Q1: What is the differential diagnosis?

Differential diagnoses include chondromyxoid fibroma, chondroblastoma, tuberculosis, chronic sclerosing osteomyelitis, simple bone cyst, aneurysmal bone cyst, clear cell chondrosarcoma, eosinophillic granuloma, fibrous dysplasia, osteoblastoma, and giant cell tumour.

Q2: How would you proceed to confirm your diagnosis?

The first aim was to differentiate chronic infection from neoplastic bone lesions. Acid fast bacilli culture excluded a tuberculous infection. Confirmation was achieved by performing an open biopsy and histopathological examination of the specimen. The final diagnosis of chondroblastoma was confirmed in view of there being a marked cellular chondroid component rather than fibrous component in the histology slides. Thorough curettage in such cases can provide an ample diagnostic specimen as well as being a definitive treatment, and may lead to complete recovery as found in this case.

Q3: How long would you follow up this patient?

It is important to maintain regular surveillance for at least five years in such cases with a diagnosis of chondroblastoma, with a view to detect any recurrence as soon as possible. Late sarcomatous change has been reported in very rare instances.

Clinical course

Open biopsy with thorough curettage was carried out. Acid fast bacilli culture was negative. The biopsy showed a cellular infiltrate of mononuclear cells and osteoclasts with areas of formation of cartilage-like stroma. There was no finding suggestive of a malignant lesion. Histology slides were reviewed by several consultants at two specialist centre because of confusion between chondroblastoma and chondromyxoid fibroma. The final diagnosis of chondroblastoma was confirmed in view of there being a marked cellular chondroid component rather than fibrous component. Her 18 month postoperative course was uneventful.

Discussion

Chondroblastoma was described as a cartilage containing giant cell tumour by Kolodny in 1927, a calcifying giant cell tumour by Ewing in 1928, and an epiphyseal chondromatous giant cell tumour by Codman in 1931. Jaffe and Lichtenstein (1942) coined the term chondroblastoma.¹ It is a benign bone tumour composed of cells arising from chondroblasts or their precursors, which tend to differentiate into cartilage cells. It is relatively rare and represents 2% of all primary bone tumours. It occurs in the second decade of life and is primarily epiphyseal in nature. It is about twice as common in males as in females.¹ Femur, humerus, tibia, pelvis, and scapulae are the five most common bones affected by chondroblastoma. Chondroblastoma occurs in less than 1%, out of which calcaneum is affected in less than 3%. A cystic component has been found to occur in 17% and a calcifying component in 30% cases.^2,3

Plain radiographs reveal well defined rarefaction of bone. They are usually less than 4 cm in size, eccentric, oval with smooth thin sclerotic border, with or without patchy calcification, mottling, and trabeculation.³ In this case, we found a scalloped margin with a trabeculated appearance initially, which later on after curettage, developed a flocculent calcific cystic appearance.

MRI scans can be confusing. Heterogenous low signal intensity with lobular internal architecture and fine lobular margins with no marrow signal change and no soft tissue extension can be suggestive of chondroblastoma. It is important to look for intra-articular extension, soft tissue invasion, and malignant transformation on MRI scan. It is well known to cause a diagnostic dilemma not only at clinical and radiological levels, and even histological findings can be confusing. Normally histological examination reveals uniform densely packed round to oval cells, scanty interstitial matrix, scattered mononuclear giant cells, fibrochondroid islands, and few mitotic figures.^4,5

Thorough curettage, with or without bone grafting, is almost always effective with an 85%–90% cure rate. Although most cases are treated successfully by conservatively performed operative procedures (such as curettage only or curettage with bone grafting), local recurrences are known to occur. These may rarely exhibit aggressive local behaviour such as extraskeletal soft tissue invasion and multifocal lesions.⁵

Final diagnosis

Calcifying cystic chondroblastoma of calcaneum.