A 4-year-old female presented with left knee pain and swelling for one year. Imaging showed a lytic lesion with sclerotic borders near the physis. MRI revealed decreased T1 signal, fluid signal centrally, and an enhancing rim, consistent with an abscess. Intraoperative curettage confirmed the diagnosis of Brodie's abscess, a localized form of osteomyelitis presenting as a well-circumscribed bone destruction surrounded by reactive sclerosis. Brodie's abscess is usually caused by hematogenous spread of bacteria like Staphylococcus aureus to the bone.

1. History: 4 year old female presents to ED with pain and limited range of motion to left knee

2. (history continued) Patient has approximately 1 year history of left knee pain and swelling. Recently treated with naproxen only.

3. What do you see? Lytic lesion with sclerotic boarders abutting the physis What is your differential diagnosis? Location…location…location

4. Benign or malignant? Well defined, sclerotic boarders, relatively narrow zone of transition… LIKELY BENIGN

5. T1 T2 T1 with Gad Decreased signal Fluid signal centrally Enhancing rim Large soft tissue collection Appearance consistent with abscess Where did it start? Bone or soft tissue? Usually hematogenous to bone with soft tissue extension

6. Extensive symptoms for a year T2

7. Intraoperative Curettage

8. Diagnosis Brodie’s Abscess

9. The differential diagnosis of Brodie's abscesses radiologically Osteoid osteoma (look for central nidus) Nonossifying fibroma Giant cell tumor Eosinophilic granuloma Chondroblastoma (occurs in the epiphysis – end of bone) Fibrous dysplasia (shows marked sclerosis) Aneurysmal bone (our lesion non-expansile) Metastatic lesion (neuroblastoma)

10. Radiographic appearance Brodie's abscess is localized form of osteomyelitis, is usually found in the cancellous tissue near the end of the long bone. A well-circumscribed area of bone destruction has a surrounding zone of reactive sclerosis, sometimes accompanied by a periosteal reaction. It may have a finger-like extension into the neighbouring bone towards the epiphyseal plate, which, when present, is pathognomonic of infection tunneling.

11. Brodie’s Abscess Acute hematogenous osteomyelitis most commonly seen in children and characterized by accumulation of the pathogenic organisms in the terminal arterioles and capillaries of the bone metaphysis. As edema and granulation occur, the intraosseous pressure may increase and result in bone necrosis due to compression of the vascular structures. These may lead to formation of a Brodie's abscess. In adults other pathogenic mechanisms of osteomyelitis are more common and include traumatic inoculation and spread from a nearby infected focus

12. Pathologically, the wall of the abscess contains large amounts of granulation tissue, accounting for pronounced rim enhancement on contrast-enhanced MRI or CT scans. The central portions are mainly constituted by necrotic fluid and pathologic organisms. Staphylococcus aureus is cultured in half of the cases. The abscess is commonly surrounded by inflammatory changes and edema of adjacent bone marrow. Transcortical fistulization may lead to soft tissue spread.

13. Brief Review of Lodwick classification system The (revised) Lodwick classification system consists of five grades: IA, IB, IC, II, and III.

14. Brief Review of Lodwick classification system Determine the type of bone destruction present in the lesion: Grade I : A lesion with geographic destruction would be defined as a lesion having a sharp, clearly defined margin. Grade II : Moth-eaten destruction is similar to moth-eaten clothes with holes of destroyed bone. Grade III: Permeative destruction ill-defined, diffuse, somewhat subtle destructive process of bone Lodwick often found it difficult to differentiate between grade II and III lesions, but it does not really matter because both grades indicate an aggressive lesion that needs further evaluation and/or treatment.

15. For Grade I Lesions: Evaluate margin of the lesion, including any cortex that the lesion abuts Evaluate lesion for expansion . If an expanded cortical shell is present and it exceeds 1 cm, then the lesion is classified as grade IB. Evaluate lesion for presence of a circumferential sclerotic margin . If the lesion has a sclerotic margin, it is classified as grade IA. Those with a nonsclerotic margin are classified as grade IB. Grade I lesions are “Geographic = sharply defined border” and Grade I lesions can be broken down into: A: Thick sclerotic rim B: Thin sclerotic rim C: No sclerotic rim

16. Rate of malignancy as predicted by classification system Grade IA is 6%, Grade IB is 48%, Grade IC is 36%, Grade II is 97%, grade III is 100%. Our case is a Grade 1A…

17. Acknowledgements This radRounds Weekly Rounds has been presented by: Laura Avery, MD Massachusetts General Hospital Harvard Medical School Boston, MA, USA