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Vascular and Interventional Radiology |
1 From the Department of Radiology, University Hospital of Strasbourg, 1 place de l'hopital, 67091 Strasbourg, France. Received August 11, 2004; revision requested October 27; revision received September 5, 2005; accepted September 23; final version accepted March 20, 2006. Address correspondence to A.G. (e-mail: gangi{at}rad6.u-strasbg.fr).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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Materials and Methods: Ethical review board approval was obtained for the retrospective study. Informed consent was waived. From June 1994 to June 2004, 114 patients (mean age, 22.3 years) suspected of having osteoid osteoma underwent ILA with a diode laser (805 nm). An optical fiber was introduced into the nidus of the osteoid osteoma, and 400–3000 J of energy was delivered, depending on the size and location of the nidus. Twelve spinal osteoid osteomas were treated; in five of these cases, the nidus was located fewer than 8 mm from the adjacent nerve roots, and slow epidural or periradicular infusion of normal saline was used to avoid thermal damage to neurologic structures. Pain was evaluated with a visual analog scale (VAS) and medication. ILA was considered successful (score, 0) when complete pain relief was achieved (VAS score, <1) without medication.
Results: One week after ILA, 112 patients had a score of 0. One week after ILA, one patient had pain that persisted for 2 months because of reflex sympathetic dystrophy. At follow-up (mean, 58.5 months; range, 13–130 months), six patients had recurrence of pain from 6 weeks to 27 months after the initial ILA. These recurrences were treated successfully with a second ILA. Only one unsuccessful treatment was encountered.
Conclusion: Percutaneous ILA is an effective treatment for osteoid osteoma.
© RSNA, 2006
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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The typical symptom is local pain that is described as severe, sharp (knifelike), boring, typically worse at night, and typically relieved with salicylates. The radiologic diagnosis is accurate when combinations of bone scintigraphy, radiography, computed tomography (CT), and magnetic resonance (MR) imaging are used (4–9). Together with clinical findings, a high-confidence, imaging-based diagnosis is possible (10–13). The major differential diagnoses are Brodie abscess and occasional stress fractures.
The disease is self-limiting, and pain may be relieved after 5–6 years of conservative medical treatment. Total removal of the nidus of the osteoid osteoma, however, is usually the treatment of choice, and surgery has been considered the definitive treatment for many years. A substantial amount of bone usually is resected with the surgical technique, and complications such as fractures could occur. The tumor can be difficult to localize precisely at surgery, and a large amount of bone often has to be resected to ensure complete tumor removal. Tumors at certain less-accessible sites may require an extensive surgical approach, which may lead to a few weeks of postoperative restriction of activity.
Percutaneous resection of the nidus with CT guidance allows precise localization of the tumor with removal of less bone than at open surgery; hence, percutaneous resection has less risk than does open surgery. The trephine needle used at percutaneous resection, however, is often large, ranging from 7 to 10 mm in internal diameter (11,12,14–19). The large size of the instrument may incur risk of neurologic and vascular injury in some anatomic regions, particularly in children. Although drills and needles of a smaller diameter (3–4 mm) have been used, the small size of the core increases the risk of incomplete nidus removal and recurrence, and multiple passes are often required to complete the resection of the nidus (15,16,18). Although immediate weight bearing is possible when a small drill or needle has been used (18), weight bearing is not allowed for 4–6 weeks to prevent risk of fracture when a large drill or needle has been used (12,19). Reported complications at percutaneous resection include skin burns at the entry site of the instrument; soft-tissue hematoma; dysesthesia; superficial and deep infection, including osteomyelitis; and fracture (12,14,19). CT-guided percutaneous drilling with subsequent alcohol injection also has been used (20), but this technique does not enable precise control of the size and morphology of the tissue damage. In this respect, thermal radiofrequency ablation or interstitial laser ablation (ILA) has the advantage of precise control of the size of the tissue damage, which is achieved with an excellent dose-response characteristic (21,22).
These disadvantages of percutaneous resection have encouraged the introduction of less invasive therapeutic methods, such as CT-guided core drill excision (11,14–19), radiofrequency ablation (21–28), alcohol injection (20), and ILA (29–34).
