Revisiting the WHO classification system of soft tissue tumours: emphasis on advanced magnetic resonance imaging sequences. Part 1

Abstract

The World Health Organisation (WHO) classification categorises musculoskeletal soft tissue tumours (STT) based on their similarity to normal adult tissue. The most recent WHO classification provides an updated scheme that integrates biological behaviour as a distinguishing feature in each subcategory; STTs are further subdivided as benign, intermediate (locally aggressive or rarely metastasising), and malignant. Although malignant STTs are infrequent in routine orthopaedic radiology practice, musculoskeletal radiologists must be familiar with the imaging appearance of malignant STTs and distinguish them from their benign counterparts for appropriate management. Magnetic resonance imaging (MRI) is the ideal modality for the detection, characterisation, and local staging of STT. This review will discuss the most recent updates to the WHO classification of STT that are relevant to radiologists in a routine clinical practice with MRI correlation. The utility of advanced MRI sequences such as diffusion weighted imaging, dynamic contrast enhanced sequences, and magnetic resonance spectroscopy to provide insight into the biological behaviour of various STTs is highlighted.

Keywords: MRI; WHO classification; sarcoma; soft tissue tumours.

Figure 1

86-year-old man with atypical lipomatous tumour. There is a 13.2 × 12.9 × 31.7 cm recurrent macroscopic fat-containing mass occupying the medial and posterior compartments of the right thigh visible on coronal (A) and axial (B) T1-weighted and fat-suppressed T2-weighted (C) magnetic resonance (MR) images. Although there is suppression of macroscopic fat on axial T2-weighted fat-suppressed MR image through the right thigh (C), MRI features concerning for atypical lipomatous tumour (ALT) including multiple thick septations, deep location, and large size are noted. Proximally and distally, the mass surrounds the sciatic nerve (dotted arrows on images A and B). There is a small defect in the right vastus lateralis muscle with intervening fat and superficial fascia with extra-compartmental extension into the subcutaneous tissues (denoted by an ^* on images B and C and anterior thigh compartment (long arrows on images B and C. On routine non-contrast and contrast-enhanced routine MRI sequences, lesion size, tissue layer (deep to the superficial fascia), location (proximal lower limb), incomplete fat suppression, or increased architectural complexity (thick septations > 2 mm and nodularity > 1 cm) have been shown to be independently predictive of ALT rather than benign lipoma

Figure 2

67-year-old woman with elastofibroma dorsi. Axial T1-weighted magnetic resonance (MR) image through the left chest wall (A) shows an unencapsulated mass-like abnormality deep to the scapula (arrow) with corresponding indistinct elevated signal on ADC map (B), late arterial enhancement on coronal DCE MIP (C), and confluent enhancement on static T1-weighted fat-suppressed post-contrast axial MR image (D). The mass was biopsied and confirmed to be elastofibroma

Figure 3

20-year-old man with right paraspinal mass. Axial T1-weighted (A), T2-weighted (B), and T1-weighted post-contrast (C) magnetic resonance (MR) images through the thoracolumbar junction show a right paraspinal soft tissue mass (arrows) that was initially characterised as an indeterminate soft tissue mass suspicious for peripheral nerve sheath tumours (PNSTs) based on target sign visible on T2-weighted image (B) and paraspinal location. The patient underwent a biopsy, which was interpreted as a malignant PNST at an outside facility. The histological slides were re-interpreted as myositis ossificans (MO), cellular phase. Sequential axial computed tomography images through the lower thoracic spine at the time of initial presentation (D), in the intermediate stage (E), and late stage (F) reveal progressive zonal ossification peripherally compatible with MO (arrows). Although the presence of extensive perilesional muscle oedema is a useful imaging feature for the distinction of early/intermediate stage MO from sarcoma, it is not always visible, as in this case

Figure 4

32-year-old man with palmar fibromatosis. There is an unencapsulated, nodular soft tissue mass arising from the proximal palmar aponeurosis extending along the subcutaneous tissues (arrows) that is isointense to skeletal muscle on axial T1-weighted (A), hyperintense to skeletal muscle on axial T2-weighted fat-suppressed (B) with confluent internal enhancement on axial (C) and sagittal (D) T1-weighted fat-suppressed subtraction magnetic resonance (MR) images through the hand. Fascial tails along the aponeurosis are visible (dotted arrow on image D). Coronal dynamic MR angiogram image (E) shows late arterial enhancement, a feature of benign disease

Figure 5

40-year-old woman with desmoid type fibromatosis. Desmoids-type fibromatosis exhibits similar signal characteristics to palmar/plantar fibromatosis with fascial tails but tends to occur in the trunk. Axial T1-weighted magnetic resonance image through the right anterior chest wall (A) shows an unencapsulated mass at the costochondral junction (arrow) with a corresponding indistinct low signal on T2-weighted image (B), and confluent enhancement on static T1-weighted fat-suppressed post-contrast axial image (C). Fascial tails (dotted arrows) along the chest wall are visible

