Differential Diagnosis for the 3mm Nodule
- Single most likely diagnosis
- Benign fatty lesion (e.g., lipoma or fibrolipoma): The fact that the radiologist described the nodule as having "matured" over the past three years suggests a benign process. The location in the fat anterior to the psoas muscle and the small size of the nodule also support a benign fatty lesion.
- Other Likely diagnoses
- Reactive lymph node: Although the nodule is small, it could represent a reactive lymph node, especially given its location near the surgical bed. However, the lack of significant change in size over three years makes this less likely.
- Fibrous pseudotumor: This is a benign lesion that can occur in the retroperitoneum and could be considered given the nodule's location and stability over time.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Metastatic disease (e.g., RCC metastasis to a lymph node): Although the primary tumor was small and low-stage, there is always a possibility of metastatic disease, especially in the context of a history of cancer. The fact that the nodule was not present before surgery but appeared shortly after raises some concern.
- Sarcomatoid transformation or dedifferentiation: Although the original tumor did not have sarcomatoid features, there is a small chance of transformation or dedifferentiation, which could manifest as a new nodule.
- Rare diagnoses
- Soft tissue sarcoma: This is a rare possibility, but it could be considered given the nodule's location in the soft tissues. However, the lack of significant growth over three years makes this less likely.
- Other rare benign lesions (e.g., schwannoma, neurofibroma): These lesions are rare and would be unlikely given the clinical context, but they could be considered if other diagnoses are ruled out.