High-grade chondrosarcoma mimicking Brodie's abscess

Abstract

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. 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. The importance of the penumbra sign in diagnosing isolated soft-tissue infection has also been mentioned. Subacute osteomyelitis of the long bones is not an uncommon disease entity, with Brodie’s abscess being the most common type. 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, in particular, the penumbra sign on MRI has been proven to 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.