The Adolescent With a Painful Knee and a Distal Femoral Lesion
A 15-year-old boy presents with 3 months of progressive right knee pain, worse at night, and a palpable firm swelling above the knee. He has no fever, weight loss, or night sweats. Radiographs show a poorly marginated lytic lesion in the distal femoral metaphysis with periosteal reaction and a soft-tissue mass. He is otherwise well, plays football, and his parents ask whether this is 'just a growth pain.'
1. What is your differential diagnosis and what is the immediate next step?
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Differential - do not anchor on benign causes:
- Osteosarcoma (most likely in a 15-year-old with a distal femoral metaphyseal lesion, aggressive periosteal reaction, and soft-tissue mass).
- Ewing sarcoma (diaphyseal/metaphyseal, permeative, often with systemic symptoms - less likely here but must be excluded).
- Chondrosarcoma (rare in this age).
- Benign mimics: aneurysmal bone cyst, giant cell tumour (usually after physeal closure), osteomyelitis (Brodie abscess), eosinophilic granuloma, stress fracture. These can mimic malignancy radiologically.
Immediate next step: MRI of the entire femur with contrast, plus staging CT chest and whole-body imaging (bone scan or PET) to look for metastases. Do not biopsy before imaging is complete and the case has been discussed at a sarcoma multidisciplinary meeting.
Key exam point: night pain and a soft-tissue mass in an adolescent are red flags. 'Growth pain' is a diagnosis of exclusion and does not cause a palpable mass or a destructive lesion.
2. The MRI shows a large extraosseous component and the chest CT is clear. How do you plan biopsy and definitive treatment?
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Biopsy:
- Perform at a specialist sarcoma centre by the surgeon who will do the definitive resection, or under their direction. A poorly placed biopsy can compromise limb salvage.
- Core needle biopsy (or incisional) through a single longitudinal tract that will be excised en bloc with the tumour. Avoid transverse incisions, drains through the compartment, and haematoma that seeds the tract.
- Send tissue for histology, immunohistochemistry, and molecular studies (MDM2 amplification for low-grade osteosarcoma/parosteal variants; EWSR1 rearrangement for Ewing).
Definitive treatment:
- Neoadjuvant chemotherapy for high-grade osteosarcoma, then limb-salvage resection with wide margins and reconstruction (custom tumour prosthesis, or allograft-prosthesis composite). Amputation is reserved for neurovascular involvement or failed reconstruction.
- Response to neoadjuvant chemo (tumour necrosis ≥90%) is a major prognostic factor.
- If Ewing sarcoma, use the appropriate Ewing protocol with chemo + local control (surgery and/or radiotherapy).
Exam point: the biopsy must be planned as part of the definitive operation, and chemotherapy usually precedes resection in osteosarcoma.
3. The family asks about limb salvage versus amputation and about fertility. How do you counsel them?
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Limb salvage vs amputation:
- Limb salvage is possible if a wide margin can be achieved with a functional reconstruction and the neurovascular bundle can be preserved. It offers better body image and function in many patients, but carries risks of infection, prosthesis failure, and local recurrence, often requiring further surgery.
- Amputation may be preferable if the tumour involves the neurovascular bundle, if margins would be inadequate, or if the reconstruction would be non-functional. Modern prosthetics allow excellent function.
- Survival is equivalent between the two when margins are adequate - this is the key message to families.
Fertility:
- Chemotherapy (especially alkylating agents) can impair fertility. Offer sperm banking before starting chemotherapy - this is time-critical and should be arranged immediately after diagnosis, before neoadjuvant treatment.
- Discuss with reproductive medicine; in female patients, oocyte/embryo cryopreservation or ovarian suppression may be options.
Communication: involve the multidisciplinary team, give written information, and allow time for questions. Document the discussion.