What is the significance of a 3mm nodule in the fat anterior to the psoas muscle, adjacent to the surgical bed, three years after a partial nephrectomy for a 1.3cm chromophobe renal cell carcinoma (RCC) with a history of clean scans, and recent findings of fat necrosis and mild chronic inflammation?

Differential Diagnosis for the 3 mm Nodule

• Single most likely diagnosis
  • Fat necrosis or a benign post-surgical change: This is the most likely diagnosis given the nodule's stable appearance over time, its location in the fat anterior to the psoas muscle, and the fact that it was visible on imaging one month after the partial nephrectomy. The improvement in imaging resolution over time, which allowed for better visualization of the nodule, supports the idea that it is a benign change related to the surgery.
• Other Likely diagnoses
  • Post-operative granuloma: This could be a possible diagnosis given the nodule's appearance and location. However, the lack of significant growth or change in density over time makes this less likely.
  • Fibrosis or scar tissue: Similar to fat necrosis, this could be a benign change related to the surgery. The nodule's stable appearance and density support this possibility.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Recurrent chromophobe RCC: Although the primary tumor was small (1.3 cm) and had a favorable histology (T1A, no sarcomatoid features, no necrosis), recurrence is still a possibility. The fact that the nodule was not present on pre-operative imaging and was visible one month after surgery increases the concern for recurrence. However, the stable appearance and lack of growth over time make this less likely.
  • Metastatic disease: Although the primary tumor was small and had a favorable histology, metastatic disease is still a possibility. The fact that the nodule is located in the fat anterior to the psoas muscle, a common site for metastases, increases the concern. However, the lack of significant growth or change in density over time makes this less likely.
• Rare diagnoses
  • Lymphoma or other malignancies: These are rare possibilities, but should be considered in the differential diagnosis. However, the lack of significant growth or change in density over time, as well as the stable appearance of the nodule, make these diagnoses less likely.
  • Vascular malformation or aneurysm: These are rare possibilities, but could be considered in the differential diagnosis. However, the nodule's density and appearance are not consistent with a vascular malformation or aneurysm.