Aneurysmal bone cyst

An aneurysmal bone cyst (ABC) is a benign, non-malignant, tumor-like vascular lesion of bone characterized by multiple blood-filled cystic spaces separated by fibrous septa containing osteoid tissue and osteoclast-like giant cells, which can expand rapidly, cause local bone destruction, and lead to pathologic fractures.^[1] These lesions are rare, accounting for 1% to 2% of all primary bone tumors, with an incidence of approximately 1 in 100,000 individuals annually, and they predominantly affect children and adolescents under 20 years of age, showing a slight female predominance (female-to-male ratio of 1:1 to 1.3:1).^[2][1] ABCs most commonly arise in the metaphysis of long bones such as the femur and tibia (about 67% of cases), followed by the spine (15%) and pelvis (9%), though they can occur in any bone.^[1] The etiology remains unclear, but evidence suggests they may develop as primary lesions due to genetic abnormalities like the t(16;17)(q22;p13) translocation involving the USP6 gene in up to 69% of cases, or as secondary reactive processes to trauma, other benign bone tumors (e.g., chondroblastoma or giant cell tumor), or vascular malformations.^[1] Clinically, patients often present with insidious onset of localized pain over weeks to months, swelling, or deformity; acute severe pain may signal a pathologic fracture, while spinal involvement can cause neurologic symptoms such as radiculopathy or myelopathy due to compression.^[3][2] Diagnosis typically involves radiographic imaging, where ABCs appear as expansile, lytic lesions with thinned "eggshell" cortical borders and internal septations; advanced modalities like MRI reveal characteristic fluid-fluid levels from blood sedimentation, aiding differentiation from malignant mimics such as telangiectatic osteosarcoma.^[1] Histopathologic confirmation via biopsy is often required, showing cavernous blood-filled spaces without endothelial lining and the presence of spindle cells, multinucleated giant cells, and reactive bone formation.^[2] Treatment is primarily surgical, with curettage and bone grafting as the standard approach to decompress the lesion and restore stability, though adjuncts like phenol or cryotherapy reduce recurrence rates (which approach 25%); alternatives include percutaneous sclerotherapy, selective arterial embolization, or denosumab for unresectable cases, particularly in the spine.^[3][1]

Overview

Definition

An aneurysmal bone cyst (ABC) is a benign, non-malignant, tumor-like vascular lesion of bone characterized by expansile, lytic destruction and the formation of blood-filled cystic spaces separated by fibrous septa.^[1][4] Histologically, ABCs consist of numerous blood-filled channels and cystic cavities lined by fibrous tissue, often featuring osteoclastic giant cells, fibroblasts, and areas of reactive woven bone formation within the septa.^[5][4] The cystic spaces may contain blood, hemosiderin, and fibrinoid material, contributing to the lesion's characteristic hemorrhagic appearance.^[1] Despite their benign histology, ABCs demonstrate aggressive local growth that can weaken the surrounding bone structure, potentially leading to pathologic fractures, though they possess no metastatic potential.^[2]

Classification

Aneurysmal bone cysts (ABCs) are classified into primary and secondary types based on their origin and association with underlying bone pathology, which influences diagnostic approaches and management strategies. Primary ABCs arise de novo without an associated preexisting lesion and account for approximately 70% of cases.^[6] These lesions are considered true neoplasms and typically exhibit characteristic genetic alterations, aiding in their distinction from other entities during histopathological evaluation. In contrast, secondary ABCs develop in conjunction with other primary bone tumors or conditions, comprising about 30% of cases, and represent reactive cystic changes within the preexisting pathology.^[7] Common associations include giant cell tumor of bone, chondroblastoma, fibrous dysplasia, osteoblastoma, and occasionally post-traumatic or post-radiation changes, which can complicate diagnosis as the cystic components may obscure the underlying lesion on imaging.^[8] The classification carries prognostic implications, as primary ABCs are uniformly benign with a focus on local control, while secondary ABCs' outcomes depend on the nature of the associated tumor, potentially requiring more aggressive intervention if the primary lesion is malignant or recurrent.^[1] Both types share a benign expansile nature but can demonstrate locally aggressive behavior, such as cortical thinning and soft tissue extension, which may mimic more aggressive processes on radiographs.^[9] Accurate subclassification often relies on correlation between clinical history, imaging features like fluid-fluid levels on MRI, and biopsy confirmation to guide treatment and predict recurrence risk, which is higher in aggressive presentations.^[4][10] Rare variants of ABC include the solid variant, which is less cystic and more fibrous in composition, comprising 3-8% of cases and often lacking the typical blood-filled spaces.^[11][12] This subtype can present with aggressive radiographic features, such as rapid growth and bone destruction, frequently mimicking malignancy like telangiectatic osteosarcoma or giant cell tumor, leading to potential misdiagnosis without histopathological review.^[13] Additionally, some ABCs exhibit unusually aggressive behavior regardless of variant, with extensive periosteal reaction or soft tissue invasion, further emphasizing the need for multidisciplinary evaluation to differentiate from sarcomas.^[14][15]