At ILA, the laser is not used for its tissue-penetrating effects but instead is used for its ability to deliver a fixed energy precisely to a target area (35). The tip of the optical fiber acts as a point heat source. The size of induced zone of ablation depends on the laser wavelength, thermal and optical properties of the tissue, total duration of energy application, power used, diameter of the fiber, fiber tip, and number of fibers used sequentially or simultaneously for ablation (35–38). Insertion of a fiber through a well-placed needle allows direct delivery of laser energy from the machine to the tumor. With a constant power of 2 W, the mean transverse diameter of coagulation varies from 3.4 mm at 200 J to 9.2 mm at 1200 J. The maximum coagulation effect is reached at 1000–1200 J, and more energy at the same location does not increase the volume of coagulation (39). These results are consistent with those demonstrating that the transmission of heat within bone is sharply limited by blood flow and that lethal temperatures cannot be sustained over great distances (37,40). ILA has also been applied clinically to treat tumors of the liver, pancreas, prostate, brain, breast, and lymph nodes (41–47).
The purpose of our study was to retrospectively evaluate the effectiveness of ILA as a curative treatment of osteoid osteoma.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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The rest of the tumors were located within the acetabulum (n = 5), talus (n = 4), calcaneus (n = 3), ilium (n = 2), navicular bone (n = 2), metacarpal bone (n = 2), ulna (n = 1), coracoid process of scapula (n = 1), acromion (n = 1), lunate bone (n = 1), hamate bone (n = 1), cuneiform bone (n = 1), and posterior sixth rib (n = 1). Thirty-two (28%) of the tumors were intraarticular (within the joint capsule) and had associated joint effusions, particularly when tumors were located within the hip and shoulder joints. The duration of pain before ILA ranged from 3 months to 10 years. All patients regularly received prostaglandin inhibitors for pain relief. Five patients were referred to us with recurrence of pain after previous surgical or percutaneous core drill resection of the nidus.
The diagnosis of osteoid osteoma was made with both clinical and suggestive imaging findings (4,6–9,13). Patients underwent various combinations of conventional radiography, bone scintigraphy with technetium 99m, CT, and MR imaging. MR imaging is an extremely sensitive modality for depiction of bone marrow and adjacent soft-tissue edema, especially with the use of fat saturation and T2-weighted sequences. The use of a T1-weighted fat suppression MR sequence with injection of paramagnetic contrast material depicts the nidus as a region of intense uptake of contrast material in many cases, but the better spatial resolution at CT allows easy localization of the nidus, particularly during ILA. Biopsy was performed only with a large (14-gauge) needle and only in nidi larger (20 mm in diameter) than usual. Altogether, 88 biopsies were performed.
Thus, before ILA the nidi were clearly identified in all patients. With the exception of tumors in three patients, the longest diameter of all tumors was smaller than 20 mm, and 99 (87%) of 114 tumors were 10 mm in diameter or smaller. Although by definition osteoid osteoma is smaller than 20 mm in diameter, three tumors measured 20 mm or larger; the largest measured 24 mm in diameter. ILA was not performed when doubt persisted in the diagnosis of osteoid osteoma. These three patients, however, had typical clinical and imaging findings of osteoid osteoma (except the size) and nonetheless underwent percutaneous biopsy before ILA.
Pain relief was the main item for evaluation of the effectiveness of our study. Pain was evaluated with a visual analog scale (VAS) and by assessing response to medication consumed before and after ILA. ILA was considered successful (score, 0) when complete pain relief was achieved (VAS score, <1) without medication. A score of 1 corresponds to VAS scores between 2 and 5, and a score of 2 corresponds to VAS scores more than 5.
The VAS score was assessed face-to-face by authors (A.G., H.A., L.W.) 24 hours and 1 week after ILA. All patients were followed up every 2 months during the first 6 months and each year thereafter (phone interview) according to the same scoring system. All patients were informed that osteoid osteoma is a bone tumor and the follow-up was necessary to confirm complete healing. The patients were informed (A.G., H.A., X.B., L.W.) that in case of any pain recurrence, they should immediately contact our department.
