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A woman with right knee pain

Case Identification
Case ID Number: 
20101001KP
Benign/Malignant: 
unknown
Clinical case information
Case presentation: 

A very pleasant 70 year old woman who has Padget's disease, a history of liver failure, and right knee pain with a worrisome mass in the distal femur.

Radiological findings:: 
The plain radiographs are notable for the absence of any visible padgetoid changes in the affected area of the right distal femur. The initial radiographs were in fact read as normal. The most significant findings are on the MRI of the knee. There is a permeative lytic destructive process in the distal femur, involving most of the lateral femoral condyles and the distal portion of the femoral shaft, sparing a portion of the medial femoral condyle. The mass has expanded into the soft tissues slightly proximal and posterior to the patella, in the process transgressing the cortex of the anterior femur just proximal to the patella. There is a circumferential mass surrounding the distal femur. In the proximal tibia there is a round lesion, roughly the size of a quarter, in the marrow of the tibial plateau, which has signal abnormalities similar to the lesion in the proximal femur, and to my eye appears to be related to the lesion in the distal femur. I think this tibial focus represent a so-called skip metastasis or regional metastasis, or a separate marrow focus of disease if this turns out to be a marrow tumor. The x-rays are not very impressive in fact were read as normal. In retrospect, the radiologist added to comment that there was a slightly sclerotic abnormality in the distal femur, but this is pretty difficult to discern. The CT scan of the chest, abdomen, and pelvis are notable for multiple nodules in the left lung fields, and a few on the right as well. A lesion is seen in the manubrium which has bony destruction and a soft tissue extension. No mass is seen in the liver, and the rest of the abdominal CT is not remarkable.
Laboratory results:: 
No labs are requested.
Differential Diagnosis: 
The unimpressive plain radiograph results plus the permeative nature of the tumor, combined with the potential multiple sites of marrow involvement by this disease, favors the possibility of a tumor such as lymphoma or myeloma. However, with the patient's history, a Padget sarcoma must be considered. Other possibilities include an adenocarcinoma metastatic to the distal femur.
Further Work Up Needed:: 
A biopsy is clearly necessary. I think in order to establish without doubt the stage of disease, both the distal right femur lesion needs to be biopsied and the right proximal tibial lesion needs to be biopsied and these can be accomplished easily through a minimally invasive techniques.
Pathology results:: 
See the images.
Special Features of this Case:: 
The history of Padget's disease, liver failure, the patients age, and the appearance of the lesion point in different diagnostic directions. Will the orthopaedic treatment this patient needs vary according to the diagnosis or not?
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