Epidemiology

Incidence and Prevalence

Aneurysmal bone cysts (ABCs) are rare benign bone tumors, accounting for approximately 1% to 6% of all primary bone tumors.^[1] This range reflects their infrequent occurrence within the spectrum of osseous neoplasms, with some studies reporting a narrower proportion of 1.4% to 2.3%.^[9] The annual incidence of primary ABCs is estimated at about 0.14 per 100,000 individuals, based on population-based epidemiologic analyses.^[16] In pediatric populations, ABCs represent roughly 1% to 2% of biopsied primary bone neoplasms, underscoring their relative rarity even among younger patients where the condition predominantly arises.^[17]

Demographics

Aneurysmal bone cysts primarily affect children and adolescents, with approximately 80% of cases diagnosed before the age of 20 years and the peak incidence occurring in the 5- to 15-year age group.^[9][18] The condition is rare in adults, with most occurrences limited to individuals under 30 years old.^[19] A slight female predominance is observed, with reported female-to-male ratios ranging from 1:1 to 1.3:1 across various studies.^[16][18] No strong ethnic or geographic predispositions have been identified in the literature on aneurysmal bone cysts.^[19][20]

Clinical Presentation

Signs and Symptoms

Aneurysmal bone cysts typically present with an insidious onset of localized pain that develops over weeks to months, often worsening at night or with physical activity.^[1][21] This pain is the most common symptom, reported in approximately 80% of cases, and is usually confined to the affected bone.^[22] Swelling or a palpable mass over the site is also frequent, occurring in about 37% of patients, reflecting the expansile nature of the lesion.^[22][1] Pathologic fractures arise due to progressive bone weakening and represent the initial presentation in 10-20% of cases, often manifesting as sudden, severe pain following minor trauma or even spontaneously.^[22][2] These fractures are more likely in long tubular bones where the cyst's cystic spaces erode the cortex, leading to structural compromise.^[1] When the spine is involved, patients may experience neurological symptoms such as radiculopathy, weakness, or paraparesis due to compression of the spinal cord or nerve roots.^[21][2] In cases affecting long bones, joint dysfunction, stiffness, or deformity can develop from the lesion's expansion, limiting range of motion in the adjacent joint.^[2]

Anatomic Sites

Aneurysmal bone cysts (ABCs) exhibit a broad distribution across the skeleton but preferentially affect certain anatomic sites, with approximately 67% occurring in the metaphysis of long bones.^[1] The most frequently involved long bones are the femur, tibia, and humerus, where lesions typically arise in the metaphyseal region and can extend into the diaphysis.^[4] These cysts are characteristically eccentric and cortical-based, often expanding the bone cortex and extending into adjacent soft tissues, which contributes to their expansile nature.^[9] Involvement of the spine accounts for 15-20% of cases, predominantly affecting the posterior elements such as the vertebral arches and spinous processes, though the vertebral bodies may also be involved in some instances.^[1] The pelvis is affected in about 9% of cases, typically involving the ilium, ischium, or pubis.^[1] ABCs in flat bones, such as the ribs, clavicle, or mandible, occur less commonly, representing a smaller proportion of overall cases.^[4] Lesions in the small bones of the hands and feet are rare, comprising fewer than 5% of reported instances, and often present unique diagnostic challenges due to their atypical location.^[9]

Etiology and Pathogenesis

Causes

The exact cause of aneurysmal bone cysts (ABCs) remains unknown, though they are widely hypothesized to represent a reactive process arising from intraosseous vascular malformations or hemorrhage that leads to cystic expansion.^[5][20] Hemodynamic disturbances are implicated in this pathogenesis, with proposed mechanisms including arteriovenous shunting or venous blockage that elevates intraosseous pressure, thereby promoting bone erosion and resorption through the formation of blood-filled cavities.^[5] A history of preceding trauma or fracture is reported in some cases, potentially serving as a trigger for initial hemorrhage and subsequent lesion development. Secondary ABCs account for 23-32% of all cases and arise in association with underlying primary bone tumors, such as chondroblastoma or giant cell tumor, or other conditions including fibrous dysplasia, where the ABC-like changes represent a reactive component to the primary pathology.^[5][23] In contrast, primary ABCs are characterized by specific genetic rearrangements that support their neoplastic nature, though non-genetic factors like those described above may contribute to their initiation.^[20]