Follow-up radiography was performed in all patients. CT or MR imaging examinations were performed in all patients a minimum of 1 year after ILA in our department or outside of the study, but the images were viewed by one of the interventional radiologists of our department (A.G., H.A., X.B., L.W.). In case of recurrence of symptoms, bone scintigraphy, CT, and MR imaging were performed systematically.
Procedure
Prior to ILA, the procedure was explained carefully to all patients and alternative treatments such as open surgery and percutaneous resection also were presented. Before ILA, informed consent was obtained from all patients (or parents for patients younger than legal age).
A CT scanner (Somatom Plus-S ICT or Volume Zoom Plus 4; Siemens Medical Systems, Erlangen, Germany) was used to localize the tumor nidus, to guide needle placement precisely within the tumor nidus, and to depict carbonization. The tumor was localized at CT by using 1-mm-thick contiguous sections through the region of interest. When bone drilling was required, combined CT and fluoroscopic guidance was used to depict the needle progression in three dimensions. At fluoroscopy, a mobile C-arm (Sire Mobile; Siemens Medical Systems) was positioned in front of the CT gantry.
A portable, continuous-wave, semiconductor diode laser (model 25 laser; Diomed, Cambridge, England) with an 805-nm wavelength was used to perform all ILA. Laser energy was delivered to the tumor by using a flexible, single-use, bare-tipped, 400-µm fiber with polymer cladding.
In typical osteoid osteoma, the needle penetration is extremely painful despite the use of local anesthesia. This point seemed to be specific in our experience with osteoid osteoma. All procedures were performed with the use of general anesthesia or regional nerve block because of the excruciating pain during the penetration of the needle into the nidus. After precise localization of the tumor with CT scanning, the skin entry point and approach were determined. The dimensions of the nidus were measured at CT, and this determined the amount of energy required to coagulate the tumor. This measurement is especially important for tumors located close to neurovascular structures. A 14-gauge bone biopsy needle (Ostycut; Angiomed/Bard, Karlsruhe, Germany) was used in nidi requiring cortical perforation (Fig 1). A 14-gauge Bonopty Penetration Set (RADI Medical Systems, Uppsala, Sweden) was used for cortical drilling in tumors surrounded by dense cortical bone (Fig 2), except in two patients for whom a 2-mm-diameter hand drill was used. An 18-gauge spinal needle (Becton Dickinson, Rutherford, NJ) was used in subperiosteal tumors and cortical nidi without major ossification (Fig 3).
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The tip of the 18-gauge spinal needle should be positioned such that all portions of the nidus are within 5–6 mm from the needle tip. Before insertion of the fiber into the needle, the fiber tip was precharred by firing the fiber in 2–3 mL of the patient's own blood to increase light absorption and reduce reflection. In the majority of cases, however, the tumor was hypervascularized with blood overflow from the needle, and charring was not necessary. The fiber was not radiopaque; therefore, before insertion into the tumor, the fiber was inserted into the 18-gauge spinal needle, and its length was adapted and marked. After confirmation of the location of the tip of the 18-gauge spinal needle within the center of the nidus with CT, the 400-µm fiber was inserted into the needle, and the needle was then withdrawn 5 mm to expose the tip of the bare fiber within the tumor. The diode laser was set to operate in the continuous-wave mode and to deliver power of 2 W for 600 seconds (total energy, 1200 J). For tumors located away from neurovascular structures, a maximum energy of 1200 J usually was applied to coagulate the nidus. For tumors located near neurovascular structures, the amount of energy delivered was calculated according to the formula (NS · 100 J) + 200 J, where NS is nidus size in millimeters. A maximum of 1200 J was used to avoid damage to surrounding structures. If a maximum energy of 1200 J was necessary to coagulate the tumor, we used a minimum distance of 8 mm between the fiber tip and the neurologic structure. If this minimum distance could not be obtained, a cooling solution was administered to the surrounding region.
