Author's personal copy Clinical Radiology (2009) 64, 944e947
CASE REPORT
High-grade chondrosarcoma mimicking Brodie’s abscess A. Datira,*, S. Lidderb, R. Pollockb, R. Tiraboscoc, A. Saifuddind a
Department of Radiology, Frenchay Hospital, North Bristol NHS Trust, Bristol, bThe London Sarcoma Unit, and Departments of c Pathology and dRadiology, Royal National Orthopaedic Hospital NHS Trust, London, UK Received 28 August 2008; received in revised form 17 March 2009; accepted 1 April 2009
Introduction The ‘‘penumbra sign’’ on unenhanced T1-weighted (T1W), spin-echo (SE) magnetic resonance imaging (MRI) is a well-recognized and characteristic finding in subacute osteomyelitis. The description of the penumbra sign was originally published by Grey et al. in 1998.1 The penumbra sign is considered to be an extremely helpful discriminator between subacute musculoskeletal infection and tumour. This sign describes a rim lining an abscess cavity with higher SI than that of the main abscess on T1W SE images. In a recent study, the average specificity and sensitivity of the penumbra sign for musculoskeletal infection has been reported as 96% (range 94e99%) and 27% (range 21e34%), respectively.2 The importance of the penumbra sign in diagnosing isolated soft-tissue infection has also been mentioned.2 Subacute osteomyelitis of the long bones is not an uncommon disease entity, with Brodie’s abscess being the most common type.3,4 The clinical diagnosis of subacute osteomyelitis may pose a challenge owing to lack of systemic illness or localizing signs of infection, and frequently normal laboratory investigations. Moreover, the presence of features like fever, localized bone pain, and elevated inflammatory markers are common to subacute osteomyelitis as well as tumour. The role of imaging, in particular MRI, is highly valuable in differentiating infection from tumour,5e7 in particular, the penumbra sign on MRI has been proven to * Guarantor and correspondent: A. Datir, Chalet No. 4, Staff Residence, Frenchay Hospital, Frenchay Park Road, Bristol, BS16 1LE, UK. Tel.: þ44 117 340 2757; fax: þ44 208 869 3907. E-mail address:
[email protected] (A. Datir).
be a helpful feature in making a diagnosis of infection. We describe a case of histopathologically confirmed grade II intramedullary chondrosarcoma of the proximal femur with MRI feature of a positive penumbra sign. The finding of the penumbra sign in chondrosarcoma has been briefly mentioned previously in the literature.1
Case report A 45-year-old woman presented with left-sided hip pain after a fall. She was admitted with a mild ache in the left hip and proximal thigh associated with fever and malaise. There was no relevant past medical history and clinical examination of the hip was normal. Preliminary investigations revealed an erythrocyte sedimentation rate of 16 mm/h, C-reactive protein of 7 mg/l, serum alkaline phosphatase of 64 IU/l, and a white blood count of 8800/mm3. Radiographs of the left thigh (Fig. 1) showed a well-defined, intramedullary, osteolytic lesion involving the proximal diaphysis of the femur and measuring 6.1 cm (craniocaudal) ! 2.7 cm (transaxial) ! 2.6 cm (anteroposterior) in size. It was associated with mild bone expansion and marginal sclerosis. The overlying cortex was thickened and intact, and there was no demonstrable soft-tissue mass or associated matrix calcification. Further evaluation using MRI (Fig. 2) demonstrated the lesion to have four separate layers: a centre, an inner ring, an outer ring, and a peripheral halo. The centre had intermediate SI on T1W SE images (Fig. 2a). This layer was hyperintense on T2W fast SE (Fig. 2b) and short tau inversion recovery (STIR; Fig. 2c) sequences. The inner ring was mildly hyperintense
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with muco-myxoid degeneration of the matrix and peripheral reactive fibroblastic proliferation were also found. There was no evidence of de-differentiation. Based on these findings, a histopathological diagnosis of grade II/III intramedullary chondrosarcoma was made. An orthopaedic surgical treatment involving proximal femoral excision and endo-prosthetic replacement was carried out. The patient subsequently made a full recovery and was walking unaided.
