Bone Tumours (1): Radiological Approach, Benign Tumours and Tumour-Like Lesions of Bone

Age at Presentation

Patient age is of huge importance in the differential diagnosis of a focal bone lesion. Primary bone tumours are rare below the age of 5 years and over the age of 40 years, with the exception of myeloma, in which 97% of patients present after the age of 40 years, and chondrosarcoma, which typically presents in the fourth to fifth decades. Metastases are the commonest lesions in patients over 40 years of age.

Location

The location of the lesion within the skeleton (appendicular, axial) and within the individual bone (epiphysis, metaphysis, diaphysis; intramedullary, intracortical, surface) must be considered in detail when individual tumours are discussed, as this has a considerable influence on the differential diagnosis.

Rate of Growth

With regard to rate of growth, the most important feature is the lesion margin. In benign and low-grade malignant neoplasms, this margin is sharp (geographical; type 1). Type 1A has a sclerotic rim ( Fig. 40.1A ); type 1B is a well-defined lytic lesion with no marginal sclerosis (see Fig. 40.1B ); and type 1C has a slightly less sharp, non-sclerotic margin (see Fig. 40.1C ). Type 2 is moth-eaten destruction, which represents the next most aggressive pattern and is characterised by multiple lucent areas measuring 2 to 5 mm in diameter separated by intact trabecular bone ( Fig. 40.2 ). Type 3 is permeative destruction, which is the most aggressive pattern and is composed of multiple coalescing tiny ill-defined lesions 1 mm or less in diameter, with a zone of transition of several centimetres ( Fig. 40.3A ). Radiographs inevitably underestimate the extent of medullary involvement, which is more clearly demonstrated by magnetic resonance imaging (MRI). Regions with an apparently intact cortex may show extra-cortical tumour masses occasionally resulting in ‘cortical saucerisation’, as the tumour, temporarily restrained by the periosteum, erodes back through the cortical bone—a feature typical of Ewing sarcoma (see Fig. 40.3B ).

Benign or low-grade malignant neoplasms tend to remain within the medullary cavity until late in their development. Typically, the cortex is not destroyed, but slow erosion of its endosteal surface (endosteal scalloping), together with periosteal new bone formation, results in expansion of bone. Conversely, high-grade malignant tumours commonly extend through the cortex by the time of presentation, resulting in cortical destruction and an adjacent extra-osseous mass (see Fig. 40.3B ).

Periosteal Reaction

Periosteal reaction is of various types with none being pathognomonic of any particular tumour; rather, the type helps to indicate the aggressiveness of the lesion. A thick, well-formed, solid periosteal reaction ( Fig. 40.4A and B ) indicates a slow rate of growth but not necessarily a benign lesion, since it may be seen with grade 2 chondrosarcoma. A laminated periosteal reaction (see Fig. 40.1B ) indicates subperiosteal extension of tumour, infection or haematoma. Lesions demonstrating periodic growth may show a multilaminated pattern (see Fig. 40.4C ). A Codman triangle indicates the limit of subperiosteal tumour in a longitudinal direction (see Fig. 40.4D ). Vertical (sunburst spiculation or ‘hair-on-end’) types of periosteal reaction are seen with the most aggressive tumours such as osteosarcoma (see Fig. 40.4E ) and Ewing sarcoma (see Fig. 40.4F ). However, the most rapidly growing lesions may not be associated with any radiographically visible periosteal response because mineralisation of the deep layer of periosteum can take 2 weeks.

Matrix Mineralisation

The matrix of a tumour represents the extra-cellular material produced by the tumour cells within which the cells themselves lie. Certain tumours produce characteristic radiographically visible matrix mineralisation, which allows the histological cell type to be predicted. Chondral calcification is typically linear, curvilinear, ring-like, punctate or nodular ( Fig. 40.5A ). Osseous mineralisation is ‘cloud-like’ and poorly defined (see Fig. 40.5B ), whereas diffuse matrix mineralisation in benign fibrous tumours produces the characteristic ‘ground-glass’ appearance (see Fig. 40.5C ), seen most commonly in fibrous dysplasia (FD). Some neoplasms, such as adenocarcinoma metastases, can provoke reactive mineralisation, whereas calcifications within an intra-osseous lipoma (IOL) are due to associated fat necrosis (see Fig. 40.5D ). Also, some bone sarcomas may develop on underlying calcified bone infarcts.

Computed Tomography and Magnetic Resonance Imaging in Diagnosis and Staging

Computed tomography (CT) is excellent for demonstrating the presence of radiographically occult matrix mineralisation ( Fig. 40.6A and B ); the persistence of a thin cortical shell indicates that the tumour still lies deep to the periosteum. CT also plays a major role in the investigation of cortical thickening, allowing demonstration of the cause, such as the nidus of an osteoid osteoma (OO; see Fig. 40.4B ) or a stress fracture.

