Revisiting the WHO classification system of bone tumours: emphasis on advanced magnetic resonance imaging sequences. Part 2

Abstract

Similarly to soft tissue tumours, the World Health Organisation (WHO) classification categorises bone tumours based on their similarity to normal adult tissue. The most recent WHO classification provides an updated classification scheme that integrates the biological behaviour of bone tumours, particularly cartilage-forming tumours, and tumours are now further subdivided as benign, intermediate (locally aggressive or rarely metastasising), and malignant. Radiologists play an important role in the detection and initial characterisation of bone tumours, with careful analysis of their matrix mineralisation, location, and overall anatomic extent including extra-compartmental extension and neurovascular invasion. Radiography remains central to the detection and characterisation of bone tumours; however, magnetic resonance imaging (MRI) is the ideal modality for local staging. This review will discuss the most recent updates to the WHO classification of bone tumours that are relevant to radiologists in routine clinical practice. The utility of advanced MRI sequences such as diffusion-weighted imaging, dynamic contrast enhanced sequences, and magnetic resonance spectroscopy that may provide insight into the biological behaviour of various bone tumours is highlighted.

Keywords: MRI; WHO classification; bone tumours; sarcoma.

Figure 1

52-year-old woman with chondroblastoma, an intermediate (rarely metastasising) chondrogenic bone tumour. Anteroposterior radiograph of the left knee (A) shows a focal lytic skeletal lesion in the distal medial femoral condyle complicated with pathological fracture (arrow). On magnetic resonance imaging (MRI), there is a distal left femoral subarticular skeletal lesion (arrow) that is isointense to skeletal muscle on T1-weighted image (B), hyperintense to skeletal muscle on axial fat-suppressed (FS) T2-weighted image (C) with confluent internal enhancement on axial T1-weighted FS post-contrast image (D) and early arterial enhancement on dynamic contrast-enhanced-maximum intensity projection (E). Axial diffusion-weighted imaging (DWI) through the distal left femur using low b-value of 50 s/mm² (F), intermediate b-value of 400 s/mm² (G), high b-value of 800 s/mm² (H) and lastly the apparent diffusion coefficient (ADC) map (I) shows qualitative restricted diffusion with lack of signal loss on DWI progressively increasing b-values as well as low ADC values (≤ 1.0 × 10-3 mm2/s) ranging from 0.8 to 1.2. MRI is optimal for local staging and readily displays cortical breach on the coronal T1-weighted image (dotted arrow on image B). Advanced MRI features (early arterial enhancement on dynamic contrast enhanced and restricted diffusion on DWI/ADC mapping) point towards aggressive biology and hypercellularity

Figure 2

25-year-old woman with grade 3 chondrosarcoma. Axial computed tomography (CT) image through the lower lumbar (A) spine shows an aggressive, permeative skeletal lesion (arrow) within the vertebral body and posterior elements with a large soft tissue component arising from the left transverse process. Axial T1-weighted (B) and T2-weighted magnetic resonance (MR) images (C) through the lumbar spine are superior to CT in demonstrating the large associated soft tissue mass (arrows) and intraspinal extension of neoplasm. Coronal post-contrast T1-weighted fat-suppressed MR image through the lumbar spine (D) shows a large extra-osseous soft tissue mass (arrow) with associated lobular and solid enhancement associated with chondrogenic tumours

Figure 3

14-year-old male with osteoblastoma and secondary aneurysmal bone cyst. Axial computed tomography image through the lower lumbar spine (A) shows an expansile, predominantly lucent, well-demarcated skeletal lesion (arrow) in the vertebral body extending posterior to involve the left pedicle and transverse process. Axial T1-weighted (B) and T2-weighted (C) magnetic resonance images through the skeletal lesion (arrow) show medial epidural extension of neoplasm (dotted arrow). Note the absence of septations and fluid-fluid levels indicative of a typical aneurysmal bone cyst (see Figure 11 for comparison)

Figure 4

66-year-old man with osteosarcoma, osteoblastic type, grade 3 (of 3) in the setting of prior radiation therapy for prostate carcinoma several years ago. Coronal (A) and axial (B) computed tomography images through the pelvis reveal a large aggressive skeletal lesion (arrow) in the right pubic body extending towards the acetabulum with osteoid matrix mineralisation. Axial T1-weighted magnetic resonance image through the pelvis (C) shows the anatomical extent of the lesion to involve the anterior right acetabulum (arrow)

Figure 5

44-year-woman with fibrosarcoma of bone. Anteroposterior radiograph of the left humerus (A) shows a lytic lesion (arrow) in the proximal humeral diaphysis complicated with a pathological fracture. Magnetic resonance imaging (MRI) shows intramedullary marrow replacement, cortical break, as well as perilesional oedema/periosteal reaction on axial T2-weighted fat-suppressed (B) and T1-weighted images (C) through the left humerus (arrows). There are no determinate features on radiographs or MRI. By definition, the diagnosis of fibrosarcoma requires absence of any recognisable line of differentiation and as such requires a thorough and representative histological sample when a biopsy is performed