Discussion
Figure 1 Anteroposterior radiograph of the left proximal femur demonstrating a well-defined, oval, lytic lesion with a thin sclerotic margin and associated solid periosteal reaction.
on T1W, T2W, and STIR images and showed strong enhancement following administration of gadolinium contrast medium (Fig. 2d and e). The outer ring was hypointense on T1W, T2W, and STIR images. The peripheral halo showed hypointensity on T1W images, and mixed high and intermediate SIs on T2W images, showing mild enhancement following contrast medium administration, consistent with reactive medullary oedema. There was also evidence of mild active periostitis. After a multidisciplinary panel discussion involving radiologists and orthopaedic surgeons, a presumed diagnosis of Brodie’s abscess was made and decision was taken to perform primary curettage of the lesion. No bacteria were grown from the curetted material on routine aerobic and anaerobic cultures, or on culture for tuberculous and fungal organisms. However, histological appearance of the obtained tissue (Fig. 3) showed tumour cells surrounded by a large amount of chondroid matrix with moderate cellularity, enlarged hyperchromatic nuclei with occasional spindle-shaped appearance and cyto-nuclear atypia. A few mitoses
Marti-Bonmati et al. 8 first described the ‘‘target’’ appearance of Brodie’s abscess on MRI, which comprised four separate layers: a centre, two rings, and a peripheral halo. The central region represents the abscess cavity with high-protein content consisting of purulent or mucoid components, and appearing hypointense on T1W and hyperintense on T2W and STIR images. The inner ring corresponds to the granulation layer, which is composed of inflammatory cells and connective tissues. On MRI, the inner ring appears isointense or mildly hyperintense to muscle on T1W and hyperintense on T2W images. Owing to its rich vascular supply, this layer shows intense enhancement after intravenous contrast medium administration. The outer ring corresponds to a zone of reactive sclerosis and spongy bone eburnation, and as one would expect, is hypointense on all pulse sequences. The peripheral halo represents the imaging equivalent of bone marrow oedema, appearing hypointense on T1W images, hyperintense on T2W and STIR sequences and showing enhancement following contrast medium administration. Grey et al. 1 first described the importance of the penumbra sign in the imaging diagnosis of subacute osteomyelitis. Occasionally, the occurrence of this sign has also been described in other disease entities, including eosinophilic granuloma 9 and intra-osseous ganglia.10 The penumbra sign refers to a zone of transition with relatively high SI on T1W images between the abscess and reactive sclerosis. This transitional zone corresponds to the inner ring described by Marti-Bonmati et al.8 On histopathological examination, it represents the vascularized granulation tissue wall with a variable water content, which in turn dictates the degree of hyperintensity on T1W images. Also, histological examination does not reveal any evidence of subacute haemorrhage to explain the hyperintensity.1 The rich vascular supply accounts for the strong enhancement of this rim following contrast medium administration.
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Figure 2 Coronal, T1W SE (a), axial T2W fast SE (b) and coronal STIR (c) MRI images through the proximal femur demonstrating the penumbra sign (arrow in a), mild peri-osseous oedema (arrows in b), and intramedullary oedema proximal and distal to the lesion (arrows in c). Coronal (d) and axial (e) contrast-enhanced, T1W SE images showing intense enhancement of the penumbra (arrows in dee) and mild enhancement of the peri-osseous oedema (arrowheads in e).
The present case clearly demonstrated the typical four-layer appearance and the penumbra sign on MRI. The penumbra sign was seen as a thin, mildly hyperintense ring with strong enhancement following intravenous contrast medium administration on T1W images. The diagnosis of Brodie’s abscess was based on these MRI findings. In addition, there were no imaging findings, such as the presence of matrix mineralisation, significant endosteal scalloping, wide zone of transition, associated soft-tissue mass, or cortical destruction, to suggest a differential diagnosis of a highgrade primary bone tumour. Chondrosarcoma is a malignant bone tumour characterized by the formation of a cartilaginous matrix by tumour cells, with conventional
Figure 3 Haematoxylin and eosin stained section showing features of high-grade chondrosarcoma.
