Diagnostic and Treatment Strategies for Adult Non-Ossifying Fibromas: A Case Report and Literature Review

Introduction

NOF is the most common benign bone tumor in children, with a reported prevalence of 30–40% in the general pediatric population and up to 50% in adolescents [1,2]. While typically regarded as a developmental defect rather than a true neoplasm, NOF is usually asymptomatic and discovered incidentally in patients aged 5–15 years. Adult presentation is uncommon (accounting for <5% of cases), often associated with neurofibromatosis type 1 (NF1) or persistence of childhood lesions into skeletal maturity [3,4]. Imaging can be used for initial diagnosis, typically showing irregular cortical defects that can later develop into sclerotic margins and eventually be replaced by bone. Primary care physicians need to have a better understanding of the imaging features of NOF [5]. Some patients with challenging diagnoses require pathological examination, and a study found that the tumor is composed of primitive mesenchymal cells that are partially mature, indicating it is classified as benign osteogenous lipoblastoma [6]. Surgical resection is both a precise diagnostic method and a treatment option [3]. Smaller non-ossifying fibromas are usually asymptomatic and do not require treatment, but pathological fractures often accompany larger tumors and can be treated conservatively or surgically, as appropriate [7].

Here, we report a case of a 28-year-old man who underwent imaging examination due to a mass found in the proximal part of the right tibia. The imaging revealed a mass lesion, but it could not accurately diagnose it, and he came to our hospital for treatment.

Case Report

We present the case of a 28-year-old man who noticed a mass proximal to the right tibia 10 days ago, about 3×4 cm in size to the naked eye, no tenderness, no radiation, no signs of redness, swelling, numbness, varicose veins, or changes in skin temperature, limited range of motion, relatively firm texture, regular margins (Figure 1), multiple light brown spots of varying sizes and several soft flesh-colored masses visible on the torso, not painful on palpation, good mobility, and regular margins (Figure 2).

He had a history of neurofibroma, but had not received treatment for it. To evaluate a potential mass near the right tibia, he underwent a computed tomography (CT) scan of the right tibia and fibula at another hospital. The results indicated that the giant cell tumor of the bone could not be distinguished from an aneurysmal bone cyst. Combined with the patient’s prior medical history and physical examination, the attending physician suspected the possibility of a non-ossifying fibroma and proceeded with anteroposterior and lateral digital radiographs (DRs) of the knee (Figure 2A, 2B). In addition, non-contrast, diffusion-weighted imaging (DWI) and contrast-enhanced magnetic resonance imaging (MRI) were performed on the proximal right tibia (Figure 2C–2E). Imaging results suggested fibroma ossification.

Differential diagnosis is required in combination with imaging, as shown in Table 1.

Sclerotic Rim on X-ray/MRI ↔ Pathological Fibrous Encapsulation: The well-defined sclerotic margin seen on radiographs (Figure 2A, 2B) and low-signal rim on MRI corresponded histologically to dense fibrous tissue with peripheral reactive bone formation, representing the body’s attempt to contain the slow-growing benign process. This sclerotic reaction is characteristic of NOF and contrasts with the ill-defined, infiltrative margins of malignant tumors. Radiolucent Lesion ↔ Cellular Fibrous Tissue: The central radiolucency on X-ray matched the hypercellular spindle cell proliferation with minimal mineralized matrix. The septations observed on MRI represented fibrous trabeculae dividing the lesion into lobules. Cortical Breach on CT/MRI ↔ Cortical Thinning/Remodeling: The apparent “cortical breach” on imaging was confirmed histologically to be cortical thinning with fibrous replacement rather than destructive cortical invasion; the outer periosteum remained intact without periosteal reaction (a feature that would suggest malignancy). This corresponds to the eccentric, expansile growth pattern typical of a non-neoplastic soft tissue mass. Despite the large size, the lesion remained intracortical with minimal soft tissue extension, histologically corresponding to contained fibrous tissue without infiltrative growth, unlike the aggressive soft tissue invasion seen in sarcomas.

Although the imaging features (eccentric metaphyseal location, well-defined sclerotic rim, and lobulated contour) were suggestive of NOF, histological confirmation was mandatory for 3 specific reasons. First, the patient’s age (28 years, skeletal maturity) is atypical for NOF, which typically undergoes spontaneous involution after puberty; persistent or enlarging NOF in adults raises concern for alternative diagnoses, particularly giant cell tumor or low-grade malignancy. Second, the lesion demonstrated cortical breach with soft tissue extension (Figure 2E), an uncommon feature in classic NOF that typically remains confined to the cortex; this finding necessitated exclusion of desmoplastic fibroma or low-grade osteosarcoma, which can mimic NOF radiographically but requires distinct oncological management. Third, given the planned surgical intervention (extended curettage and internal fixation), a preoperative tissue diagnosis was essential to avoid inappropriate intra-lesional treatment of a potential malignancy, which could result in tumor dissemination and compromised oncological outcomes. Therefore, ultrasound-guided core-needle biopsy was performed to establish a definitive diagnosis prior to definitive treatment.

