Arterial embolization of primary sacral aneurysmal bone cyst

Aneurysmal bone cyst (ABC) is a rare and benign disease that demonstrates active biological behavior, can promptly expand, and exhibits an invasive or destructive capability.1 Currently, the major therapy for ABC is complete curettage of the lesion.1,2 However, the therapy for sacral ABC has risks such as severe bleeding and nerve root damage.2 Arterial embolization is used to treat large sacral giant cell tumors and ABCs and can promptly relieve pain, reduce lesion volume, and promote osteogenesis.3 The present study reviewed nine cases of primary sacral ABC treated with arterial embolization between January 2007 and March 2011, compared the clinical presentations and imaging changes after therapy, and analyzed the therapeutic effects. METHODS Patients Nine patients with primary sacral ABC underwent arterial embolization in Beijing Jishuitan Hospital between January 2007 and March 2011. All patients were examined with regular anteroposterior and lateral pelvic X-rays, computed tomography (CT), or magnetic resonance imaging (MRI) to measure the lesion range. The lesion in nine patients spanned across more than two vertebral bodies, and the diameter was more than 5 cm in at least two directions. The lesion was located within the sacrum in five patients, extruded outside the sacrum in four patients, and invaded the sacroiliac articulation (Figure 1A-1C) in two patients. A typical fluid-fluid level (Figure 1D) was found in seven patients. The diagnosis was confirmed in all patients using needle biopsy.Figure 1.: An 11-year-old female patient visited the clinic for evaluation of a 6-month history of a slight limp accompanied by sacrocoxalgia. The diagnosis was an aneurysmal bone cyst, and arterial embolization was performed repetitively for five times without recurrence during 2 years of follow-up. A-E: Plain X-rays, computed tomography (CT), magnetic resonance imaging (MRI), and angiography before embolization. A: Plain X-rays showing an expansive and osteolytic sacral lesion. B: CT showing extrasacral extrusion of the lesion into the pelvic cavity and invasion of sacroiliac articulation (arrow). C: Enhanced CT showing obvious signal enhancement. D: Axial T2 MRI showing cystic lesion and fluid-fluid level (stars). E: Angiography showing supplying arteries and an enriched blood supply within the lesion. 1. Lateral sacral arteries. 2. Middle sacral artery. 3. Iliolumbar arteries. 4. Lumbar arteries. F-H: Plain X-rays and CT 6 months after embolization. Partial calcification within and surrounding the lesion, and reduced signal enhancement on contrast enhanced CT. I-L: CT and MRI 2 years after embolization. I-J: CT and enhanced CT showing nearly complete calcification and disappearance of signal enhancement. K-L: T1WI and T2WI MRI showing the decreased lesion and fluid-fluid level (star).Arterial embolization After confirmation of the diagnosis, angiography and selective embolization were performed every 4-6 weeks in three consecutive sessions. The therapeutic aim was blockage of all supplying arteries. The arterial embolization was performed via the femoral artery pathway. This involved angiography of the distal abdominal aorta for confirmation of the arterial source (Figure 1E) and distribution of vessels surrounding and within the lesion and evaluation of the ABC blood supply (Table 1), selective intubation and embolization with 100-μm polyvinyl alcohol particles (embolized arteries included the lateral sacral arteries, lumbar arteries, iliolumbar arteries, and middle sacral arteries), avoidance of unnecessary embolization of the internal iliac artery and superior gluteal artery, and repetitive angiography for evaluation of the embolism. Subsequent angiography and embolization usually revealed lateral branches of vessels, which were embolized to avoid omission.Table 1: Clinical and imaging comparison of primary sacral aneurysmal bone cyst before and after therapyFollow-up and evaluation of therapeutic effects The effect of embolization was evaluated according to clinical presentations and imaging examinations. Pelvic X-rays, CT, or MRI were performed every 3-6 months to observe changes in the lesion size and the development of calcification surrounding or within the lesion (Figure 1F-1G). An intra-lesion calcification range of 50%-75% was considered to be partial calcification, and that greater than 75% was considered to be complete calcification.4 If clinical signs deteriorated or enlargement of the lesion and obvious enhancement were found on imaging examinations (Figure 1H), angiography and embolization were repeated. Statistical analysis Data are expressed as mean ± standard deviation. SPSS software version 17.0 (SPSS Inc., USA) was used for statistical analysis. A paired t-test was used to compare the pre-therapeutic and 6-month post-therapeutic pain scores. Pearson’s correlation coefficient was used to evaluate the correlation between the calcification at the 2-year follow-up and the blood supply and embolization times of the ABCs. A P value of less than 0.05 was considered to be statistically significant. RESULTS Four male and five female patients (age range, 11-43 years; mean (24.22±11.66) years) were included in this study. All nine patients demonstrated moderate to severe sacrocoxalgia with a palpable lump, and two patients had nerve root damage as demonstrated by hypesthesia and movement disorders. The pain assessment results using a numerical rating scale are shown in Table 1. Embolization was performed three to seven times ((4.21±1.27) times) for every patient