Because of the limitation in coagulation size (maximum thermal ablation diameter, 16 mm) that can be produced in bone by using precharred fiber (39), two—or in one case—three 18-gauge spinal needles were placed in six patients with nidi 15–24 mm in diameter. The 18-gauge spinal needles positioned 5–6 mm apart were inserted in the nidus to ensure adequate tumor ablation. In these cases, the fibers were fired simultaneously with a 1 x 4 fiber splitter (Diomed), and a total energy of 2000–3000 J (1000 J delivered through each fiber) was delivered to each nidus. In two patients, one 14-gauge Bonopty needle was used for cortical drilling and two 18-gauge spinal needles then were successively inserted through the lumen of this 14-gauge needle, with modification of the direction of the needles.
In the patient who had three 18-gauge spinal needles placed within a large sacral nidus, which measured 21 x 13 x 13 mm, a total energy of 3000 J was delivered. The nidus was located fewer than 8 mm from the adjacent sacral nerves. To avoid neurologic damage, a cool-bath technique was used. This technique consisted of slow infusion of normal saline at room temperature at a rate of 70 mL/hr with a 22-gauge spinal needle (Becton Dickinson) placed into the epidural space adjacent to the tumor. A similar cool-bath technique with normal saline infusion into the epidural space also was used in four other cases of osteoid osteoma located within the vertebral lamina to avoid damage to adjacent nerves.
In two patients—one with a 5-mm-diameter tumor and one with a 6-mm-diameter tumor within the third metacarpal bone that were close to the tendons and neurovascular structures—5–10 mL of a solution of normal saline and 1% lignocaine was injected into the surrounding fat plane to separate these structures from the nidus prior to ILA. The normal saline acted as a buffer to avoid thermal damage to these structures during ILA. The 1% lignocaine was injected for added pain relief.
In 41 patients with an intraartricular or juxtaarticular nidus, 5–10 mL of normal saline was infused into the joint space prior to ILA. This was administered to reduce thermal injury to the cartilage and capsule.
At the end of ILA, 5–10 mL of rupivacaine at a dose of 2.5 mg per milliliter of saline was administered subperiosteally in 68 patients to ease postprocedural local pain.
The average total procedural time was 1 hour. All patients were either treated on an outpatient basis or hospitalized for 1 night after ILA, depending on the severity of the postprocedural pain. For patients treated with general anesthesia, intravenous nonsteroidal antiinflammatory drugs and analgesics were given before the patient was awakened to reduce the intense postprocedural pain experienced in the first hours after ILA. All patients were allowed to bear weight and resume daily and sport activities as comfort allowed immediately after ILA.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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Thus, we encountered only one unsuccessful treatment (score of 2) in a 16-year-old male patient who had a superficial intraarticular nidus located at the posterior surface of the femoral neck. During the 1st minute of ILA, the patient became agitated, a minimal dose of only 300 J was delivered, and ILA was prematurely terminated. The patient's agitation was presumably caused by the neuroleptic drugs. His parents declined a subsequent proposal for a second ILA with the use of general anesthesia and opted for surgical resection 3 weeks later with the use of general anesthesia. After this incident, only general anesthesia or regional nerve block was used.
No neurologic complication was observed in any of our patients. All 12 spinal osteoid osteomas were treated successfully with no neurologic deficit. By using the epidural normal saline cool-bath technique, we were able to successfully treat five cases of spinal tumor nidus located fewer than 8 mm from adjacent neurologic structures without any neurologic complication.
Follow-up
The follow-up period for patients in our series was 13–130 months (mean, 58.5 months). We were unable to follow up 16 patients after a minimum of 3 years of follow-up. Six patients had recurrence of pain occurring from 6 weeks to 27 months after the initial ILA (four patients with a score of 1 and two patients with a score of 2). Two recurrences occurred at 6 weeks and at 4 months after the initial ILA, and these were a result of imprecise needle positioning within the tumor nidus, which led to incomplete tumor ablation. The other four recurrences occurred 12–27 months after the initial ILA. Two of these cases of recurrence (children younger than 10 years) had previous recurrence after surgical resection despite histologic findings of complete resection margins, and both cases were located within the tibial shafts. In four cases of recurrence, the diameter of the nidus was 10–24 mm, and this large tumor size may account for the recurrence (24,48). All of these patients underwent a successful second ILA in the residual or recurrent nidus and remained pain free (score of 0) at recent follow-up.
