Discussion
OS has distinct subtypes including TOS, small cell osteosarcoma, low-grade central osteosarcoma, parosteal osteosarcoma, periosteal osteosarcoma, high-grade surface osteosarcoma, chondroblastic osteosarcoma, and pagetic osteosarcoma.2 TOS is a rare subtype of OS that accounts for approximately 1% of OS.3 It often occurs in the metaphysal area of long bones including, the distal femur (41.6%), proximal tibia (16.9%) and proximal humerus (9.2%).4 The incidence of TOS in pelvis is rare, only 3.1%.5 The most common symptoms of TOS included a local pain, swelling, and tenderness in the affected bone area.6 Although our patient had a limp for two years, the absence of pain delayed her presentation and the diagnosis.
The characteristic radiological features of TOS are aggressive-asymmetric expansion, tumour ossification (fluffy, cloud-like) and osteolytic lesion with a permeative destruction with rapid growth tumour and visually minimal peripheral sclerosis.2 A CT scan is an accurate tool to detect peripheral mineralization and septal enhancement within TOS. MRI is the investigation of choice to identify vascular spaces and fluid- fluid levels within the tumour.2 Histologically, TOS is often composed of blood-filled or empty cystic spaces resembling ABC, and sometimes this similarity results in misdiagnosis. Blood-filled spaces mimic pseudo vascular spaces within the tumour. The septa are populated by a mixture of malignant osteoid producing OS cells and osteoclast like multinucleate giant cells.1
TOS patients often present with multiple lung metastasis at the time of diagnosis, particularly when the diagnosis has been delayed for 2 years.7, 8 our patient showed no lung metastasis. She has diabetes, was taking daily Metformin. According to some studies Metformin inhibits metastasis and cell proliferation in MG63 and U2-OS OS cell lines. 9, 10 However, this effect remains unproven in the clinical setting.
The common treatment for TOS is a combination of neoadjuvant chemotherapy followed by limb-salvage surgery.11 Most patients who received chemotherapy and post-surgery radiotherapy for contaminated margins developed multiple lung or liver metastases and died.8 The prognosis of TOS was initially very poor, because of misdiagnosis or delayed diagnosis. The 10-year survival rate of people with TOS is approximately 60%, early diagnosis in TOS with an appropriate treatment is crucial for disease-free survival.7, 8
Normally, sarcoma patients present either with pain or a mass, but not in this instance, indicating that limited mobility can be an important presenting symptom. TOS remains an enigma, labelled an OS because of the osteoid producing malignant cells, the distinguishing features of vascular lakes and abundant multinucleate osteoclast -like giant cells characterise TOS histopathology. There is a need to investigate the underlying mechanism of TOS to understand pathophysiology and ontology of this rare sarcoma.