Genetic Factors

Aneurysmal bone cysts (ABCs) are classified into primary and secondary forms based on their genetic profiles, with primary ABCs characterized by specific chromosomal rearrangements involving the USP6 gene. The most common alteration is the t(16;17)(q22;p13) translocation, which fuses the promoter region of the cadherin 11 (CDH11) gene on chromosome 16q22 to the entire coding sequence of the ubiquitin-specific protease 6 (USP6, also known as TRE17) gene on chromosome 17p13, resulting in USP6 oncogene upregulation through promoter swapping.^[24] This fusion drives neoplastic transformation in primary ABCs by enhancing USP6 transcription, a mechanism first identified in seminal studies on ABC cytogenetics.^[24] USP6 rearrangements occur in approximately 69% of primary ABCs, as demonstrated by fluorescence in situ hybridization (FISH) analysis targeting USP6 and CDH11 loci, while variant fusions with other partners (e.g., COL1A1, ZNF9) account for additional cases.^[25] Overexpression of USP6 promotes osteolysis, the hallmark bone destruction in ABCs, by disrupting osteoblastic maturation through autocrine dysregulation of bone morphogenetic proteins (BMPs) and enhancing osteoclast activity via stimulation of matrix metalloproteinase production and inflammatory signaling pathways.^[26] These molecular effects underscore the neoplastic nature of primary ABCs, distinguishing them from reactive processes.^[27] In contrast, secondary ABCs—those arising in association with other bone tumors such as giant cell tumor or chondroblastoma—typically lack USP6 rearrangements, though they may exhibit genetic alterations inherited from the underlying primary neoplasm.^[25] This absence supports the view that secondary ABCs represent reactive cystic changes rather than independent neoplasms.^[27] The detection of USP6 rearrangements holds significant diagnostic utility, as FISH or polymerase chain reaction (PCR)-based assays can confirm primary ABC and differentiate it from mimics like telangiectatic osteosarcoma, which may show overlapping radiographic and histologic features but lack USP6 fusions.^[28][29] Such molecular testing is particularly valuable in challenging cases, enabling precise classification and guiding management.^[30]

Histopathology

Aneurysmal bone cysts (ABCs) are characterized grossly by a well-demarcated, expansile, spongy, and hemorrhagic lesion composed of multiloculated cystic spaces filled with blood and friable tissue. These spaces range in size from less than 1 mm to several centimeters and are separated by thin, fibrous septa, often surrounded by a delicate "eggshell" shell of reactive bone. The lesion may exhibit irregular borders and variable amounts of solid components, particularly in certain anatomic sites like the digits, with overall dimensions varying based on location and duration. Hemosiderin deposits from chronic hemorrhage contribute to the dark, mottled appearance of the cut surface.^[4][20] Microscopically, ABCs feature blood-filled cystic cavities lacking an endothelial or epithelial lining, instead separated by cellular fibrous septa composed of bland fibroblasts, osteoclast-like multinucleated giant cells, and reactive woven bone trabeculae. The septa often contain areas of hemorrhage, hemosiderin-laden macrophages, and mixed inflammatory cells, with clusters of giant cells concentrated near the cystic spaces. Mitotic figures are common in the stromal cells but lack atypia, and necrosis is rare. A distinctive feature is the presence of calcified, basophilic, reticulated chondroid-like material within the septa. In the solid variant of ABC, cystic spaces are reduced, with a predominance of spindle-shaped stromal cells and more prominent fibro-osseous tissue, though giant cells and reactive bone persist.^[4][20][31] Immunohistochemically, the multinucleated giant cells express CD68, consistent with their osteoclast-like nature, while the cystic walls are negative for endothelial markers such as CD31 and CD34, confirming the absence of a true vascular lining. Stromal cells may show positivity for RANKL, and giant cells for RANK, supporting the lesion's reactive osteoclastic activity, though no specific panel is diagnostic for ABC. Genetic confirmation can be achieved through detection of USP6 gene rearrangements in approximately 65% of cases via fluorescence in situ hybridization.^[4][31][32]

