Teaching Case

A 13-year-old boy was referred to the department of radiology for a Magnetic Resonance Imaging (MRI) of the right knee because of posteromedial knee pain for three months. Further clinical history was unremarkable.

MRI showed a well-delineated lesion in the posteromedial tibial epiphysis of the right knee. The lesion was isointense to muscle on T1-weighted image (WI) and hyperintense on T2-WI. The lesion had a multilobular contour and there was surrounding bone marrow oedema. The margins of the lesion showed an intimate relationship with the posteromedial and articular cortex of the proximal tibia (Fig. 1).
After comparison with the MRI images, plain radiography showed a barely visible osteolytic lesion in the posterior tibial epiphysis of the right knee (Fig. 2).
Subsequent computed tomography (CT) was performed for more precise evaluation of the lesion extent towards the growth plate, the posterior cortex and the articular surface. CT showed an osteolytic lesion with a partial sclerotic rim and cortical breakthrough of the articular and posterior cortex. The lesion was confined to the tibial epiphysis. There were subtle intralesional calcifications (Fig. 3).

Based on the imaging findings, the diagnosis of chondroblastoma was suggested, which was confirmed after histopathological examination of a surgical bone biopsy.

Chondroblastoma is a rare benign cartilaginous tumour with a prevalence of 1-2% of primary bone tumours [1]. It affects young patients, usually under the age of 20 years and prior to the closure of the growth plate. Histologically, chondroblastoma consists of highly cellular and relatively undifferentiated chondroblast-like cells and chondroid matrix [2].

Symptoms are usually insidious and non-specific and include joint pain, muscle wasting, decreased joint mobility and swelling [3].

The tumour is typically located at the epiphysis or more rarely the apophysis. The most commonly affected site is the femur, followed by the humerus and tibia [3]. Approximately 10% are found in hand and feet [1].

Plain radiography shows a well-defined lucent lesion with a sclerotic rim, typically confined to the epiphysis.

CT may be used to evaluate the local extent of the lesion particularly towards the adjacent joint and cortex. Furthermore, CT may be useful to evaluate potential intralesional chondroid calcifications. Because of radiation concerns CT is not routinely performed in paediatric patients.

MRI is the image modality of choice for preoperative characterisation of the lesion. It typically shows a well-delineated, multilobular lesion, which is hyperintense on T2-WI with surrounding bone marrow oedema.

The differential diagnosis includes other epiphyseal lytic lesions such as giant cell tumour, intraosseous ganglion cysts and clear cell chondrosarcoma, which are seen in adulthood after closure of the growth plate. Moreover, giant cell tumours are often located at the metaphysis with epiphyseal extension [4]. A Brodie’s abscess is typically located in the metaphysis.

The standard of care is curettage and bone grafting. Chondroblastoma is a benign tumour but can behave locally aggressive and recurrence after surgery is not uncommon [5].