Differential Diagnosis for US L Knee Findings

• Single most likely diagnosis
  • Evolving subcutaneous hematoma: This is considered the most likely diagnosis due to the recent trauma and the location of the palpable abnormality, which correlates with the cystic-appearing lesion on the comparison MRI. The ultrasound findings of a nonspecific collection at the site of trauma support this diagnosis.
• Other Likely diagnoses
  • Lipoma: Although less likely than a hematoma, a lipoma could present as a cystic-appearing lesion on MRI and could be considered if the clinical history and ultrasound findings were not so strongly suggestive of trauma.
  • Phlegmon: This is an inflammatory process that could present with similar imaging findings, especially if there's an infectious component. However, the recent trauma history makes this less likely than a hematoma.
• Do Not Miss diagnoses
  • Soft tissue sarcoma: Although rare and less likely, missing a diagnosis of soft tissue sarcoma could have significant consequences. Any persistent or growing mass, especially with atypical features on imaging, warrants further investigation.
  • Abscess: An abscess could present similarly to a hematoma or phlegmon and has significant clinical implications if not treated promptly. The presence of fever, increased white blood cell count, or other signs of infection would increase the likelihood of this diagnosis.
• Rare diagnoses
  • Hemangioma: A vascular malformation could present as a cystic lesion on MRI, but this would be less likely given the acute presentation following trauma.
  • Lymphangioma: Similar to hemangioma, this is a congenital vascular malformation that could appear as a cystic lesion but is rare and less likely in the context of recent trauma.
  • Other rare soft tissue tumors: Various benign or malignant tumors could present as a cystic-appearing lesion on MRI, but these are less common and would be considered based on specific clinical and imaging features not mentioned in the scenario provided.