**Abstract**

Giant cell tumor of bone (GCTB) is mostly a benign disease of the bone, although with high local recurrence rate and potential for metastatic spread, namely to the lungs. It is also a locally aggressive tumor, associated with severe morbidity and functional impairment due to bone destruction. Treatment is therefore required and should be offered at an early stage to allow complete resection, minimizing functional sequelae and local recurrence. Surgical resection is the mainstay of treatment, often followed by intralesional adjuvant therapy. GCTB has a particular biology, in which RANKL represents a key factor in tumor pathogenesis, thus making this molecule a valuable therapeutic target. Monthly administration of denosumab, a fully human monoclonal antibody directed against RANKL, has been studied in several clinical trials and shown a high rate of local control with favorable safety profile. In this chapter, current medical management, ongoing studies, and future directions in GCTB will be discussed.

**Keywords:** denosumab, giant cell tumor of bone, RANKL, sarcoma, sarcomatoid transformation

### **1. Introduction**

Giant cell tumor of bone (GCTB) is a primary tumor of bone usually arising in the meta-epiphysis of long bones, although potentially also occur in other parts of the skeleton, such as the spine or pelvis [1]. GCTB more often affects young patients in the second to forth decades of life [1, 2] and those living in urban (rather than rural) areas [3]. It is also associated with Paget's disease [4]. The condition presents with localized pain, tenderness to touch, palpable mass, and decreased range of motion, as well as mechanical pain and joint swelling in patients with presentation near joints [5]. Rarely, it may present with pathological fracture [4].

GCTB is mostly a benign disease, but local recurrence rates are high and there is a small risk of metastatic spread, namely to the lungs [1, 6]. Risk factors for pulmonary metastases include young age, Enneking stage 3, local recurrence, and axial disease, but not treatment modality [6, 7]. Pulmonary metastases most often appear three to four years after initial diagnosis and rarely are the cause of death [8]. However, when GCTB metastasizes, mortality rate rises to 14–25% [8, 9].

Despite being a rare event, GCTB can also undergo malignant sarcomatoid transformation [10]. In these cases, malignant GCTB can present with three histologic subtypes: osteosarcoma, fibrosarcoma, or undifferentiated pleomorphic

sarcoma. This usually occurs following multiple recurrent lesions (e.g. Paget's disease) and/or radiation therapy [5].

elongated mononuclear stromal cells, RANK-positive mononuclear cells of myeloid linage, and RANK-positive osteoclast-like giant cells, reflecting a physiopathology intimately linked to the RANKL/RANK pathway [17, 21, 22] (**Figure 1**). Small areas of osteoid matrix deposition, woven bone, and occasionally new bone are also observed in about 50% of GCTB samples, with different studies reporting an

The characteristic giant cells in GCTB are osteoclastic in nature [24–28] and represent the reactive component responsible for GCTB aggressive lytic behavior, leading to GCTB designation as osteoclastoma. These cells express RANK but not RANKL [26]. Profiling studies have shown that giant cells in GCTB are the result of CD33+ pre-osteoclast fusion that further fuse with CD14+ mononuclear cells [27] and express tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor,

However, in GCTB neoplastic cells are ovoid stromal cells, displaying markers of mesenchymal stem cells derived from the osteoblast lineage, but minimal expression of fully differentiated osteoblasts, like osteocalcin, alkaline phosphatase, osteoprotegerin (OPG), and TRAIL [29–33]. Twist-mediated downregulation of RUNX2, a major osteogenic regulator, has been shown to interrupt osteoblastic

GCTB stromal cells express high levels of RANKL [27] and also produce other

Because G34W mutations occur more frequently than chromosomal abnormalities and can be causative risk factors for chromosomal structural remodeling in DNA synthesis, it has been hypothesized that this driver mutation causes chromosomal instability and defects, contributing to pleiotropic effects on cell cycle-related expression, immature osteoblastic differentiation, and chemokines, cytokines, and

*Representative images of RANKL (A) and RANK (B) immunohistochemistry in formalin-fixed and*

osteoclastogenic cytokines, like interleukin (IL)-1, 6, 11, and 17, tumor necrosis factor-alpha (TNF-α), and macrophage colony-stimulating factor (M-CSF), through which osteoclast differentiation is stimulated from precursor cells [26]. Other characteristics supporting their neoplastic nature include dominance, increased proliferative potential, abundance of genetic alterations, and expression of more differentiation markers than multinucleated giant cells [22]. GCTBs are polyclonal in nature, with inconsistent chromosomal changes and telomere associations occurring in up to 72% of cases, although lacking prognostic value [35–38]. Mononuclear stromal cell-exclusive mutation p.G34W (or p.G34L, p.G34M, p. G34R, or p.G34V in a small sub-set of cases) in the *H3F3A* gene, encoding histone 3.3 (H3.3) variant implicated in epigenetic reprogramming and memory, has been

osteoclast markers, being capable of lacunar resorption [29].

identified as GCTB-specific driver mutation [30].