The pathological results confirmed the diagnosis of non-ossifying fibromas. Pathological tissue storiform arrangement, with the presence of foam cells and hemosiderin. The combination of cytologically flat, pleomorphic pre-lamellar structures, lack of high-risk histological features (necrosis, vascular invasion, invasive boundaries), and negative immune markers of sarcoma (H3.3G34W, H3K36M, IMP3) definitively excluded low-grade osteosarcoma, giant cell tumor of bone, fibrous dysplasia, and aneurysmal bone cyst (Figure 3). Due to the large size of the tumor (36×37×53 mm), which had broken through the bone cortex and could lead to pathological fractures, we decided to perform surgical treatment.

Given the patient’s age (28 years) and activity demands, prolonged protected weight-bearing (required for 3–6 months if autografting were used) or prolonged observation with activity restriction would result in unacceptable functional limitations and cost.

Formal risk stratification was conducted to determine the appropriateness of operative intervention. The lesion measured 36×37×53 mm in 3 planes, occupying >50% of the tibial diameter at the metaphyseal level. According to established criteria (Cai et al [7], 2024; Herget et al [8], 2016), NOF lesions exceeding 33–35 mm in length or involving >33–50% of the cortical thickness carry a significant risk of pathological fracture, particularly in weight-bearing bones. In this case, 3 high-risk features were identified:

The calculated fracture risk score (based on lesion size relative to bone diameter and cortical integrity) indicated a >50% probability of pathological fracture with continued weight-bearing, requiring prophylactic stabilization rather than observation.

The surgical focus was on the tumor, with an arched incision made along the anterolateral side of the proximal tibia, and once sufficient exposure is achieved, the tumor was carefully excised at the border of the tumor and surrounding pathological bone. The resulting defect was then filled with bone cement and fixed using plates and screws of appropriate size (Figure 4). After the surgical procedure, all tumors underwent pathological biopsy, and the results confirmed a diagnosis of non-ossifying fibroma (Figure 5). One week after the operation, follow-up digital radiography (DR) of the right tibia and fibula (Figure 6) demonstrated proper positioning of the bone plate and complete filling of the defect with bone cement. The patient was discharged without complications. Two months later, during a follow-up evaluation, he reported no pain in the right lower limb and exhibited a normal range of motion. A subsequent right tibiofibular DR (Figure 7) showed adequate internal fixation and no signs of tumor recurrence.

Discussion

To contextualize this case, we systematically compared our experience with reported cases of large NOF (>3 cm) and adult-onset presentations (Table 2).

While non-ossifying fibroma (NOF) is consistently reported as the most common benign bone lesion in children (prevalence 30–40% in the general pediatric population, up to 50% in adolescents) [1,2], adult presentation is distinctly uncommon. The persistence of NOF into adulthood, as seen in this 28-year-old patient, is well-documented but rare, occurring in approximately <5% of cases, typically in the context of neurofibromatosis type 1 (NF1) or Jaffe-Campanacci syndrome [4,9]. We emphasize that this case represents an atypical demographic, and the diagnostic approach must differ from pediatric cases, where characteristic imaging often suffices for diagnosis.

NOFs demonstrate a characteristic metaphyseal-to-diaphyseal migration as the skeleton matures [10]. Originating in the metaphysis adjacent to the physis (growth plate), the lesion appears to “move” away from the joint line toward the shaft due to longitudinal bone growth – a phenomenon distinct from true lesion migration. This explains why pediatric NOF naturally becomes diaphyseal and eventually obliterates with remodeling, whereas in our adult patient, the lesion remained metaphyseal-proximal due to growth plate closure, creating the radiographic illusion of an aggressive, expanding mass. This distinction is critical: epiphyseal lesions (GCTB) grow toward the joint surface, while NOF migrates away from it; recognizing this directional growth pattern prevents misdiagnosis.

Unlike pediatric NOF, which is typically asymptomatic and discovered incidentally or via pathological fracture during play/trauma [8], adult NOF presents distinct challenges. Our 28-year-old patient exemplifies the adult phenotype: (1) persistent growth beyond skeletal maturity (documented increase from 36 mm to 55 mm), (2) predominantly painless mass rather than acute fracture, and (3) cortical breach without significant trauma. This case underscores that while NOF is nearly universal in the pediatric population (as fibrous cortical defects), adult presentation is rare but clinically significant, particularly in patients with neurofibromatosis type 1 (NF1). Gill et al [4] and others have documented that NOF in adults with NF1 often presents as larger, more aggressive-appearing lesions that fail to undergo the spontaneous involution seen in childhood. The prevalence of NOF in NF1 patients is substantially higher than in the general population, and these persistent lesions may require surgical intervention due to continued growth and fracture risk in skeletally mature bone [11]. Crucially, the diagnostic criteria applied to children (characteristic metaphyseal location, cortical-based lucency) may be insufficient in adults. This case demonstrates that in patients over 25 years of age, biopsy is mandatory to exclude GCTB and low-grade sarcoma, as the “classic” NOF presentation in an adult is a diagnostic outlier that mimics more aggressive pathology.