The major role of Magnetic resonance imaging (MRI) is in local staging, particularly for high-grade malignant tumours such as osteosarcoma, where the intra- and extra-osseous extent, identification of ‘skip’ lesions and relationship to the neurovascular bundle and adjacent joint can all be assessed with great accuracy. Such information is essential for planning surgical management, be it limb salvage or amputation. Dynamic contrast-enhanced MRI has been advocated for determining chemotherapeutic response and may have a role in differentiating benign from malignant bone lesions, particularly in combination with other functional MRI techniques such as chemical shift and diffusion-weighted imaging (DWI). However, its role in routine patient management is still unclear.

In the presence of a purely lytic lesion, several MRI features may help in further lesion characterisation. The presence of profound low signal intensity (SI) on T ₂ weighted fast spin echo (T ₂ W FSE)/T ₂ * weighted gradient-echo (T ₂ W WGE) images indicates chronic haemorrhage and may be seen with giant cell tumour (GCT). MRI is very sensitive to the presence of fluid-fluid levels (FFLs), the extent of which is related to histological diagnosis. Lesions that are completely filled with FFLs are almost always aneurysmal bone cysts (ABCs). MRI can also demonstrate a fatty matrix, as seen with vertebral haemangioma and IOL. The vascular nature of renal metastases has been demonstrated by the presence of the ‘flow-void’ sign. MRI is very sensitive to reactive medullary and soft-tissue oedema, which characterises certain lesions such as osteoid osteoma (OO), osteoblastoma (OB), chondroblastoma (CB) and a Brodie abscess.

Bone scintigraphy now plays little role in the diagnostic work-up of a suspected primary bone tumour. However, whole-body bone scintigraphy is still useful for the identification of skeletal metastases, although this role has recently been challenged by whole-body MRI and fluoro­deoxyglucose-positron-emission tomography (FDG-PET)/CT. The combined functional and anatomical imaging capabilities of single photon emission CT (SPECT-CT) increases diagnostic confidence in OO and OB, particularly when these occur in atypical locations or with misleading or equivocal characteristics on conventional imaging techniques.

The value of MR spectroscopy, PET and FDG-PET/CT in the routine management of suspected bone tumours is still unclear. PET has a role in identification of local recurrence in osteosarcoma due to its high sensitivity and lack of susceptibility to metallic artefact; PET-MRI shows promise as a hybrid modality for tumour characterisation and staging as well as evaluation of treatment response, whereas whole-body diffusion-weighted MRI (WB-DWI) is highly sensitive for the identification of skeletal metastases.

Finally, the use of ultrasound and CT for image-guided needle biopsy is well established, and MR-guided biopsy has also been developed for targeting subtle marrow lesions.

Osteochondroma

Recent genetic studies indicate that osteochondroma (OC), also termed cartilage-capped exostosis, is a true neoplasm which may also arise following total body radiotherapy, Salter–Harris fractures and surgery. It accounts for 20%–50% of benign bone tumours. OC presents from 2 to 60 years, but the highest incidence is in the second decade, with a M:F ratio of around 2.5 : 1.

Long bones are commonly affected, especially around the knee (∼40%), the commonest locations being the distal femur, proximal humerus, proximal tibia and proximal femur. The commonest flat bones affected are the ilium and scapula. Lesions may be classified as either pedunculated, when they have a thin stalk that typically points away from the adjacent joint ( Fig. 40.7A ), or sessile, when they arise from a broad base (see Fig. 40.7B ). They are usually metaphyseal in location.

Diaphyseal aclasis (hereditary multiple exostoses [HMEs]) constitutes a rare autosomal dominant disorder in which the exostoses may be larger than the solitary variety and may lead to shortening or deformity of the affected limbs. The metaphyses in this condition are also typically widened and dysplastic (see Fig. 40.7C and D ).

OC presents with mechanical problems such as an enlarging mass, pressure on adjoining structures (muscles, vessels) or, rarely, with fracture of the bony stem. Mechanical irritation of overlying soft tissues may result in muscle oedema or adventitial bursa formation, which can mimic sarcomatous degeneration. MRI is highly accurate in the assessment of symptomatic OC. The incidence of chondrosarcomatous change in the cartilage cap is very small in a solitary OC (probably <1%), whereas malignant degeneration in HME is estimated at 3% to 5%.

Radiological Features

Continuity between the medullary cavity of the lesion and that of the underlying bone is essential for the diagnosis and is best demonstrated with either CT or MRI ( Fig. 40.8A and B ). The cartilage cap is optimally demonstrated on axial proton density–weighted (PDW) or T ₂ weighted FSE sequences, when the hyperintense cartilage contrasts well against the adjacent iso-/hypointense muscle (see Fig. 40.8B ). The thickness of the cartilage cap typically measures 1–3 cm in children and up to a few millimetres in adults but should not exceed 2 cm in adults. The cartilage cap can also be visualised and measured on ultrasound, where it appears hypoechoic in contrast to the brightly reflective bone surface (see Fig. 40.8C ). Complications associated with OC include soft-tissue impingement ( Fig. 40.9A ), bursa formation (see Fig. 40.9B ), neurovascular compromise and, rarely, pseudoaneurysm formation (see Fig. 40.9C ). Lesions arising on paired bones may result in chronic deformity of the adjacent bone (see Fig. 40.9D ) or synostosis across the adjacent joint (see Fig. 40.9E ).

