A 36-year-old female patient without any relevant past medical history presented to her primary care physician with insidious complaints consisting mainly of pain during flexion of the right knee. Physical examination was generally unremarkable, except for a slight tumefaction in the right popliteal region without associated inflammatory signs.
A plain film was requested, revealing a dense, lobulated, predominantly sclerotic mass with irregular margins arising from the posterior surface of the distal femoral metaphysis, measuring 6.5 x 3.3 x 5.5 cm (Fig. 1).
CT, MR and a bone scintigraphy were performed (Figs. 2-5), as well as a core needle biopsy, and a diagnosis of juxtacortical osteosarcoma was made.
The patient underwent surgery with excision of the distal 12 cm of the right femur and total knee arthroplasty.
The pathologic analysis of the surgical specimen (Figs. 6-8) resulted in the diagnosis of an intermediate-grade parosteal osteosarcoma, with small foci of dedifferentiation and chondroblastic differentiation. No invasion of the medullary cavity was seen. The surgical margins were histologically negative.
The patient entered a long-term follow-up schedule with no evidence of relapse so far.
Osteosarcoma follows myeloma in frequency of primary bone malignancy among all ages (15% to 20%). Parosteal osteosarcoma is a surface lesion that represents the second most common type of osteosarcoma, typically manifesting in the 2nd and 4th decades of life. This surface osteosarcoma is found most frequently at the posterior distal femoral metaphysis; other common locations are the proximal tibia and proximal humerus [1, 2].
Osteosarcoma is a malignant tumour of connective tissue that produces osteoid matrix and variable amounts of cartilage matrix and fibrous tissue. These tumours can be histologically quite heterogeneous, and pathologists further classify the lesions on the basis of predominant features: osteoblastic (50%), chondroblastic (25%) or fibroblastic (25%).
Parosteal osteosarcoma usually arises from the outer fibrous layer of the periosteum and is most often a low-grade lesion, exhibiting minimal fibroblastic stromal atypia and extensive bone matrix [1, 3].
Classically, the radiologic appearance of parosteal osteosarcoma is often characterised by a large, lobulated, exophytic and juxtacortical mass, revealing extensive ossification. A radiolucent cleavage plane separating the tumour and adjacent normal cortex (the string sign) has been reported in approximately 30% of cases at radiography and in 65% of cases at cross-sectional imaging, corresponding to the periosteum interposed between the cortex and the tumour mass. Cortical thickening with a relative lack of aggressive periosteal reaction is often apparent. The ossified regions of the tumour show high attenuation on CT, while at MR imaging they are predominantly hypointense on both T1- and T2-weighted images, following the imaging behaviour of the cortex [1, 2, 4].
Parosteal osteosarcomas tend to be slow growing and low grade, but with inadequate excision may recur locally in a more aggressive form; hence, wide surgical excision is required to prevent recurrence. Parosteal osteosarcomas are often amenable to limb preservation due to their location, low-grade histologic features and lack of local invasiveness. Higher-grade parosteal osteosarcomas may warrant neoadjuvant chemotherapy because of the risk of metastasis. MR imaging and CT are essential in treatment planning, helping to detect the high-grade component and guiding the biopsy site location.
The prognosis for parosteal osteosarcoma is better than that for conventional osteosarcoma, as the 5-year overall survival rate is 86%-91% for the former but 53%-61% for the latter. Metastases to the lung in parosteal osteosarcoma occur both later and with considerably less frequency compared with metastases in conventional osteosarcoma [1, 2].
Parosteal osteosarcoma
The patient is a 36-year-old female with main complaints of pain in the right knee joint during flexion. Imaging (including X-ray, CT, and MRI) primarily shows:
Based on the imaging findings and the patient’s clinical information, the primary differential diagnoses include:
Considering the patient’s age, clinical symptoms, and the aforementioned imaging features (a protrusion on the cortical surface with heavy osteosclerotic appearance, well-defined borders, typically located on the posterior aspect of the distal femur), along with pathology suggesting a low-grade malignant osteogenic process, the diagnosis strongly correlates with Parosteal Osteosarcoma.
Most Likely Diagnosis: Parosteal Osteosarcoma.
Rehabilitation should be tailored to the patient’s surgical procedure, healing progress, and individual condition. A general outline is as follows:
Please follow the FITT-VP principle (Frequency, Intensity, Time, Type, Volume, Progression) throughout the rehabilitation process. Under safe conditions, and in accordance with medical advice and recommendations from professional rehabilitation specialists, properly adjust weight-bearing and exercise amplitude, ensuring a balanced and orderly progression.
If bone fragility or poor overall health is present, consult with a physician promptly, reduce exercise load, and reinforce supervision as needed.
This report is a reference analysis based on existing data and does not substitute in-person consultation or professional medical advice. Specific treatment plans should be determined by a specialist, taking into account the patient’s actual condition.
Parosteal osteosarcoma