*paraffin-embedded GCTB samples. (unpublished data).*

**Figure 1.**

**57**

differentiation and depress osteoblast lineage markers in GCTB [34].

incidence between 22 and 52% [23].

*Medical Therapy of Giant Cell Tumor of Bone DOI: http://dx.doi.org/10.5772/intechopen.97114*

Multicentric GCTB in another rare form of tumor presentation, characterized by two or more distant lesions of histologically confirmed disease [11]. These lesions can present as synchronous (more common) or metachronous. Although multicentric GCTB appears to have demographic differences (patients are young and more commonly female), disease behavior including local recurrence rates, pulmonary metastases pattern, and malignant transformation seems to be similar to solitary GCTB [11].

Radiologically, GCTB presents as an osteolytic lesion with characteristic radiolucent and geographic (well-circumscribed) appearance and fading cortical bone, rarely showing periosteal reaction. GCTBs are usually eccentric masses in the epiphyseal region extending to subchondral bone (sclerotic metaphyseal margin) [5, 12].

Besides a high degree of suspicion in radiological exams (plain films, computed tomography [CT], and magnetic resonance imaging [MRI]), GCTB diagnosis must be histologically confirmed by core-needle or open biopsy [5]. Still, plain radiographs, CT scan, and MRI are useful for diagnosis and local staging. MRI is often performed to delineate neoplasm extent, namely soft tissue extension. Additionally, bone scintigraphy helps ruling out other asymptomatic bone lesions and chest CT scan should be performed to exclude lung involvement and guide treatment.

Based on radiological findings and according to Enneking and later Campanacci grading systems, GCTB can be classified in three grades [7, 13]:


This surgical staging system allows preoperative planning. Post-operatively, GCTB can also be graded based on histological features in grade 1 (typical), grade 2 (aggressive), or grade 3 (malignant) [14].

Due to lack of long-term follow-up data, GCTB prognosis is not well established to date [15]. However, the overall prognosis of benign GCTB is generally favorable. Recurrence rates are estimated at 25% [15] and can be as high as 50% after curettage alone [16]. Systemic treatment with bisphosphonates or denosumab seems to lower these figures [17]. Although secondary lung involvement is rare in benign GCTB and very uncommonly the cause of death, mortality rate is higher in these patients (14–25%) [8, 9, 18]. Regarding malignant GCTB (either primary or secondary), overall survival at 5 years is about 85% and poorest in older patients and those with distant disease at diagnosis, according to a Surveillance, Epidemiology and End Results (SEER) study involving 117 cases of malignant GCTB [19]. Smaller studies may indicate worse survival rates [20].

## **2. GCTB biology and pathogenesis**

#### **2.1 Histopathology**

GCTB is histologically characterized by diffuse growth of receptor activator of nuclear factor kappa-B ligand (RANKL)-positive, round-to-oval polygonal or

#### *Medical Therapy of Giant Cell Tumor of Bone DOI: http://dx.doi.org/10.5772/intechopen.97114*

sarcoma. This usually occurs following multiple recurrent lesions (e.g. Paget's dis-

Multicentric GCTB in another rare form of tumor presentation, characterized by two or more distant lesions of histologically confirmed disease [11]. These lesions can present as synchronous (more common) or metachronous. Although multicentric GCTB appears to have demographic differences (patients are young and more commonly female), disease behavior including local recurrence rates, pulmonary metastases pattern, and malignant transformation seems to be similar to solitary GCTB [11]. Radiologically, GCTB presents as an osteolytic lesion with characteristic radiolucent and geographic (well-circumscribed) appearance and fading cortical bone, rarely showing periosteal reaction. GCTBs are usually eccentric masses in the epiphyseal region extending to subchondral bone (sclerotic metaphyseal margin) [5, 12]. Besides a high degree of suspicion in radiological exams (plain films, computed tomography [CT], and magnetic resonance imaging [MRI]), GCTB diagnosis must be histologically confirmed by core-needle or open biopsy [5]. Still, plain radiographs, CT scan, and MRI are useful for diagnosis and local staging. MRI is often performed to delineate neoplasm extent, namely soft tissue extension. Additionally, bone scintigraphy helps ruling out other asymptomatic bone lesions and chest CT scan should be performed to exclude lung involvement and guide treatment.