Two patients had recurrence of pain less than 1 year after the initial ILA. The patient with recurrence of pain at 6 weeks had a residual nidus of 3 mm at the external part of the initial femoral neck nidus depicted at follow-up CT. Retrospective analysis of the CT scan at the initial ILA showed that the needle tip was not centered precisely enough in the nidus (9 mm in diameter), and residual nidus had occurred 4 mm from the needle tip. The other patient had recurrence of pain 4 months after the initial ILA. Follow-up CT depicted a 7-mm-diameter residual nidus at the periphery of the initial 12-mm-diameter intraarticular tumor located within the L5 lamina that was depicted at initial CT. Recurrence of pain in this case was postulated to result from a combination of inadequate energy and imprecise positioning of the needle (48).
Four patients had pain recurrence 1 year or later after the initial ILA. Two of these patients had previous recurrence after surgical resections despite histologic findings of complete resection margins. Both patients were referred to us to undergo ILA treatment after surgical recurrence, and both tumors were located within tibial shafts. At the initial ILA, one tumor was given 600 J of energy and the other was given 3000 J of energy because of the large nidus size (24 mm in diameter). Pain recurrence occurred 18 months and 15 months, respectively, after the initial ILA, and recurrent tumors were demonstrated at the same sites at follow-up CT. Both tumors were treated with a second ILA by using 1200 and 3000 J of energy, respectively, and both patients remained pain free 71 and 46 months, respectively, after the second ILA. The two remaining recurrences were located at the left femoral neck and left fibular shaft and occurred 27 and 12 months, respectively, from the initial ILA. Both tumors were treated successfully with no recurrence of pain 61 and 76 months, respectively, after the second ILA. All these recurrences were in patients younger than 16 years. Three of the six recurrences occurred at an intraarticular location; two occurred within the hip joint and one occurred within the zygoapophyseal joint.
Complications
Complete or partial sclerosis of the nidus was observed after 12 months on CT scans in 79 (69%) of 114 patients, and signs of bone marrow and soft-tissue edema were not seen on T2-weighted and STIR MR images in patients after 1 year.
In all 114 patients, no severe complications (eg, pathologic fracture, neurovascular or adjacent tissue damage, infection) were encountered at ILA. One patient with an osteoid osteoma within the lunate bone had mild reflex sympathetic dystrophy of the wrist that occurred 1 week after ILA, and this consisted of burning pain, hyperalgesia, hyperesthesia, and vasomotor disturbances. A short course of high-dose prednisone and a ß-blocking agent were given in addition to physical therapy. The patient's symptoms were relieved completely after 2 months.
Postprocedural CT scans (evaluated by A.G., H.A., X.B., L.W.) confirmed the lack of soft-tissue swelling, edema, or hematoma. No late complications were observed. One hundred twelve patients were able to return to normal activities within 1 week.
Histologic Findings
When biopsy was performed, histologic confirmation of osteoid osteoma was determined in 67 (76%) of 88 patients. For the remaining patients, no histologic diagnosis could be determined because of insufficient material.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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ILA proved to be effective in 112 of 114 patients; pain relief occurred within 1 week of ILA in 112 patients. Although we had six recurrences in our series, the two early recurrences (<4 months) were a result of imprecise needle positioning within the tumor nidus, which led to incomplete tumor ablation. The other four recurrences occurred 12–27 months after the initial ILA. The tumors in two of these late cases of recurrence (children younger than 10 years) had previous recurrence after surgical resection despite histologic findings of complete resection margins, and both tumors were located within the tibial shafts. We postulate that these cases of recurrence with a pain-free interval of 12 months or longer after the initial ILA may have been triggered by the regeneration of a new tumor at the same site. This may explain the two cases of repeat recurrence, one occurring after complete surgical resection and one occurring after ILA in similar locations. Three of the six recurrences occurred at an intraarticular location. This is a higher-than-expected proportion in view of the 28% of intraarticular tumors in our series. The higher recurrence rate at an intraarticular location was not explained by any technical difficulties.