Diagnosis

Imaging Characteristics

Aneurysmal bone cysts (ABCs) typically present on plain radiographs as eccentric, expansile lytic lesions located in the metaphysis of long bones, with a characteristic "blown-out" appearance due to cortical thinning and a thinned sclerotic rim.^[33] Internal septations may be visible, imparting a multiloculated or soap-bubble appearance, while aggressive lesions can show periosteal reaction or pathologic fractures.^[34] These features reflect the lesion's vascular and cystic nature, often without matrix mineralization unless secondary changes are present.^[35] Magnetic resonance imaging (MRI) is the modality of choice for characterizing ABCs, revealing multiloculated cystic spaces with fluid-fluid levels due to sedimentation of blood products, a characteristic finding seen in a majority of cases and highly suggestive of the diagnosis.^[33] On T1-weighted images, the lesion shows variable signal intensity from hemorrhage, while T2-weighted sequences demonstrate hyperintense cystic components with a hypointense rim; post-contrast enhancement occurs in the thin septa and nodular solid areas, aiding in assessing lesion extent and soft-tissue involvement.^[34] Surrounding bone marrow and soft-tissue edema may also be evident, indicating reactive changes.^[35] Computed tomography (CT) complements radiography and MRI by better delineating cortical thinning, internal fluid levels, and subtle matrix mineralization in expansile lesions, particularly in complex sites like the spine or pelvis.^[33] It highlights the geographic, lytic nature with a well-defined sclerotic margin and can identify periosteal reactions or fractures not fully appreciated on plain films.^[34] No significant soft-tissue mass is typically seen unless associated with fracture.^[35] Bone scintigraphy demonstrates increased radiotracer uptake in ABCs, reflecting active bone turnover, hyperemia, and osteoblastic activity, often with a peripheral "doughnut" pattern of intense rim uptake and central photopenia corresponding to the cystic spaces.^[34] This nonspecific finding helps assess lesion activity but is less diagnostic than MRI.^[35] While imaging strongly suggests ABC, histological confirmation via biopsy is generally required for definitive diagnosis.^[33]

Biopsy Findings

Biopsy is indicated for the definitive diagnosis of aneurysmal bone cyst (ABC), particularly to exclude malignant mimics such as telangiectatic osteosarcoma, when imaging findings are suggestive but not conclusive.^[1][36] Percutaneous image-guided core needle biopsy is the preferred initial method due to its minimally invasive nature, high diagnostic yield, and safety for most primary bone tumors, including ABC; open biopsy with frozen section analysis may be employed if the needle biopsy is nondiagnostic or yields insufficient tissue.^[37][36] Histological examination of biopsy specimens reveals characteristic blood-filled cystic cavities separated by fibrous septa, lacking an endothelial lining and thereby confirming the lesion as a non-vascular malformation rather than a true aneurysm or hemangioma.^[1][36] The septa typically contain fibroblasts, spindle cells, multinucleated giant cells, and areas of reactive osteoid formation, with mitotic activity present but no atypical cells or significant pleomorphism.^[1][36] Due to the highly vascular nature of ABC, biopsy carries a risk of significant hemorrhage, which can complicate the procedure and potentially affect limb salvage.^[36][38] Preoperative selective arterial embolization may be performed to reduce intraoperative bleeding and facilitate safer tissue sampling.^[39][38] Molecular testing for USP6 gene rearrangements on biopsy tissue is integrated into the diagnostic workup to distinguish primary ABC, which harbors these alterations in approximately 65-70% of cases, from secondary ABC or other cystic lesions lacking such clonal changes.^[1][36][40]

