CRA61 SILENT LUNGS, ACHING KNEE: ISOLATED KNEE PAIN AS THE INITIAL PRESENTATION OF LUNG ADENOCARCINOMA

Fui Bee Woo¹, Shing Wei Chew², Nai-Chien Huan³, Joon Hi Tham⁴

¹Respiratory Unit, Department of Internal Medicine, Hospital Tuanku Ja’afar, Negeri Sembilan, Malaysia
²Department of Oncology, Hospital Wanita Dan Kanak-Kanak Sabah, Sabah, Malaysia
³Department of Respiratory Medicine, Hospital Queen Elizabeth, Sabah, Malaysia
⁴Department of Pathology, Hospital Queen Elizabeth, Sabah, Malaysia

Introduction
Lung cancer is a leading cause of cancer-related mortality globally, with most patients being diagnosed at advanced stages. While bone metastasis is common, the presentation of a solid long bone tumour as the first sign of lung adenocarcinoma is unusual.

Case presentation
A 66-year-old female presented with gradual onset of right knee pain following a fall six months ago. Magnetic resonance imaging (MRI) of the right knee revealed a destructive lesion at the distal end of the femur. Bone biopsy confirmed metastatic adenocarcinoma originating from the lung, with epidermal growth factor receptor (EGFR) mutation identified. The patient showed significant improvement following a combination of targeted therapy and radiotherapy. Follow-up computed tomography (CT) revealed a reduction in the size of the primary tumour and regression of the bone lesion, rendering the patient symptom-free.

Discussion
Proximal femur is a common site of bone metastases due to the abundant red bone marrow, while involvement of the distal femur should raise suspicion for a primary bone tumour. In this case, the patient’s sole symptom was knee pain localised to the distal femur. The lack of other systemic features of lung cancer with normal chest radiography and blood work, made it challenging to promptly identify the diagnosis.

Conclusion
This case highlights the unusual presentation of bone metastasis in lung adenocarcinoma and the need for a high index of suspicion when diagnosing metastatic disease in unusual sites.