الفهرس 
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Abstract
Giant cell tumor of bone (GCTB) is an osteolytic, locally aggressive neoplasm, with a high recurrence rate and usually occurs in the long bones of young adults.
This tumor is composed of: mononuclear round to spindled stromal cells, admixed with multinuclear giant cells uniformly distributed throughout the tumor.
Aneurysmal bone cyst (ABC) is an expansile benign lytic lesion of the bone, composed of multiple blood-filled cysts separated by fibrocellular tissue septa including spindled stromal cells admixed with osteoclast-like giant cells in variable amounts.
Occasionally low grade GCTB closely resembles solid variant ABC giving a difficult diagnosis. Moreover, the association of giant cell tumor of bone (GCTB) with secondary aneurysmal bone cysts (ABC) may affect the final correct diagnosis. Hence the need for a specific immunostaining marker is essential to make the correct distinction. In most cases of GCTB, mutations of H3F3A was found, reflecting its involvement in genes activation during oncogenic transformation.
Moreover, GCTB is known to have a high recurrence rate, therefore the need for a marker to predict the tumor recurrence is of prognostic value.
P63 has a role in cancer pathogenesis due to its effect in the regulation of cell cycle arrest and DNA damage response. P63 has been identified to be highly expressed in GCTB.
The aim of this study was to explore and validate the specific expression pattern of Histone 3.3 as a clinical biomarker for the diagnosis of giant cell tumor of bone and to differentiate it from aneurysmal bone cyst and to detect the expression of p63 in cases of GCTB and correlate its expression with tumor recurrence.
Both histopathological and H3.3 immunostaining were examined in 30 cases of GCTB and 30 cases of ABC. P63 expression was examined in 30 cases of GCTB including both primary (22) cases and recurrent (8) cases.
The study enrolled thirty cases of GCTB. Their mean age was 27.13. Seventeen cases (56.7%) were females and sixteen cases (53.3%) were located in the distal femur. Twenty two cases (73.3%) were primary while eight cases (26.7%) were recurrent. In six cases (20%) secondary aneurysmal bone cyst was identified.
The thirty cases of ABC included in this study have mean age of 15.57 years. Sixteen cases (53.3%) were females and eight cases (26.7%) were located in the tibia. Twenty two cases were primary (73.3%) while eight cases (26.7%) were recurrent. Eleven cases (36.7%) were of the solid variant type.
There was statistically significant difference between histone 3.3 expression in giant cell tumor of the bone and aneurysmal bone cyst (p<0.001). H3.3 was positive in 29 (96.7%) GCTB cases, and negative in 28 (93.3%) ABC cases.
In the studied cases it was found that 1 case diagnosed by H&E stain as GCTB gave negative expression for H3.3 thus shifting the diagnosis to ABC. In addition 2 cases diagnosed by H&E stain as ABC gave a positive expression for H3.3 immunostaining, hence shifting the diagnosis to GCTB.
Statistical analysis revealed that Histone 3.3 sensitivity in diagnosing GCTB is 96.67%; as it had the capability of identifying 29/30 cases of GCTB. Its specificity was also 93.33%; as 28/30 cases of aneurysmal bone cyst were negative for histone 3.3 and only 2 cases were positive. Therefore, histone 3.3 can be considered a highly sensitive and specific diagnostic marker for GCTB and giving a clearcut answer to equivocal cases .
Concerning P63 expression, there was statistically significant difference between p63 expression in the two GCTB subgroups (primary and recurrent). As all recurrent cases (100%) were strongly positive for P63 (p<0.001). reflecting its role in tumor recurre