The decision to operate on NOF hinges on fracture risk stratification. Herget et al [8] established that lesions >33–35 mm in length involving >50% of cortical diameter carry significant fracture risk – a threshold our patient exceeded (55 mm, >50% diameter). However, treatment strategies diverge based on patient age and lesion biology. In pediatric patients, Cai et al [7] demonstrated successful healing with curettage and autologous bone grafting, leveraging the regenerative capacity of immature bone. In adult patients, our case and Gill et al [4] show that prophylactic stabilization is preferred in mature bone due to limited remodeling potential. We selected PMMA cement with plate fixation based on the rationale that (1) the defect volume (36×37×53 mm) exceeded safe autograft harvest limits (~30 cm3 from iliac crest), (2) the patient required immediate mechanical strength for early weight-bearing, and (3) mature bone lacks the osteogenic potential for rapid integration of cancellous grafts [12].

Traditional management of large NOF involved curettage with autologous bone grafting or allograft [7]. However, for adult patients with large defects, structural stability takes precedence over biological remodeling. Parwaz et al [12] introduced vascularized fibular graft for femoral neck NOF, addressing both mechanical and biological needs in a high-stress location. Our approach with PMMA bone cement and rigid plate fixation is a pragmatic alternative for the tibial metaphysis, where cement provides compressive strength (~80 MPa) equivalent to cortical bone, allowing immediate full weight-bearing [12] and plate fixation protects against stress risers at the cement-bone interface. This avoids donor site morbidity and the prolonged protected weight-bearing required for autograft incorporation (3–6 months). This strategy aligns with the treatment goals in skeletally mature patients: functional restoration over biological reconstruction.

Long-term outcomes in large NOF are generally favorable, with recurrence rates <10% after adequate curettage [13,14]. However, adult cases require distinct follow-up parameters. While pediatric NOF monitoring focuses on spontaneous regression via serial radiographs, adult postoperative surveillance must confirm the mechanical integrity of the reconstruction and the absence of cement loosening or implant failure [1]. Our 2-month follow-up demonstrated stable fixation without recurrence, consistent with reports by Bouaicha et al [9] in aggressive adult NOF cases. Notably, unlike malignant lesions, NOF requires no adjuvant therapy, and the risk of malignant transformation is negligible, distinguishing it from fibrous dysplasia or other fibro-osseous lesions. This case contributes to the limited but growing literature on adult NOF management by demonstrating that: (1) Size thresholds for surgical intervention (>33 mm) established in pediatric populations are equally applicable to adults, but the treatment goal shifts from biological healing to immediate mechanical stability; (2) NF1-associated NOF in adults is a distinct clinical entity requiring proactive intervention rather than observation; and (3) PMMA cement with rigid fixation offers a rational alternative to autografting in large metaphyseal defects of mature bone. Future comparative studies should specifically evaluate functional outcomes between biological (graft) and synthetic (cement) reconstruction methods in this rare adult population.

We acknowledge that this case is an atypical presentation of a common childhood lesion. NOF is the most frequently encountered benign bone lesion in children aged 5–15 years, but adult persistence – as seen here – is rare (<5% of cases) and typically associated with NF1. The metaphyseal location (not epiphyseal) is the hallmark anatomical feature, although growth-related migration can create confusion with epiphyseal tumors in adults. These epidemiological and anatomical nuances are critical for accurate diagnosis and treatment planning.

Conclusions

This case challenges the conventional perception of non-ossifying fibroma (NOF) as exclusively a childhood lesion by demonstrating that adult persistence of NOF, particularly in neurofibromatosis type 1 (NF1) patients, requires a fundamentally different diagnostic and therapeutic approach. Our findings establish 3 evidence-based principles for managing large NOF in skeletally mature patients. (1) In patients >25 years presenting with metaphyseal lytic lesions, NOF must be actively distinguished from giant cell tumor of bone and low-grade sarcoma through mandatory biopsy, even when imaging appears characteristic. Adult NOF lacks the predictable natural history of pediatric lesions and can mimic aggressive neoplasms. (2) Lesions exceeding 33 mm in length or involving >50% of cortical diameter in weight-bearing bones warrant prophylactic surgical intervention regardless of symptoms. In mature bone, observation is contraindicated due to the absence of spontaneous involution potential and high pathological fracture risk. (3) For large defects (>30 cm3) in adult long bones, polymethylmethacrylate (PMMA) cement augmentation with rigid plate fixation offers biomechanical advantages over autologous grafting, providing immediate compressive strength (≈80 MPa) that facilitates early weight-bearing and functional restoration.