Subungual Exostosis

Subungual exostosis is histologically distinct from OC and demonstrates no medullary continuity. Lesions develop on the dorsal surface of the distal phalanx and are thought to be the result of previous trauma or infection, most commonly involving the big toe.

Radiological Features

Subungual exostosis has a similar plain radiographic appearance to that of a classical OC; it is seen as a bony spur extending into the nail bed ( Fig. 40.10 ). A fibrocartilaginous cap, which is typically larger than the base of the lesion, may be seen on MRI.

Enchondroma

An enchondroma is an intramedullary neoplasm comprising lobules of benign hyaline cartilage; it is the second most common benign chondral lesion after OC, accounting for approximately 8% of all primary bone tumours and tumour-like lesions in a large biopsy series. However, the true prevalence of enchondromas is unknown because the majority are asymptomatic. Incidental enchondromas have been identified in approximately 3% of routine knee MRI studies. Enchondromas affect the tubular bones of the hands and feet in 40%–65% of cases and present either when they become symptomatic, due to increasing size, as pathological fractures (60% of cases) or as incidental findings. The majority arise in the proximal phalanges (40%–50%), followed by the metacarpals (15%–30%) and middle phalanges (20%–30%). The small bones of the feet are involved in 7% of cases. Approximately 25% arise in the femur, tibia and humerus; other sites are very rare. The age range is 10–80 years, with most presenting in the second to fourth decades. There is equal M:F prevalence.

Radiological Features

Most enchondromas arise centrally in the phalanges and metacarpals. Lesions are typically metaphyseal or diaphyseal, with epiphyseal location accounting for approximately 8%. Enchondromas are often eccentric and 75% are solitary. They are typically well-circumscribed, lobular or oval lytic lesions, which may expand the cortex ( Fig. 40.11A ). Size at presentation ranges from 10 to 50 mm and chondral-type mineralisation may be identified within the matrix (see Figs 40.5A and 40.6A ).

The term enchondroma protuberans has been used to describe an eccentrically placed enchondroma with an associated extra-osseous component, which is usually covered by a thin shell of intact cortical bone (see Fig. 40.11B ). Most cases arise in the fingers or toes; they may be difficult to distinguish from periosteal chondroma, although this may not be clinically relevant because both lesions have the same management.

The differentiation between a relatively large enchondroma and a grade 1 chondrosarcoma can be difficult. Lesion size above 5–6 cm and deep endosteal scalloping are suggestive of chondrosarcoma, but prominent scalloping can also be seen with an eccentrically placed chondroma, which has been termed an ‘endosteal chondroma’ (see Fig. 40.11C ). However, this differentiation may not be of clinical relevance, as both can be treated in the same way with either careful clinical and imaging follow-up or curettage with cementation. Low-grade chondral tumours have characteristic MRI features, showing a lobular margin with intermediate T ₁ weighted and T ₂ weighted SI ( Fig. 40.12A ), and hyperintensity on fat-suppressed PDW/T ₂ weighted/short tau inversion recovery (STIR) images without surrounding reactive oedema (see Fig. 40.12B ). Matrix mineralisation manifests as punctate areas of signal void, while areas of fat SI may be seen due to trapped medullary fat. A hypointense rim and septa may also be seen, the latter showing enhancement following intravenous administration of a gadolinium contrast medium.

Less Common Varieties of Chondroma

Periosteal chondroma.

Periosteal chondromas, affecting children and young adults, are rare lesions located in the metaphyses of tubular bones, most commonly the proximal humerus followed by the femur and tibia. They are also seen in the small bones of the hands and feet, in which case some extension into the underlying medullary cavity commonly occurs. Radiologically, the lesion appears as a well-defined area of cortical erosion typically measuring 1–3 cm with a mature periosteal reaction and sometimes a thin external shell of bone. Cartilaginous matrix mineralisation is observed in half the cases ( Fig. 40.13A ). MRI shows the features of a chondral lesion with a lobular hyperintense mass on T ₂ weighted images adjacent to the cortex (see Fig. 40.13B ). The differential diagnosis includes periosteal chondrosarcoma and periosteal osteosarcoma. Malignant transformation has not been reported.

Enchondromatosis (Ollier disease).

This is a rare disease with prevalence estimated at around 1 : 100,000. Ollier disease most commonly arises in the long bones, although it can also occur in the hands, feet and pelvis, often predominantly involving one side of the body. In addition to multiple enchondromata ( Fig. 40.14A ), flame-like rests of cartilage in the metaphyses impede bone growth and may result in bowing, angulation and bone enlargement (see Fig. 40.14B and C ). Malignant change is reported in 5%–30% of cases.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here