Figure 6

11-year-old female with Ewing sarcoma. Oblique radiograph of the left ankle (A) shows an aggressive lytic lesion (arrow) in the distal tibia involving the diametaphysis with cortical destruction and associated extra-osseous extension. Magnetic resonance imaging shows large skeletal lesion (arrow) measuring 3.5 × 2.6 × 6.2 cm with demonstration of its intramedullary extent as well as large associated soft tissue mass on coronal T1-weighted (B), axial T2-weighted fat-suppressed (FS) and (C) axial T1-weighted FS post-contrast images (D) through the left distal tibia (arrows)

Figure 7

18-year-old male with giant cell tumour of bone. Anteroposterior radiograph of the left knee (A) shows a focal, subarticular lytic lesion in the proximal tibia involving the tibial spine (arrow). Magnetic resonance imaging (MRI) shows a focal skeletal lesion (arrow) on coronal T1-weighted (B), coronal T2-weighted fat-suppressed (FS), and (C) coronal T1-wighted FS post-contrast images (D) through the left proximal tibia with marked perilesional bone marrow and soft tissue oedema and enhancement. MRI can play an important role in locoregional staging of giant cell tumour of bone and selection of intralesional margins to strike a balance between optimal oncological and functional outcome. On MRI, giant cell tumour of bone margins can be classified as clear (as seen here), relatively clear, or blurred, which correlate with local tumour cell infiltration on histology

Figure 8

56-year-old male with C3 chordoma. Axial T2-weighted (A), T1-weighted (B), and GRE-based (C) magnetic resonance images through C3 vertebra show an expansile, T2 hyperintense, marrow replacing lesion (arrow) involving the right C3 vertebral body with a ventral soft tissue component. There is no intra-lesional mineralised matrix on the coronal computed tomography image (D) or GRE

Figure 9

16-year-old female with polyostotic epithelioid haemangioendothelioma (EHE) in the right lower extremity. Lateral radiograph of the right ankle (A) shows multifocal mixed lytic and sclerotic skeletal lesions in the distal tibia (long arrow), posterior calcaneus (dotted arrow, complicated with pathological fracture), and talus (short arrow). On magnetic resonance imaging (MRI), there are multifocal skeletal lesions in the distal tibia (long arrow), posterior calcaneus (dotted arrow, complicated with pathological fracture), and talus (short arrow). These lesions are isointense to skeletal muscle on sagittal T1-weighted image (B) and hyperintense to skeletal muscle on axial T2-weighted fat-suppressed image (C). Axial diffusion-weighted imaging (DWI) through the distal left tibia (arrow) using low b-value of 50 s/mm² (D), intermediate b-value of 400 s/mm² (E), high b-value of 800 s/mm² (F), and the apparent diffusion coefficient (ADC) map (G) show qualitative free diffusion with progressive signal loss on DWI with progressively increasing b-values as well as high ADC values (≤ 1.0 × 10-3 mm2/s) ranging from 1.5 to 2.2. Coronal dynamic contrast enhanced maximum intensity projection (H) shows multifocal sites of early arterial enhancement in the distal tibia (arrow), fibula, calcaneus, and talus, a distinguishing MRI feature of EHE when compared with epithelioid haemangioma

Figure 10

15-year-old male with right second rib aneurysmal bone cyst. Anteroposterior chest radiograph (A) shows an expansile lytic skeletal lesion involving the right second lateral rib (arrow) with a large intrathoracic subpleural component. On axial computed tomography image (B) there is no internal mineralised matrix (arrow). On magnetic resonance imaging, there are several fluid-fluid levels on axial T2-weighted fat-suppressed (FS) image (C), presence of T1 hyperintensity medially – probably representing haemorrhagic products on axial T1-weighted FS pre-contrast image (D) with thin septal enhancement on axial T1-weighted FS post-contrast imaging (E) compatible with primary aneurysmal bone cyst (arrows)

Figure 11

8-year-old female with right femoral neck Langerhans cell histiocytosis. Frog leg lateral radiograph of the right hip (A) shows a focal lytic skeletal lesion (arrow) in the right femoral neck. On magnetic resonance imaging (MRI), there is a proximal right femoral neck intramedullary lesion (arrow) that is isointense to skeletal muscle on coronal T1-weighted image (B) and hyperintense to skeletal muscle on axial T2-weighted fat-suppressed (FS) image (C) with moderate perilesional bone marrow oedema extending to the proximal femoral diaphysis. Axial apparent diffusion coefficient (ADC) map (D) shows lack of restricted diffusion with ADC values (≤ 1.0 × 10-3 mm2/s) ranging from 0.8 to 1.2. There is late arterial enhancement of this lesion (arrow) on dynamic contrast-enhanced-maximum intensity projection (E) and confluent internal enhancement (arrow) on coronal T1-weighted FS post-contrast magnetic resonance image through the right femur (F). Of note, imaging features of the langerhans cell histiocytosis, particularly in the axial skeleton, can overlap with infection as well as malignancy because it demonstrates aggressive MRI features including perilesional oedema, perilesional enhancement, periostitis, endosteal scalloping, and a soft tissue mass