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.