A 17-year-old male without previous trauma was admitted to the orthopedic outpatient clinic with the complaint of pain in the right knee. Plain radiography indicated a lobule-shaped, lytic lesion with a sclerotic rim in the proximal metaphyseal part of the tibia. Computed tomography (CT) revealed a soft tissue lesion measuring 46x17 mm at the diaphyseometaphyseal region of the tibia, causing a periosteal reaction with destruction of the cortex and forming a lytic area in the medulla. A metaphyseal tibial mass with accompanying diffuse edema was detected in the intramedullary area. Fluid interpositions and increased signals were found in the adjacent muscular structures and fascia, with an increase of reactive thickness in the periosteum on magnetic resonance imaging (MRI). The edematous findings were interpreted as secondary to the pathological fracture. The histopathology of the biopsy from this lesion was compatible with non-ossifying fibroma (NOF), but concomitant osteoblastic activity was also evident. A retrospective radiological examination was performed due to the persistence of nocturnal pain. In addition to NOF, a lytic lesion with an interior mineralized matrix was seen on CT, and a cortical-located lesion with a central hypointense area was detected on proton density-weighted MRI. An open biopsy was performed and the patient received a histological diagnosis of concurrent NOF and osteoblastoma.

NOF is the most common benign bone lesion and is usually asymptomatic. NOF is typically located in the metaphysis of long bones and can lead to pathological fractures when it is large.^1,2

In contrast, osteoblastoma is an uncommon osteogenic tumor >2 cm in size and histologically equivalent to osteoid osteoma.¹ The most common symptoms of osteoblastoma are pain, tenderness, and increased temperature in the area of the lesion.³

Co-existence of these tumors in the exact location is extremely rare and osteoblastoma can be overlooked with an accompanying pathological fracture and the inflammatory findings.⁴

To date, a few case reports have been presented in which osteoid osteoma and NOF co-existed.4 However, in this case, we present a patient with osteoblastoma that could have been overlooked due to the pathological fracture caused by NOF.

Informed Consent: Written informed consent was obtained from the patient.

Authorship Contributions: Concept- A.A., F.E.U., U.U., M.Ç., F.Ü.; Design- A.A., F.E.U., U.U., M.Ç., F.Ü.; Data Collection or Processing- M.Ç.; Analysis or Interpretation- F.E.U.; Literature Search- A.A., U.U.; Writing- A.A.

Conflict of Interest: No conflict of interest was declared by the authors.

REFERENCES

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2. WHO Classification of Tumours Editorial Board. WHO Classification of Tumours, 5th Edition. Soft Tissue and Bone Tumours. (2020) ISBN: 9789283245025
3. Berry M, Mankin H, Gebhardt M, Rosenberg A, Hornicek F. Osteoblastoma: A 30-Year Study of 99 Cases. J Surg Oncol. 2008;98:179-183.
4. Bisadi A, Hosseininejad M. Co-existence of Non-ossifying Fibroma and Osteoid Osteoma in one Tibia: a Case Report. Int J Pediatr. 2022;10:15577-15582.