during the follow-up period of 24-47 months ((32.44±7.18) months). Partial and complete lesion calcifications were found in five and four patients respectively and the improvements in signs lasted longer than 2 years, meeting the cure standard proposed by Rossi.4 The pain score (Table 1) six months after embolization significantly decreased from moderate/severe (6.11±1.05) to mild pain (1.33±1.00, P <0.001); medication was not required for pain control. Of the two patients with nerve root damage, one showed signs of relief after the first embolization and almost complete disappearance of the ABC after three embolizations. The other patient failed to show clear improvement of the nerve damage and still displayed obvious hypesthesia and movement disorder after 2 years. Different extents of osteogenesis surrounding and within the lesion were found on X-ray and CT among the nine patients, six months after embolization; the ABC wall was thickened, the bone crest was widened, and the cyst cavity had decreased in size and gradually filled with irregular new compact bone (Figure 1I-1L). At 2 years, partial and complete calcifications were found in five and four patients respectively. The blood supply in arterial angiography was moderate and enriched in six and three patients respectively (Table 1). There was a weak correlation between the blood supply and embolization time (r=-0.46, P=0.213) or calcification (r=0.316, P=0.407). One patient developed hip pain and paralysis after one embolization session. These complications were considered to be the results of ischemia induced by the embolization and were gradually relieved in 3 months with symptomatic treatment. DISCUSSION Primary ABC is a benign tumor-like disease that accounts for 1% of primary bone tumors and the cause remains unclear.2 Sacral ABC is very rare, accounting for <4% of sacral tumors.1 On plain X-rays, sacral ABC mainly displays cystic, osteolytic destruction, occasionally with an invasive or malignant appearance. CT and MRI can show important signs such as separation, cysts, and a fluid-fluid level. By combining plain X-rays, CT, and MRI, most cases can be diagnosed correctly.2 In general, ABC is treated using the therapeutic principles for benign tumors with the aim of complete resection of the lesion to avoid recurrence and repeat surgery. The most commonly used method is intra-lesional excision (curettage), followed by local inactivation and bone grafting. Large bone openings, high-speed drills, and adjunct inactivation can be helpful for local recurrence.2,4 Because of the specific anatomy of the sacrum, there are risks associated with curettage of large sacral ABCs, such as intraoperative bleeding, infection, nerve root damage, and, in pediatric patients, growth plate damage. The recurrence rate of sacral ABC is higher than that of limbic ABC after curettage, reaching up to 22%.5 Some researchers recently proposed that ABC is not tumorous in nature2 and that blockade of destructive lesion growth and improvement in the spontaneous formation of new bone should be the therapeutic aim.1,2 Therefore, it was proposed that after comprehensive consideration of the therapeutic effect, complications, and recurrence, minimally invasive or noninvasive techniques should be the first choice to avoid invasive curettage.4,5 Arterial embolization or embolization combined with curettage and percutaneous injection of a sclerosis agent or osteogenic inducer can promptly control pain and result in osteogenesis and lesion shrinkage.2,4-6 Selective arterial embolization is used initially to reduce signs of sacral tumors and intraoperative bleeding for smooth surgical performance; it is also used as palliative therapy for sacral tumours.3 The key to treatment success is to completely block the arterial supply to the tumors.3 In the present study, nine patients with ABCs underwent embolization and showed good control of clinical signs, with pain relief for moderate/severe to mild pain (P <0.001) and no requirement for medication. The area of ossification on imaging at 2 years was >50% in all patients and 75% in four patients. The improvements in signs and imaging results lasted more than 2 years, reaching the cure standard.4 One patient developed hip pain and hypesthesia after therapy; these complications were considered to be the result of ischemic damage induced by the embolization. This situation suggests that it is important to avoid tissue necrosis during the operation, particularly in children, although there were no serious complications involving necrosis. Because of the small sample in the present study, the correlation among the blood supply, embolism time, and extent of calcification is not accurate. Donati et al5 performed embolism therapy in four patients with sacral ABCs. They considered that embolization was ineffective in three patients who underwent one session because no effect was seen.5 In our study, two patients showed no clear improvement after the first embolization and demonstrated obvious enhancement on CT imaging. The reason for this was considered to be a missed arterial supply because of incomplete embolization, opening of lateral branches, or neovascularization. For such patients without clear effect, continuous embolization to completely block the arterial supply is necessary. According to our current experience, arterial embolization of sacral ABCs has advantages such as a definite effect, low complication rate, and good repeatability. Affected patients showing no clear effect after a single embolization can achieve satisfactory results after multiple treatments.