Based on radiological findings and according to Enneking and later Campanacci

• Grade I (latent): well-defined margin (thin rim of mature bone) and intact

• Grade II (active): relatively well-defined margins but no radiopaque rim; cortex is thinned and moderately expanded. Grade II lesions with fracture are

• Grade II (aggressive): indistinct borders and cortex destruction, suggesting

This surgical staging system allows preoperative planning. Post-operatively, GCTB can also be graded based on histological features in grade 1 (typical), grade 2

Due to lack of long-term follow-up data, GCTB prognosis is not well established to date [15]. However, the overall prognosis of benign GCTB is generally favorable. Recurrence rates are estimated at 25% [15] and can be as high as 50% after curettage alone [16]. Systemic treatment with bisphosphonates or denosumab seems to lower these figures [17]. Although secondary lung involvement is rare in benign GCTB and very uncommonly the cause of death, mortality rate is higher in these patients (14–25%) [8, 9, 18]. Regarding malignant GCTB (either primary or secondary), overall survival at 5 years is about 85% and poorest in older patients and those with distant disease at diagnosis, according to a Surveillance, Epidemiology and End Results (SEER) study involving 117 cases of malignant GCTB [19]. Smaller studies may indicate worse survival rates [20].

GCTB is histologically characterized by diffuse growth of receptor activator of

nuclear factor kappa-B ligand (RANKL)-positive, round-to-oval polygonal or

grading systems, GCTB can be classified in three grades [7, 13]:

ease) and/or radiation therapy [5].

*Recent Advances in Bone Tumours and Osteoarthritis*

cortex (not deformed).

rapid and permeated growth.

(aggressive), or grade 3 (malignant) [14].

**2. GCTB biology and pathogenesis**

**2.1 Histopathology**

**56**

graded separately.

elongated mononuclear stromal cells, RANK-positive mononuclear cells of myeloid linage, and RANK-positive osteoclast-like giant cells, reflecting a physiopathology intimately linked to the RANKL/RANK pathway [17, 21, 22] (**Figure 1**). Small areas of osteoid matrix deposition, woven bone, and occasionally new bone are also observed in about 50% of GCTB samples, with different studies reporting an incidence between 22 and 52% [23].

The characteristic giant cells in GCTB are osteoclastic in nature [24–28] and represent the reactive component responsible for GCTB aggressive lytic behavior, leading to GCTB designation as osteoclastoma. These cells express RANK but not RANKL [26]. Profiling studies have shown that giant cells in GCTB are the result of CD33+ pre-osteoclast fusion that further fuse with CD14+ mononuclear cells [27] and express tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor, osteoclast markers, being capable of lacunar resorption [29].

However, in GCTB neoplastic cells are ovoid stromal cells, displaying markers of mesenchymal stem cells derived from the osteoblast lineage, but minimal expression of fully differentiated osteoblasts, like osteocalcin, alkaline phosphatase, osteoprotegerin (OPG), and TRAIL [29–33]. Twist-mediated downregulation of RUNX2, a major osteogenic regulator, has been shown to interrupt osteoblastic differentiation and depress osteoblast lineage markers in GCTB [34].

GCTB stromal cells express high levels of RANKL [27] and also produce other osteoclastogenic cytokines, like interleukin (IL)-1, 6, 11, and 17, tumor necrosis factor-alpha (TNF-α), and macrophage colony-stimulating factor (M-CSF), through which osteoclast differentiation is stimulated from precursor cells [26]. Other characteristics supporting their neoplastic nature include dominance, increased proliferative potential, abundance of genetic alterations, and expression of more differentiation markers than multinucleated giant cells [22]. GCTBs are polyclonal in nature, with inconsistent chromosomal changes and telomere associations occurring in up to 72% of cases, although lacking prognostic value [35–38]. Mononuclear stromal cell-exclusive mutation p.G34W (or p.G34L, p.G34M, p. G34R, or p.G34V in a small sub-set of cases) in the *H3F3A* gene, encoding histone 3.3 (H3.3) variant implicated in epigenetic reprogramming and memory, has been identified as GCTB-specific driver mutation [30].

Because G34W mutations occur more frequently than chromosomal abnormalities and can be causative risk factors for chromosomal structural remodeling in DNA synthesis, it has been hypothesized that this driver mutation causes chromosomal instability and defects, contributing to pleiotropic effects on cell cycle-related expression, immature osteoblastic differentiation, and chemokines, cytokines, and

#### **Figure 1.**

*Representative images of RANKL (A) and RANK (B) immunohistochemistry in formalin-fixed and paraffin-embedded GCTB samples. (unpublished data).*

surface markers expression [22, 37]. Additionally, mutations in cyclin D1, p53, and MET have been linked to malignant transformation and GCTB recurrence [22].