Differential Diagnosis

The differential diagnosis of aneurysmal bone cyst (ABC) includes several benign and malignant bone lesions that may present with expansile lytic changes on plain radiography.^[8] Distinguishing ABC from these mimics relies on imaging features, such as the presence of fluid-fluid levels on MRI, which are characteristic of ABC due to its blood-filled cystic spaces, and genetic testing for USP6 gene rearrangements, found in approximately 70% of primary ABC cases but absent in most differentials.^[9][41] Giant cell tumor of bone (GCTB) is a key benign mimic, typically occurring in the epiphysis of long bones in adults aged 20-40 years, presenting as a more solid, eccentric lytic lesion without the multiloculated cystic spaces or fluid-fluid levels seen in ABC on MRI.^[9] Unlike ABC, GCTB harbors the H3-3A gene mutation (H3.3 pGly34Trp), which can be confirmed via immunohistochemistry to differentiate the two.^[9] Telangiectatic osteosarcoma (TOS), a rare malignant variant of osteosarcoma, closely resembles ABC clinically and radiographically, affecting similar age groups (adolescence) and sites (long bones), with both showing aggressive, expansile lytic destruction and cystic components.^[42] However, TOS exhibits permeative bone margins, soft tissue extension, and lacks USP6 rearrangements; definitive distinction requires biopsy revealing anaplastic sarcomatous cells and osteoid production, absent in ABC.^[4][42] Unicameral bone cyst (UBC), also known as simple bone cyst, is a benign lesion primarily in the metaphysis or diaphysis of long bones in children, appearing as a non-expansile, unilocular lytic defect with thin walls and serous fluid content, lacking the internal septations, blood-filled cavities, and fluid-fluid levels of ABC on cross-sectional imaging.^[43] UBC typically does not cause cortical thinning or pathologic fractures as aggressively as ABC.^[44] Fibrous dysplasia (FD) may mimic ABC in its monostotic form, presenting as an expansile lesion with ground-glass matrix opacity on radiography due to irregular bony trabeculae in a fibrous stroma, contrasting with the purely cystic, radiolucent appearance of ABC.^[27] FD often affects a broader age range and lacks USP6 fusions, though secondary ABC changes can occur in up to 10% of FD cases, requiring histologic correlation to identify the fibro-osseous component.^[27][41] Chondroblastoma, a rare benign cartilage tumor predominantly in the epiphysis of long bones in adolescents, can resemble ABC when secondary aneurysmal changes develop (in 10-20% of cases), but primary chondroblastoma features chondroid matrix and calcifications on imaging, with histologic evidence of chondroblasts, distinguishing it from the vascular, spindle-cell stroma of ABC.^[1][23]

Management

Surgical Treatments

Surgical treatments for aneurysmal bone cysts (ABCs) primarily involve operative interventions aimed at removing the lesion while preserving bone function, with intralesional curettage serving as the cornerstone approach for most cases.^[45] This method is preferred due to its relative preservation of surrounding bone stock compared to more radical procedures.^[46] Adjuvant therapies are commonly incorporated to enhance local control by targeting residual tumor cells.^[45] Intralesional curettage involves scraping out the cystic contents and fibrous septa under direct visualization, often using high-speed burrs to extend the margins into healthy bone.^[45] Adjuvants such as phenol for chemical cauterization, cryotherapy with liquid nitrogen to induce cellular necrosis, or polymethylmethacrylate (PMMA) cement for thermal ablation and structural support are applied to reduce the risk of local recurrence, particularly in aggressive lesions.^[46][45] This technique is suitable for accessible sites like long bones and is the standard for initial management.^[47] Following curettage, the resulting bone defect is typically filled with autogenous bone grafts from the iliac crest or rib, allografts, or synthetic materials like PMMA to restore structural integrity and promote healing, especially in weight-bearing areas.^[47] These fillers help prevent pathological fractures and support early mobilization.^[45] For highly aggressive, recurrent, or spinal ABCs where curettage margins are inadequate, en bloc resection provides complete excision of the lesion with a margin of normal bone, often requiring spinal fusion and instrumentation for stability.^[47] This approach is technically demanding and reserved for cases where preservation of function is feasible, such as select mobile spine locations.^[47] Preoperative selective arterial embolization is frequently employed to devascularize the lesion, using agents like polyvinyl alcohol particles or N-2-butyl-cyanoacrylate to minimize intraoperative blood loss, particularly in hypervascular tumors of the pelvis or spine.^[47][45] It is performed 24-48 hours prior to surgery via percutaneous catheter access to feeding arteries.^[47] In young patients, these surgical strategies are essential to address the lesion's potential for rapid growth while minimizing long-term skeletal deformities.^[45]

