joint, and one within the zygoapophyseal joint. This is of a higher-than-expected proportion in view of the 32% of intra-articular tumors in our series. It seems that the intra-articular location may be predisposed to the recurrence, but the underlying mechanism is unclear. All these recurrences happened in patients under 16 years of age.

Reflex sympathetic dystrophy was the only major complication occurring in one of our patients. There is, however, no correlation between the type and severity of the injury and the occurrence or course of this entity. There is evidence that regional analgesia, including sympathetic blockade, during the intervention provides pain relief and improves circulation of the injured part and helps prevent reflex sympathetic dystrophy (36,37).

There was no neurological complication observed in all our patients. All 10 spinal osteoid osteomas were successfully treated, with no neurological deficit. In our previous publications, we had limited ILP treatment to tumor nidus that occurred at least 8 mm away from vital tissues (e.g., neurological and tendinous structures) to avoid tissue damage (39,40). This limitation, however, had been overcome with the use of epidural normal saline cool bath technique. This technique consisted of slow infusion of normal saline at room temperature at a rate of 70 mL/hr via a 22-gauge spinal needle placed into the epidural space or foramina adjacent to the tumor (Fig. 4). The maximum coagulation time is 10 minutes. With this technique, we were able to successfully treat four cases of spinal tumor nidus that were located less than 8 mm from adjacent neurological structures without any neurological complications or tumor recurrence (follow-up, 24-48 months).

There are many advantages in the ILP treatment of osteoid osteoma. The fiber used for delivering the laser energy is flexible and thin (400 pm), and this can be put into a small caliber needle (18 gauge) with minimal alteration to the existing structure (39-43). The thin fiber also

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FIGURE 4 Osteoid osteoma of the spine with use of epidural normal saline cool bath technique. (A) Localization of the nidus. (B) A 22-gauge spinal needle placed into the epidural space close to the tumor. In difficult cases, a thermocouple could be used with the same technique for thermal monitoring to avoid any accidental coagulation. (C) Percutaneous interstitial laser photocoagulation of the tumor.

allows precise placement within tumors and is particularly useful in small, deep-seated osteoid osteomas. Because the fiber is a single-use fiber, there is reduced risk of contamination. The semiconductor laser is simple to use and does not require watercooling or three-phased electrical power. It is small, compact, and portable. With low power (typically 2 W), the laser source can produce a predictable size of necrosis in proportion to the energy delivered, which is much more precise than alcoholization. Most lesions are treated effectively by use of one fiber. For lesions larger than 15 mm in diameter, up to four fibers can be placed in the nidus and simultaneously fired with a 1 x 4-coupler fiber splitter for complete ablation. Nd:YAG or diode laser is usually available in the majority of medium- to large-sized hospitals, because it is also used by specialists of other disciplines. The cost of the procedure will be further reduced if the laser machine can be shared among specialists in the hospital. This technique is less expensive than surgical intervention. Most patients either are treated on an outpatient basis or require one night of hospitalization with ILP. Overall, this is more cost effective when compared to the costs incurred with several days of hospitalization usually required for surgical intervention. ILP has less associated morbidity compared to surgical resection or percutaneous resection, with maximum work interruption of two weeks and usually return to normal activities in one week for most of our patients.

The drawbacks of this technique are lack of routine histological verification and the use of quite expensive material. When biopsy was performed in surgical resection or radio-frequency ablation, the confirmation of osteoid osteoma is obtained in 57% to 79% of cases (5,6,9,11). We do not routinely obtain histological confirmation, because the small 18-gauge needle introduced does not allow sufficient material to be obtained to confirm the diagnosis. However, with a 14-gauge needle or in atypical cases, when the size of the nidus exceeds 2 cm, histological confirmation was obtained.

Thermal ablation of osteoid osteoma by percutaneously placed radio-frequency electrodes has also been described (10-17). It is a safe and effective treatment, with the failure rate of single radio-frequency treatment being 7.5% (13), and this is comparable to that of ILP in our series. This method has also been used for treatment of osteoid osteoma in the spine (15-17).

ILP is safe and effective for the treatment of osteoid osteomas (19-22,31). The small size of the needle required for accessing the tumor, no or short hospital stay, a quick postprocedural recovery, and the lack of significant complications are major advantages over open or percutaneous surgical resection. Recurrences can be treated easily and effectively with repeat ILP. We propose that all osteoid osteomas should be treated by thermal ablation techniques, radio-frequency ablation, or ILP, rather than surgical or percutaneous resection. Surgical resection should only be performed in the very rare cases of percutaneously inaccessible osteoid osteomas.

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