Figure 6

35-year-old male with dermatofibrosarcoma protuberans (DFSP). There is a 4.8 × 2.0 × 0.9 cm multilobulated T2 hyperintense (A), homogeneously enhancing dermal-based lesion (arrow) on axial T1-weighted post-contrast fat-suppressed image (B) through the pelvis located in the region of the left inguinal crease, with plaque-like thickening of the skin. DFSP, previously classified as a skin tumour, is a new addition in the intermediate (rarely metastasising) sub-group of fibroblastic/myofibroblastic soft tissue tumours

Figure 7

66-year-old man with malignant fibroblastic/myofibroblastic tumour. Axial T2-weighted fat-suppressed magnetic resonance image through the right proximal thigh (A) shows an encapsulated 12 cm heterogeneous centrally higher signal intensity soft tissue mass located deep to the superficial fascia (arrow) with corresponding low-signal/restricted diffusion on ADC map (B), early arterial enhancement on coronal DCE MIP (C), and heterogeneous enhancement with intralesional necrosis on static coronal T1-weighted fat-suppressed post-contrast coronal image (D). The mass was biopsied and con- firmed to be grade 2 myxofibrosarcoma

Figure 8

16-year-old girl with neurofibroma. Axial T2-weighted fat-suppressed (A) and T1-weighted fat-suppressed post-contrast (B) magnetic resonance( MR) images through the left proximal thigh show a small enhancing soft tissue mass along the sciatic nerve (arrows) with a target sign seen in (A). DWI using low b-value of 50 s/mm² (C), intermediate b-value of 400 s/mm² (D), high b-value of 800 s/mm² (E), and lastly the ADC map (F) (arrows) show a soft tissue mass with a target sign: suggesting preserved zonal histological architecture of a peripheral nerve sheath tumours (PNST). A morphological target sign can be visible on fluid-sensitive and contrast-enhanced MRI sequences and indicates a benign PNST. The qualitative presence of target sign is more often seen on DWI using high b-values and ADC mapping based on the intrinsic T2 properties of a PNST when compared with routine MRI sequences, and suggests the preservation of the PNST’s internal histological architecture. Mean PNST diameter (> 4.2 cm) and minimum ADC values (≤ 1.0 × 10-3 mm2/s) have been found to accurate quantitative metrics that can be used to distinguish malignant PNSTs from benign PNSTs

Figure 9

17-year-old male with reported right-sided neck swelling found to be synovial sarcoma. On coronal (A) and axial (B) fluid-sensitive fat-suppressed magnetic resonance (MR) images through the neck, there is a circumscribed somewhat heterogeneous high signal intensity mass centred within the right parapharyngeal and submandibular spaces (arrow), measuring approximately 3.6 × 3.8 × 8.1 cm (AP by TR by CC). C) The mass demonstrates avid slightly heterogeneous enhancement on the T1-weighted fat-suppressed axial MR image (arrow). By routine MRI sequences, the lesion is characterised as an indeterminate solid soft tissue mass that requires biopsy

Figure 10

30-year-old woman with recurrent clear cell sarcoma. Axial T2-weighted magnetic resonance (MR) image (A) and ADC map (B) through the upper lumbar spine show a right paraspinal unencapsulated soft tissue mass (arrow) of heterogeneously low signal intensity. There is qualitative restricted diffusion on DWI using low b-value of 50 s/mm² (C), intermediate b-value of 400 s/mm² (D), and high b-value of 800 s/mm² (E) with lack of signal loss on DWI with increasing b-values. On the axial ADC map (B), there is qualitative restricted diffusion or low signal and quantitatively low ADC values, compatible with malignancy. Sagittal STIR MR image (F) shows perilesional oedema and internal septations within the mass (arrow). There is early arterial enhancement on DCE MIP (G) and heterogeneous enhancement on axial T1-wighted fat-suppressed post-contrast axial MR image (H) (arrows). Features of malignancy include restrictive diffusion on DWI/ADC mapping and early arterial enhancement

Figure 11

61-year-old woman with grade 3 undifferentiated pleomorphic sarcoma. There is a large 9.6 × 1.5 × 7.4 cm soft tissue mass in the left gluteus maximus muscle (arrows). The mass is hyperintense relative to skeletal muscle T2-weighted fat-suppressed magnetic resonance (MR) image through the left hemipelvis (A) and isointense to skeletal muscle on T1-weighted image (B) with moderately heterogeneous enhancement on post-contrast T1-weighted fat-suppressed image (C). Aggressive MRI features include presence of tumoural necrosis and perilesional oedema/enhancement. There is no extra-compartmental extension