Author's personal copy High-grade chondrosarcoma mimicking Brodie’s abscess
intramedullary chondrosarcoma being the most common type of primary chondrosarcoma. Patients with conventional chondrosarcoma most commonly present in fourth to fifth decades of life with a male-to-female ratio of 1.5e2 to 1. Clinical symptoms are non-specific, with pain being the most frequent, occurring in at least 95% of patients. The most common skeletal location for conventional chondrosarcoma is the long tubular bones, with the femur being the single most commonly affected bone (20e45% of cases). Long tubular bone lesions most commonly involve the metaphysis (49%), followed by diaphysis (36%) and are more common in a proximal location. As for subacute osteomyelitis, MRI is the best method for depicting the extent of marrow involvement by conventional intramedullary chondrosarcoma. On T1W images, marrow replacement appears low to intermediate SI, whereas on T2W images the non-mineralized components of chondrosarcoma have high SI corresponding to the high water content of hyaline cartilage. Matrix mineralization is common in conventional chondrosarcoma and is seen in approximately 79% of cases. The bone response in an attempt to contain the intramedullary chondrosarcoma in the marrow cavity is evidenced by cortical remodelling, cortical thickening, and periosteal reaction. Perilesional oedema is a common finding in both high-grade conventional chondrosarcoma and subacute osteomyelitis. Usually, the differentiation of high-grade intramedullary chondrosarcoma from sub-acute osteomyelitis is straightforward. This can usually be achieved on imaging by the presence of matrix mineralization, a lobular growth pattern, and associated soft-tissue mass. However, these features are not present in all cases of conventional chondrosarcoma, particularly for grade 1 and 2 intramedullary chondrosarcoma,11 and were not identified in the current patient. The penumbra sign is considered to be a characteristic imaging feature of subacute osteomyelitis with a high degree of specificity. It consists of a thin hyperintense rim on T1W images, which can be mimicked by subacute haemorrhage. The appearance of the slightly hyperintense transitional zone (‘‘the penumbra’’) on T1W is rendered more conspicuous by the relative lack of signal on either side. On the external side, it is caused by reactive sclerosis, which may occur in both subacute osteomyelitis and conventional chondrosarcoma. In subacute osteomyelitis, the relative central hypointensity on T1W is due to the central abscess cavity with purulent or mucoid components. This can be compared with the low to intermediate SI
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seen in conventional chondrosarcoma due to marrow replacement by malignant chondroid matrix. The presence of four separate layers in a case of grade II/III intramedullary chondrosarcoma may be compared with subacute osteomyelitis. The central zone represents the abscess cavity in subacute osteomyelitis and primary tumour mass in conventional chondrosarcoma. Also, the outer ring of sclerosis and perilesional oedema is a known finding in both entities. Peripheral ‘‘rim’’ enhancement is also a feature of chondrosarcoma, but is typically also associated with internal linear enhancement due to the presence of fibrovascular septa around and between lobules of chondroid matrix. The latter internal septal enhancement was not seen in the current case, adding to the difficulty in suggesting the correct diagnosis. In summary, we present a case of grade II intramedullary chondrosarcoma with the MRI feature of a positive penumbra sign, resulting in a misdiagnosis of Brodie’s abscess. The penumbra sign on MRI is considered to facilitate the diagnosis of subacute osteomyelitis. However, the reporting radiologist should be aware of a possible, although uncommon, differential diagnosis of conventional chondrosarcoma. Whenever doubt exists, biopsy of the lesion should be favoured over primary curettage, even when the diagnosis is thought to be infection rather than tumour.
References 1. Grey AC, Davies AM, Mangham DC, et al. The ’penumbra sign‘ on T1-weighted MR imaging in subacute osteomyelitis: frequency, cause, and significance. Clin Radiol 1998;53:587e92. 2. McGuinness B, Wilson N, Doyle AJ. The ’penumbra sign‘ on T1-weighted MRI for differentiating musculoskeletal infection from tumour. Skeletal Radiol 2007;36:417e21. 3. King DM, Mayo KM. Subacute haematogenous osteomyelitis. J Bone Joint Surg Br 1969;51:458e63. 4. Stephens MM, MacAuley P. Brodie’s abscess; a long-term review. Clin Orthop 1988;234:211e6. 5. Tang JSH, Gold RH, Bassett LW, et al. Musculoskeletal infection of the extremities: evaluation with MR imaging. Radiology 1988;166:205e9. 6. Quinn SF, Murray W, Clark RA, et al. MR imaging of chronic osteomyelitis. J Comput Assist Tomogr 1988;12:113e7. 7. Boriani S. Brodie’s abscess: a study of 181 cases, with reference to radiographic diagnostic criteria. Ital J Orthop Traumatol 1980;6:373e83. 8. Marti-Bonmati L, Aparisi F, Poyatos C, et al. Brodie abscess: MR imaging appearance in 10 patients. J Magn Reson 1993; 3:543e6. 9. Davies A, Grimer R. The penumbra sign in subacute osteomyelitis. Eur Radiol 2005;15:1268e70. 10. Williams H, Davies A, Allen G, et al. Imaging features of intraosseous ganglia: a report of 45 cases. Eur Radiol 2004;14:1761e9. 11. Murphey MD, Walker EA, Wilson AJ, et al. Imaging of primary chondrosarcoma: radiologicepathologic correlation. RadioGraphics 2003;23:1245e78.
