Xanthogranulomatous osteomyelitis (XO) is a very rare chronic granulomatous inflammation. It can mimic malignant bone tumors on its clinical presentation, gross features, and radiological imaging. However, histopathological examination can differentiate it from malignancy. We describe the case of a 20-year-old male presented with fever and pain in the right knee joint for 4 months. Plain radiography and magnetic resonance imaging of the right knee joint revealed osteolytic lesion in the lower metaphyseal region of femur. With this clinical presentation and radiological imaging, a diagnosis of primary bone tumor was made. However, XO was confirmed by histopathological examination. This case highlights the rare occurrence and also it can mimic as bone tumor.

Introduction

Xanthogranulomatous reaction is a rare form of chronic inflammation, histologically consisting of collections of foamy histiocytes admixed with polymorphonuclear leukocytes, lymphocytes, and activated plasma cells.[1],[2] This entity has been described in various organs; however, rarely, it can occur in organs such as lung, brain, and bone.[1],[3] The presence of xanthogranulomatous inflammation in the bone is known as xanthogranulomatous osteomyelitis (XO). It can present mass-like lesion extending to adjacent structures and can mimic infiltrative carcinoma.[4] To the best of our knowledge, only 10 cases involving bone have been reported so far in literature.[1],[2],[3],[5],[6],[7],[8],[9] We report the 11th case of XO with a brief review of literature.

Case Report

A 20-year-old male presented with fever, pain, and swelling in the right knee joint for 4 months. Pain aggravated on walking with no history of night pain/trauma. On local examination, tenderness was present in the lower end of right femur with no joint line tenderness and normal range of movements of knee. Systemic examination was normal. Complete blood count was within normal range. Serum parathormone and alkaline phosphatase were also normal. X-ray of right knee joint showed suspicious lytic lesion with sclerotic margin in the lower metaphyseal region of the femur [Figure 1]a. Magnetic resonance imaging of the right knee joint revealed hyperintense lesion in the lower metaphyseal region of the femur with well-defined sharp margin and minimal hyperintensity in the adjacent part of the femur [Figure 1]b. A clinical suspicion of primary bone tumor was entertained. The involved bone was curetted and specimen was sent for histopathological examination and also for culture.{Figure 1}

Grossly, the specimen was consisting of two gray brown bony tissue bits. Microscopically, there were sheets of foamy macrophages, exuberant lymphoplasmacytic inflammation along with dead bone [Figure 1]c. There was no evidence of malignancy or granuloma. Periodic acid–Schiff stain was positive for foamy macrophages [Figure 1]d. Ziehl–Neelsen stain for tissue of acid-fast bacilli was negative. Microbiological culture of the tissue sent at the time of surgery failed to reveal the growth of organisms after 48 h of aerobic incubation. The postoperative period was uneventful.

Discussion

The first two cases of this entity were described by Cozzutto in 1984.[1] The previously reported cases of XO are summarized in [Table 1]. In most of the previously reported case reports, it was clinically as well as radiologically mimicking malignancy. Hence, histopathological conformation is mandatory for proper diagnosis and management of patients.{Table 1}

XO has to be differentiated from the following conditions to arrive at the correct diagnosis: Langerhans cell histiocytosis (LCH), Erdheim-Chester disease, xanthoma, storage disease, malakoplakia, and metastatic renal cell carcinoma.[1],[2],[3]

LCH microscopically consists of Langerhans cells having reniform nuclei and eosinophils infiltration. X-ray and computed tomography scan typically reveal circumscribed lytic bony lesions without surrounding sclerosis.[1],[10],[11]

Erdheim–Chester disease is a multifocal disorder with frequent involvement of extraskeletal tissues. Histology shows foamy histiocytes, cholesterol clefts, and fibrosis without neutrophilic infiltrations.[1]

Xanthoma can occur in bone, secondary to hyperlipidemia. However, it may not show suppurative inflammation.[1]

Malakoplakia is described in bone, kidney, and other organs, and more common in immunosuppressed individuals. Histologically, it shows large foamy macrophages and Michaelis–Gutmann bodies, which was not seen in our case.[12]

In case of storage diseases, clinical background, foamy macrophages within the bone marrow, and absence of suppurative component can differentiate the XO.[1]

Metastatic renal cell carcinoma can be easily differentiated by history and histologic tumor patterns.[1]

Conclusion

We present this case primarily due to its rarity, curability, and importantly, it can mimic primary or secondary tumor on clinical, radiological, and gross examination. Hence, radiologists must be aware of the possibility of the XO, and all the cases require histopathological confirmation for proper management. Currently, histopathological examination is most specific for the diagnosis of XO.

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Conflicts of interest

There are no conflicts of interest.