History of right distal femur replacement for osteosarcoma. Knee pain with cystic mass over anteromedial knee. Evaluation of the knee and surrounding soft tissues is markedly limited by metallic susceptibility artifact arising from the patient's longstem total knee arthroplasty endoprosthesis device. There is a mass within the subcutaneous fat along the anteromedial aspect of the proximal tibial metaphysis, immediately medial to the anterior tibial tubercle. This extraosseous mass is approximately 3.5 x 3 cm in transverse dimension and 3.5 cm in craniocaudal dimension. This mass is of predominantly low signal intensity on T1-weighted images, high signal intensity on T2-weighted images, and shows peripheral enhancement following gadolinium administration. There is mild edema in the adjacent subcutaneous fat. The mass permeates the adjacent tibial cortex, extending into the medullary space anterior to the stem of the prosthesis. This intramedullary component of the mass is slightly more heterogeneous on T2-weighted images, and of intermediate signal intensity on T1-weighted images, showing little enhancement following gadolinium administration other than along the its periphery. This intramedullary process also extends posteromedially along the stem of the tibial component forming an additional lobule that shows peripheral enhancement following gadolinium administration. Overall, the intraosseous component is approximately 4.5 x 3 cm in transverse dimension and approximately 4 cm in craniocaudal dimension. Additionally, there is a subcentimeter focus of abnormal signal intensity within the lateral tibial plateau; I'm uncertain whether not this represents the same process.More proximally, evaluation of the soft tissues is extremely limited by susceptibility artifact. However, there is intermediate signal intensity on T1-weighted images and high signal intensity on the T2-weighted images in Hoffa's fat pad which could conceivably reflect an extension of the aforementioned process along the anterior margin of the endoprosthesis construct. More proximally, there is abnormal signal intensity along the medial aspect of the condylar portion of the femoral component that is of similar signal intensity to the aforementioned process and shows both nodular and rim enhancement following gadolinium administration. Accurate measurement of this component of the process is difficult but I suspect that it is approximately 4 x 1 cm in transverse dimension and 3 cm in craniocaudal dimension.
Soft tissue mass along the anteromedial aspect of the proximal tibial metaphysis that extends into the proximal tibia, and possibly superiorly into Hoffa's fat pad and along the medial aspect of the condylar portion of the femoral component. Although there is relatively minimal enhancement of this mass, I cannot exclude recurrence of osteosarcoma given the relatively aggressive nature of the extension of this process. Alternatively, this may reflect infection or a peculiar foreign body/particle reaction.