In GCTB, stromal cell-derived monocyte chemoattractant protein-1 (MCP-1/ CCL2) recruits bone marrow-derived CCR2/CXCR4-expressing monocytic osteoclast precursors from peripheral blood [45, 46]. Other soluble factors within GCTB microenvironment are chemotactic for myelomonocytic cells, including stromal cell-derived factor 1 (SDF-1/CXCL12), macrophage inflammatory protein 1-alpha (MIP-1α/CCL3), and M-CSF1 [26, 47]. Osteoclast precursors localized at GCTB

Different pre-clinical studies have shown that GCTB stromal cells with circulating mononuclear cells co-culture induces differentiation of osteolytic giant cells [41–43]. For differentiation to occur, RANKL expression in stromal cells is regu-

RANK pathway activation in giant cells leads to up-regulation of nuclear factor

MMPs have an important role in GCTB physiopathology. Apart from the abovementioned role in giant cell migration via TGF-β activation, MMPs influence other major aspects within the tumor microenvironment, like angiogenesis, invasion, and metastatic development. MMP-2 and MMP-9 are key in all these processes [22]. In GCTB, the extracellular matrix metalloproteinase inducer (EMMPRIN) is responsible for inducing MMP expression. Higher EMMPRIN expression at multinuclear osteoclast-like giant cells has been observed in stage III GCTB, probably regulated

As previously mentioned, metastases are extremely rare in GCTB and there are

Pathophysiology of GCTB progression remains unclear and prognostic factors,

Histologically, ambiguous giant cell-rich lesions including benign GCTB, chondroblastoma, aneurysmal bone cyst, central giant cell granuloma of the jaw, and malignant giant cell–rich osteosarcoma are often found, especially as small biopsy or curettage specimens [22]. In these cases, *H3F3A* gene p.G34W mutation can be used in the differential diagnosis, as it is almost GCTB-exclusive [30, 55]. Approximately 90% of GCTBs display the p.G34W mutation, with minor subsets (<2%) displaying p.G34L, p.G34M, p.G34R, or p.G34V variants. H3F3B p.K36M is the H3.3 mutation found in the vast majority (90–95%) of chondroblastomas [30]. H3.3 p.G34W mutant-specific immunohistochemistry (IHC; clone RM263, commercially available) is a highly sensitive and specific surrogate marker of H3F3A p.G34W mutation in GCTB [56–58], being useful for practical diagnosis in primary [58] or recurrent, metastatic, and secondary malignant GCTB [59]. Although denosumab therapy may decrease p.G34W expression [22], evidence

no clues on molecular or physiopathological events related with GCTB

treatment targets, and predictive biomarkers represent unmet needs.

microenvironment differentiate into active, bone resorbing, osteoclasts.

lated by CCAAT/enhancer-binding protein beta (C/EBPβ), found to be

growth factors (IGF) I and II [51].

*Medical Therapy of Giant Cell Tumor of Bone DOI: http://dx.doi.org/10.5772/intechopen.97114*

by RANKL from stromal-like tumor cells [54].

metastization to date.

**2.3 Tumor markers**

**59**

overexpressed in GCTB [48], and also by parathyroid hormone-related peptide (PTHrP) in an autocrine manner [49]. Next, RANKL-induced cell fusion is costimulated by M-CSF and IL-34 [26] and enhanced by specific transmembrane proteins overexpressed in GCTB [50] and coupling components, like insulin-like

of activated T cells c1 (NFATc1), an auto-regulated key transcription factor responsible for regulating expression of important genes involved in bone resorption, like cathepsin K or β3-integrin [52]. Cathepsin K is involved in initial steps of bone resorption, degrading collagen type I and remodeling the bone matrix, allowing migration. As bone resorption starts, TGF-β entrapped in bone matrix is activated by matrix metalloproteases (MMPs), stimulating giant cell migration [46], which is mediated by αvβ3 integrin attachment to the bone matrix [53].

Biologically, Wnt/β-catenin and transforming growth factor beta (TGF-β) signaling pathways mediate the exacerbated proliferation of stromal cells in GCTB. βcatenin, cyclin D1, and p21 have been shown to be overexpressed in the nuclei of GCTB stromal cells [39]. Additionally, one study showed that protease activated receptor-1 (PAR-1) is also upregulated in GCTB downstream of TGF-β, via Smad3 and Smad4 [40]. In the study, PAR-1 knockout in GCTB stromal cells inhibited tumor growth, angiogenesis, and osteoclastogenesis in vitro and PAR-1 inhibition suppressed tumor growth and giant cell formation *in vivo*.
