1 Background

Osteosarcoma is a primary bone malignancy occurring due to abnormal proliferation of bone forming mesenchymal cells. It usually effects the metaphysis of long bones.¹ It can be composed of a variety of diverse histological types including classical, osteoblastic, chondroblastic, chondromyxoid, epitheloid, clear cell, giant cell, telangiectatic and small cell variants. Chondroblastic variant histologically resembles chondroblastoma but has more cellular atypia, local aggressiveness, and osteoid matrix production.² The management of osteosarcoma has seen a major shift from amputation towards limb salvage surgery (LSS) in the past two decades.³ LSS involves resection of the tumor and subsequent reconstruction. This reconstruction is quite challenging as there are large swaths of missing bone. Tumors occurring near the knee joint make restoring a functional joint an arduous task. Mega prosthesis has revolutionised the management of such challenging cases. We present a case of osteosarcoma of distal femur which did not have the classical histology, and was managed with a megaprosthesis.

2 Case report

A 21 year old lady had complaints of pain in the left thigh for one year. The pain was diffuse, dull aching, and intermittent. The frequency of the pain had increased over the past six months. There was no history of diurnal variation of pain, aggravating or relieving factors. She had no history of fever, loss of weight or appetite. She had no previous medical conditions.

3 Clinical findings

On examination, a hard mass was palpable over the lateral aspect of the distal thigh. It found to be fixed to the distal end of the femur, and extending till the superior border of the patella. The mass was tender and non-mobile. The borders were ill defined and the skin above the swelling was normal. The knee joint movements were full and free. No other swellings were palpable elsewhere in the body.

4 Diagnostic assessment

All routine blood investigations were within normal limits. Radiographs (Fig. 1) revealed an ill-defined ‘fluffy’ osseous lesion involving the metaphyseal region of distal femur. The lesion was seen extending into the soft tissue laterally and posteriorly. Computed tomography (CT) scan (Fig. 2) showed mixed lytic and blastic regions in the distal femur with extension into soft tissue. CT guided core needle biopsy (Fig. 3) was performed. Despite the lesion appearing predominantly over the lateral aspect of the distal thigh, the CT guided biopsy was done using a medial transmuscular approach through the vastus medialis muscle. This was done keeping in my mind any future limb salvage surgery planned in the future, most of which would involve a medial incision over the distal thigh and knee joint. Utilising the medial approach for the biopsy, would enable the biopsy tract to be excised at the time of the definitive surgery. Histopathological examination (Fig. 4) showed cellular lesion with chondroblastic differentiation and focal lace like osteoid formation. Individual cells showed pleomorphism and hyperchromatic nucleus. By immunochemistry, the atypical cells were positive for Vimentin, SATB2, CD99, and negative for S100. A diagnosis of Chondroblastic Osteosarcoma of the distal femur was made.

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Fig. 1 Radiographs showing an ill-defined lesion involving the metaphyseal region of distal femur. Medullary bone destruction (blue arrow), wide zone of transition, periosteal reaction (yellow arrow) and soft tissue extension are noted. Fluffy calcifications (red arrow) are seen within the matrix of the lesion. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 2 CT showing mixed lytic (green arrow) and blastic regions (red arrow) in the distal femur with extension into soft tissue (yellow arrow). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 3 CT guided core needle biopsy of distal femur lesion using a medial transmuscular approach through the vastus medialis muscle.

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Fig. 4 Histopathological examination of specimen showing cellular lesion with chondroblastic differentiation (yellow arrow) and focal lace like osteoid formation (black arrow). Individual cells showed pleomorphism and hyperchromatic nucleus (green arrow). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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5 Differential diagnosis

It posed a diagnostic challenge as chondrosarcoma, and chondroblastoma were close differentials. Presence of bone-forming tumor matrix, aggressive periosteal reaction, metaphyseal origin and young patient age helped us differentiate chondroblastic osteosarcoma from its closest differential of chondrosarcoma.⁴ Immunohistochemistry was confirmatory of the diagnosis.

