Soft tissue abscess masquerading as soft tissue sarcoma: a case report

Dr Sivakumar S P  ·  Dr Muthulakshmi S  ·  Dr Dheenadhayalan J  ·  Dr Rajasekaran S

Department of Orthopaedics, Ganga Hospital, Coimbatore, Tamil Nadu, India

Corresponding author: Dr Sivakumar S P · drspsiva@gmail.com

 

Keywords

Soft-tissue abscess; soft-tissue sarcoma; tumour mimic; musculoskeletal infection; MRSA; Staphylococcus aureus; MRI; PET-CT; diagnostic pitfall; image-guided biopsy and culture; source control; primary ciliary dyskinesia; Kartagener syndrome; bone and joint infection.

Abstract

• Background — Deep musculoskeletal infection can closely mimic soft-tissue sarcoma, both clinically and on imaging; misinterpretation risks either an unnecessary oncological resection or a dangerous delay in source control.
• Case — A 26-year-old woman presented with two weeks of insidious, progressive right distal-thigh / knee pain and swelling, worse at night. She was afebrile with digital clubbing; WBC was 16,400/µL (81.5% neutrophils) and CRP 156.56 mg/L.
• Imaging — MRI showed a multilobulated soft-tissue mass (10.2 × 7 × 5.4 cm) of the posterolateral distal femur, and PET-CT an FDG-avid necrotic lesion (7.5 × 4.8 cm) — both reported as an aggressive soft-tissue sarcoma (? synovial).
• Turning point — Image-guided biopsy yielded frank pus; culture grew MRSA. Repeat MRI showed a thick-walled complex collection (≈11 × 8.4 cm).
• Management & outcome — Drainage, debridement and lavage with culture-directed antibiotics; the infection resolved and the patient became ambulatory. Underlying primary ciliary dyskinesia (Kartagener's syndrome) was confirmed on genomic sequencing.
• Conclusion — No clinical, biochemical or imaging feature — including PET-CT — reliably separates infection from sarcoma. Biopsy sent for both histology and culture is decisive.

Background

Deep musculoskeletal infections can closely mimic soft-tissue sarcomas, both clinically and on imaging — each may present as an enlarging, deep, painful mass showing necrosis, rim enhancement and intense FDG uptake. This overlap is a well-recognised diagnostic pitfall: misinterpretation risks either an unnecessary oncological resection or a dangerous delay in source control. We report a young woman whose thigh collection was reported as an aggressive sarcoma on both MRI and PET-CT, yet proved to be a methicillin-resistant Staphylococcus aureus (MRSA) abscess.

Case presentation

History

• 26-year-old female schoolteacher.
• Right distal-thigh / knee pain for 2 weeks; insidious, progressive.
• Worse on walking and at night, with swelling.
• No fever, weight loss or anorexia.
• Lifelong recurrent respiratory infections; hypothyroid; treated for infertility.

Examination

• Afebrile, haemodynamically stable.
• Diffuse, warm swelling over the lateral distal thigh.
• Painful, restricted range of motion.
• Digital clubbing; neurovascularly intact.

Investigations

WBC: 16,400 /µL (neutrophils 81.5%)

CRP: 156.56 mg/L

Temperature: Afebrile (37.0 °C)

Markedly raised inflammatory markers despite being afebrile.

Imaging — an aggressive-looking mass

• MRI: multilobulated soft-tissue mass, 10.2 × 7 × 5.4 cm, posterolateral aspect of the distal femur; no intra-articular extension.
• PET-CT: FDG-avid necrotic lesion, 7.5 × 4.8 cm; no metabolically active disease elsewhere.

Radiological impression: aggressive soft-tissue sarcoma (? synovial sarcoma) — on both MRI and PET-CT.

The turning point — biopsy and culture

Image-guided biopsy yielded frank pus. Repeat MRI showed a large, thick-walled, lobulated complex intra/inter-muscular collection (≈11 × 8.4 cm) with central diffusion restriction — an infective collection.

Specimen: Pus (intra-operative)

Organism: MRSA (Staphylococcus aureus)

Sensitive: Vancomycin, teicoplanin, linezolid

Final diagnosis: MRSA soft-tissue abscess.

Management & outcome

• Source control — abscess drainage, debridement and thorough wound lavage.
• Culture-directed therapy — guided by organism sensitivities.
• Outcome — infection resolved; wound healed and the patient became fully ambulatory.
• Underlying cause — primary ciliary dyskinesia confirmed on genomic sequencing; referred for long-term respiratory and fertility care.

Underlying diagnosis

Situs inversus totalis, chronic sinusitis / bronchiectasis and infertility, with clubbing and lifelong infections, indicated Kartagener's syndrome (primary ciliary dyskinesia) — a ciliary defect predisposing to recurrent, deep infection.

Discussion — relevance to musculoskeletal infection practice

• Imaging can't separate them — musculoskeletal infection mimics sarcoma clinically, on MRI and on PET-CT; metabolic avidity does not discriminate.
• Biopsy is decisive — send tissue for both histology and microbiology; culture yield is lost if it goes to histopathology alone.
• Use the MDT — where doubt exists, a combined sarcoma–infection team should review before any definitive resection.
• Stay suspicious — marked neutrophilia and CRP with a soft-tissue mass should raise infection even in an afebrile patient.

“Biopsy all your cultures, and culture all your biopsies.”

Key message

No clinical, biochemical or imaging feature — including PET-CT — reliably separates soft-tissue infection from sarcoma. Biopsy sent for both histology and culture is the decisive step, and is central to every bone & joint infection service.

Conclusion

An aggressive-looking thigh mass, reported as sarcoma on MRI and PET-CT, proved to be an MRSA abscess. Biopsy with culture averted an unnecessary oncological resection. Keep infection in the differential of every soft-tissue mass, and confirm with tissue for histology and culture.

Figure legends

Figure 1. (a) Axial T2 and (b) axial T1 MRI: a well-defined, multilobulated soft-tissue mass in the distal thigh. (c) PET-CT: solitary FDG-avid necrotic lesion.

Figure 2. (a) Intra-operative view of the thigh; ≈300–400 ml frank pus was drained. (b) Repeat MRI: thick-walled complex collection.

Figure 3. Chest radiograph: dextrocardia (situs inversus).

References

1.  Lex JR, Gregory J, Allen C, Reid JP, Stevenson JD. Distinguishing bone and soft tissue infections mimicking sarcomas requires multimodal multidisciplinary team assessment. Ann R Coll Surg Engl. 2019;101(6):405–410.

2.  Dion E, Forest M, Brasseur JL, et al. Epithelioid sarcoma mimicking abscess: review of the MRI appearances. Skeletal Radiol. 2001;30(3):173–177.

3.  Aydın Ö, Çelik A, Batıbay SG, et al. Tumor-mimicking musculoskeletal infectious lesions: experience of a single referral center. Srp Arh Celok Lek. 2022;150(5-6):295–301.

Written informed consent was obtained for publication of this case and images.