Non-Surgical Options

Non-surgical options for aneurysmal bone cysts (ABCs) are particularly valuable in cases involving difficult surgical access, such as spinal or pelvic locations, or when preserving bone growth in pediatric patients is prioritized. These approaches aim to induce cyst obliteration, inhibit lesion progression, or monitor stable lesions without intervention, often serving as alternatives or adjuncts to surgical curettage, which remains the first-line treatment for most accessible ABCs.^[48] Percutaneous sclerotherapy involves the image-guided injection of sclerosing agents directly into the cyst to promote fibrosis and endothelial damage, leading to gradual obliteration of the cystic spaces. Commonly used agents include polidocanol, a sclerosant that achieves cyst consolidation through multiple outpatient sessions, with reported cure rates up to 97% in long-term follow-up.^[49] Doxycycline, another effective agent, is injected percutaneously to induce inflammation and sclerosis, demonstrating low recurrence rates of approximately 5% after more than 24 months in treated cases.^[50] This minimally invasive procedure typically requires 2–6 sessions spaced 4–6 weeks apart, under fluoroscopic or CT guidance, and is well-tolerated with minimal complications such as transient pain or skin irritation.^[48] Radiotherapy is reserved for inoperable ABCs, such as those in the spine or sacrum, or for recurrent lesions where surgery poses high risk. It delivers targeted ionizing radiation to arrest lesion growth and promote ossification, achieving local control rates exceeding 90% when used as an adjuvant or primary modality with doses of 26–30 Gy in fractionated regimens.^[51] However, its use is limited due to risks including secondary malignancies, such as radiation-induced sarcoma, and potential growth plate disturbances in children.^[52] Medical therapy with denosumab, a monoclonal antibody that inhibits RANKL to suppress osteoclast activity, offers a targeted option for unresectable or recurrent ABCs, particularly in axial skeletons. Administered subcutaneously at a weight-based dose (e.g., 70 mg/m² monthly after loading doses) in pediatric patients, it has shown radiographic evidence of lesion sclerosis and volume reduction within 3–6 months, with clinical improvement in pain and function.^[53][54] This approach is especially beneficial in pediatric cases where surgical morbidity is a concern; recent reports (as of 2024) also support its use for recurrent lesions via re-challenge. However, long-term data on durability remain limited, and side effects include hypocalcemia during treatment as well as severe rebound hypercalcemia upon discontinuation, necessitating careful monitoring and potential bridging with bisphosphonates.^[48][55][56] For small, asymptomatic ABCs in children, active observation with serial imaging (e.g., MRI or radiographs every 3–6 months) allows monitoring for spontaneous involution, which occurs in some cases without intervention. This conservative strategy is supported by reports of cyst healing post-biopsy alone in active lesions, avoiding unnecessary treatment in stable presentations.^[57] Progression or symptom development prompts escalation to other therapies.^[58]

Prognosis

Outcomes

Aneurysmal bone cysts (ABCs) exhibit an excellent overall prognosis following appropriate treatment, with cure rates ranging from 90% to 95% in most cases.^[5] These benign lesions carry a low mortality risk, as they do not metastasize.^[1] However, if left untreated, ABCs can lead to significant morbidity, primarily through pathologic fractures, bone deformity, and disruption of normal skeletal growth.^[59] In pediatric patients, who comprise the majority of cases, preservation of the growth plate during intervention is crucial to prevent limb length discrepancies and angular deformities.^[60] Successful outcomes are influenced by factors such as the completeness of lesion excision and whether the ABC is primary or secondary to another bone pathology.^[5] Primary ABCs generally demonstrate better prognoses compared to secondary forms, particularly in spinal locations among adults.^[61] Recent pharmacologic options, such as denosumab followed by bisphosphonates, have shown promise in improving outcomes for aggressive or recurrent cases, potentially lowering recurrence rates.^[62] Long-term expectations include restored function and minimal complications when these elements are addressed effectively.

Recurrence and Monitoring

Aneurysmal bone cysts exhibit a recurrence rate of 10-20% following initial treatment, with rates approaching 19% in cases of incomplete excision or curettage.^[1][63] Most recurrences manifest within the first year post-intervention, though delayed cases beyond two years have been documented.^[64] Several factors elevate the risk of recurrence, including incomplete curettage during surgery, patient age under 10 years, lesion size exceeding 5 cm, and involvement of the spine or pelvis.^[65][66][36] These elements contribute to higher local persistence due to the lesion's aggressive, expansile nature and anatomical challenges in certain sites.^[38] Recommended monitoring protocols emphasize serial imaging to facilitate early detection. This includes MRI or plain X-rays performed every 3-6 months for the initial two years after treatment, transitioning to annual assessments up to five years to track potential regrowth or complications.^[2][67] Recurrent lesions are generally managed through repeat surgery, such as extended curettage with adjuvants or sclerotherapy, depending on location and extent; malignant transformation remains exceedingly rare, occurring in fewer than 1% of cases.^[38][1]