6 Therapeutic intervention

After discussion with the oncologist and parents of the child, it was decided to proceed with limb salvage surgery and chemotherapy. Chemotherapy was given was per the European and American Osteosarcoma Study (EURAMOS-1) protocol,⁵ consisting of Methotrexate, Adriamycin and Cisplatin. She was initiated on two cycles of neoadjuvant chemotherapy. She then underwent wide resection of the tumor and reconstruction of distal thigh, knee joint and proximal tibia with modular megaprosthesis (Link Mega system C- Tumor/Revision System {Hamburg, Germany}) (Fig. 5). She was started on knee bending exercises, quadriceps and hamstring strengthening exercises the day after the surgery. She was mobilised two days following the surgery. She was given nine cycles of adjuvant chemotherapy. The resected specimen on histopathological examination showed classical features of chondroblastic osteosarcoma, 15% tumor necrosis and margins to be free of malignancy.

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Fig. 5 Radiograph performed 1 year following surgery showing megaprosthesis in situ, no evidence of aseptic loosening of implant or recurrence of tumor.

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7 Follow up and outcomes

She was followed up regularly. Radiographs were performed every six months during the first two years, followed by yearly radiographs for five years. She was able to mobilise without any aids. She was found to have 90° of knee flexion. No recurrence of the tumor occurred. She was noted to have an excellent Musculoskeletal Tumor Society (MSTS) Score of 26 at last follow up.

8 Discussion

Chondroblastic osteosarcoma is a rare, aggressive variant of osteosarcoma. It accounts for less than a quarter of the cases of osteosarcoma.⁶ The usually occurs in the second decade of life, with most common bones involved being the tibia and femur.⁷ It poses a diagnostic challenge as chondrosarcoma and chondroblastoma are close differentials. It is important to identify this histological variant of osteosarcoma and differentiate it from classical osteosarcoma as chondroblastic osteosarcoma is associated with a higher recurrence rate and metastatic potential compared to classical osteosarcoma, hence has a poor prognosis. In fact, the overall survival has been found to be as low as 53% at 2 years and 16% at 4 years follow up. Median overall survival is reported to be 28 months.⁸ Considering the short life expectancy, we had to prioritise providing satisfactory quality of life, along with attempting for complete resolution of the malignancy.

Limb salvage surgery has advanced manifold in the past few decades with improvement in reconstructive surgical technique, implants used and effective chemotherapy. In recent studies LSS have shown better 5 year survival and functional outcomes compared to amputation.^9,10 LSS employs one of four main types of reconstructive procedures-endoprosthesis/megaprosthesis, autograft, bulk allograft, or allograft prosthetic composite.¹¹ Each technique has its own advantages and associated complications. Megaprosthesis have shown an excellent functional outcome with average MSTS scores as high as 22.9/30, five-year and eight-year survival rates of 71% and 67.2% respectively.¹² The causes of failure of megaprosthesis have been aseptic loosening, local recurrence of tumor, infection and structural failure.¹³ Considering the fact that amputation has not shown no additional survival benefit over LSS in the management of osteosarcoma around the knee joint, we decided to proceed with LSS. Reconstruction with Megaprosthesis was favoured as in this case providing quality of life was the priority considering the short life expectancy. The range of movements and early mobilisation offered by megaprosthesis made us choose this option over an arthrodesis. The patient has been disease free and passed the survival expectancy of 84% cases of chondroblastic osteosarcoma. This makes this case report one of the very few cases of such a long term survival and follow up in a patient with chondroblastic osteosarcoma.

9 Conclusion

Chondroblastic Osteosarcoma is a rare variant of osteosarcoma which can be distinguished from classical osteosarcoma by histopathology. It is important to recognise this variant as it is associated with a higher recurrence rate and metastatic potential. Limb salvage surgery with megaprosthesis provided excellent functional outcome with no recurrence in our case.

Informed consent

Informed consent was obtained from the patient for this publication.

Declaration of patient consent form

Patient's parents declare no conflict of interest in publishing the case report for academic purposes.

Financial support and sponsorship

Nil.

Author contributions

SNP obtained the patient data and digital imaging of the patient. SNP also drafted the initial manuscript. GM was the chief operating surgeon. GM and GH reviewed the manuscript. All authors read and approved the final manuscript.