Fine needle aspiration cytology of chondroblastoma of the fibula.

Chondroblastoma is a rare benign cartilaginous tumor typically seen in long bones, especially femur, tibia and proximal humerus. Extracortical soft tissue invasion or metastasis is rarely seen. We report here a unique case of chondroblastoma of the fibula with extracortical soft tissue invasion. Differential diagnosis on cytology is discussed.

Introduction

Chondroblastoma is a rare tumor entity that comprises approximately 1% of all benign bone tumors.[1] In up to 95% cases, it occurs in the second decade of life with a male:female ratio of 2:1.[2] The most frequent localizations of chondroblastoma are epiphyses of long bones (proximal humerus, proximal femur, distal femur and proximal tibia). Patella, pelvis, fibula, small bones of hands and feet, ribs, skull, mandible, maxillae, vertebrae, scapulae and sternum are considered as atypical localizations. Although fine needle aspiration cytology (FNAC) is gaining acceptance as very convenient and reliable method for the rapid diagnosis of bone lesions, very few cases of chondroblastoma diagnosed on cytology have been reported in the literature.

Case Report

An 18-year-old-female presented with pain in left leg for last 1-month and progressively increasing swelling around the knee joint for last 15 days. There was no history of trauma. On examination, the patient had restricted left knee joint movements with tenderness. Radiograph showed a lytic lesion in the head and upper shaft of the fibula.

Various investigations like routine blood and urine examination, serum acid and alkaline phosphatase, serum calcium and phosphorus were within normal limits.

Magnetic resonance imaging of leg showed abnormal marrow infiltration in medullary cavity of head and upper shaft of fibula hypo-intense on T1-weighted T2-weighted sequences and hypo-to hyper-intense on short tau inversion recovery (STIR) sequence, the vertical length being 8 cm approximately. There was cortical destruction with extra-osseous extension of abnormal marrow signals abutting soft tissue and musculature namely peroneal, extensor digitorum longus, flexor hallucis longus, tibialis anterior and posterior, and soleus muscle. The extra-osseous component was large and measured 7.3 cm × 4.7 cm approximately. Tibialis anterior vessels and deep peroneal nerve were encased. Tibia and femur were normal [Figure 1].

Figure 1

Magnetic resonance imaging knee joint short tau inversion recovery Coronal reveals abnormal hyperintense medullary marrow signal in the head of fibula (epiphysis) extending inferiorly into upper shaft of the fibula. There is associated cortical destruction with sotissue components

The FNAC smears, stained with giemsa stain, showed neoplastic mononucleated and binucleated chondroblasts, multinucleated osteoclast-like giant cells, and chondromyxoid matrix fragments representing early cartilage in hemorrhagic background. The chondroblasts were distributed singly in smears, had round to oval nuclei with indentation and lobulation, and fine evenly distributed chromatin. Their cytoplasm was dense with rounded well-defined borders. Many binucleated forms were also present along with multinucleated osteoclast-like giant cells. The giant cells were indistinguishable from those seen in other bone neoplasms [Figure 2a and c]. Characteristic magenta colored fragments of chondroid matrix were seen [Figure 2b]. Diagnosis of chondroblastoma was made on cytology.

Figure 2

(a) Photomicrograph showing chondroblasts distributed singly (yellow arrow) in smear, have round to oval nuclei with indentation, fine chromatin and dense cytoplasm with rounded well-defined borders. Binucleated forms (green arrow) and multi nucleated osteoclast-like giantcells (red arrow) are also evident (Giemsa, ×400) (b) Photomicrograph of Fine needle aspiration smear showing chondroid matrix (Giemsa, ×100) (c) Photomicrograph showing chondroblasts with binucleated forms (green arrow) and multi nucleated giant cells (Giemsa, ×400)

Subsequent intralesional curetting was done with bone autografting. Histopathology revealed tumor composed of sheets of polyhedral chondroblasts with round to ovoid, indented and lobulated nuclei and thick sharply defined cell membrane. At places, multinucleated giant cells were seen. Thin network (chicken-wire like) to dense obvious deposits of calcification was evident. Histopathology confirmed the diagnosis of chondroblastoma. There was no recurrence observed on 2-year follow-up of the case.

Discussion

Chondroblastoma was first described by Codman in 1931, who designated it as “epiphyseal chondromatous giant cell tumor”.[3] Jaffe and Lichtenstein in 1942 differentiated chondroblastoma from giant cell tumor and established the term “benign chondroblastoma”.[4] Very few cases with extracortical soft tissue invasion or metastasis are reported.[5] Atypical cases of chondroblastoma are reported, especially in older patients, tend to involve unusual sites and have a greater tendency to expand the affected bone.[1] The present case was a young female having atypical localization (fibula) of chondroblastoma with extracortical soft tissue invasion.

The histological diagnosis of typical chondroblastoma is not difficult due to their characteristic appearance with rounded or polygonal chondroblasts, multinucleated giant cells and eosinophilic chondroid extracellular matrix with focal chicken-wire calcification.[6]

Although aspiration cytology is very convenient and reliable method for the rapid diagnosis of bone lesions, the failure rate is high. High failure rate of aspiration cytology in bone tumors is mainly due to the hardness and fibrous nature of the tumors and if they are guarded by thick cortex leading to difficulty in piercing the needle.[7]

Many times, chondroblastoma is mistaken as giant cell tumor on aspiration cytology. FNAC of giant cell tumor also shows two population of cells; dominant histiocytoid mononuclear cells and multinucleated giant cells. Presence of chondroid matrix is believed to be the decisive feature to differentiate chondroblastoma from giant cell tumor, which is seen better in Giemsa-stained smears than in Papanicolaou-stained smears.[8] Also, mononuclear cells in chondroblastoma have irregular ridged nuclei with longitudinal clefts, while, in giant cell tumor, the cells resemble histiocytes with nuclei similar to those of the giant cells.[9] In difficult cases, the demonstration of S100 positivity in chondroblasts is helpful.[10]

Another entity to differentiate from chondroblastoma is Langerhan's cell histiocytosis (LCH). Cytologically histiocytes of LCH are like chondroblasts of chondroblastoma but not associated with any type of matrix production.[1]

Conclusion

Fine needle aspiration cytology provides a simple, quick, easy and reliable method of preoperative diagnosis of chondroblastoma and can be used as a substitute of surgical biopsy. The cytologic features of the chondroblasts and chondroid matrix are the diagnostic hallmarks of chondroblastoma.