Reflex sympathetic dystrophy was the only complication that occurred 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 ILA provides pain relief, improves circulation to the injured part, and helps prevent reflex sympathetic dystrophy (49,50).
In our previous publications, we had to limit ILA to tumor nidi that occurred at least 8 mm from vital tissues (eg, neurologic or tendinous structures) to avoid tissue damage (29,31). This limitation, however, has been overcome with the use of the epidural or periradicular normal saline cool-bath technique. With this technique, we were able to successfully treat five cases of spinal tumor nidus located fewer than 8 mm from adjacent neurologic structures (34).
Currently, a single-use polymer fiber costs 
120, and a laser machine with a 20-W laser costs 20 000. An Nd:YAG or diode laser is usually available in the majority of medium to large hospitals because these lasers are also used by specialists in other disciplines.
Thermal ablation of osteoid osteoma by using percutaneously placed radiofrequency electrodes has also been described (21–27). It is a safe and effective treatment, with the failure rate for a single radiofrequency treatment being 7.5% (26). This rate is comparable to that of ILA in our series. Radiofrequency ablation has also been used for treatment of osteoid osteoma in the spine (23,25).
There are many advantages to ILA of osteoid osteoma, as follows: (a) a 400-µm optical fiber can be introduced through an 18-gauge spinal needle, (b) a predictable size of necrosis is produced in proportion to the energy delivered when compared with that at alcohol injection, (c) there is no need to use a neutral electrode and no current in the patient body when compared with that at radiofrequency ablation, (d) there is no interaction with stimulators and pacemakers and metallic structures when compared with that at radiofrequency ablation, (e) the lower price of disposable optical fiber, (f) the laser acts as a point heat source causing direct coagulation and destruction of tissue from the center to the periphery, (g) the instrument is more MR imaging compatible when compared with that at radiofrequency ablation, (h) single-use fibers reduce the risk of contamination, (i) infrared lasers are already available in many hospitals, (j) recovery is on an outpatient basis or requires only 1 night of hospitalization, (k) there is a quick postprocedural recovery and lack of substantial complications, and (l) recurrences can be treated easily and effectively with follow-up ILA.
Overall, ILA is more economical when compared with the costs incurred during the several days of hospitalization usually required after surgery. ILA has less associated morbidity than surgical resection or percutaneous resection and has maximum work interruption of 2 weeks and normal activity interruption of 1 week.
At surgical resection or radiofrequency ablation, confirmation of osteoid osteoma is obtained in 57%–79% of cases when biopsy is performed (12,14,18,22). We do not routinely perform biopsy for histologic confirmation because the small 18-gauge spinal needle does not allow sufficient material to be obtained to confirm the diagnosis. In our series, however, 88 biopsies were performed and the diagnosis of osteoid osteoma was confirmed in 76% of cases. These results are compatible with those of previous studies (4,7,8,12).
Our study had two limitations. First, we did not have routine histologic confirmation. Second, there was a need for insertion of two to three fibers for lesions longer than 15 mm in diameter to achieve complete ablation.
To date, we are aware of other studies of ILA for the treatment of osteoid osteoma (32,33); however, to our knowledge, this study is the largest series of ILA of osteoid osteoma, with 114 patients and a mean follow-up of 58.5 months. Percutaneous ILA is a safe and effective treatment of osteoid osteoma.
We propose that all osteoid osteomas be treated with thermal, radiofrequency, or laser ablation, rather than with surgical or percutaneous resection. We believe surgical resection should be performed only in rare cases of percutaneously inaccessible osteoid osteoma.
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FOOTNOTES |
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Abbreviations: ILA = interstitial laser ablation • STIR = short inversion time inversion recovery • VAS = visual analog scale
Author contributions: Guarantors of integrity of entire study, A.G., H.A., J.D., C.R.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, A.G., H.A., L.W., J.D.; clinical studies, A.G., X.B., J.D., C.R.; and manuscript editing, A.G., H.A., L.W., C.R.
Authors stated no financial relationship to disclose.
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