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:
  • Fibrosis or scar tissue: The nodule's appearance has "matured" over time, and its density is similar to the psoas muscle, suggesting a benign process. The fact that it was visible on early scans and has remained stable in size also supports this diagnosis.
• Other Likely diagnoses:
  • Lymph node: Although no lymph nodes were submitted during the partial nephrectomy, it's possible that this nodule represents a reactive or benign lymph node.
  • Granuloma: A small granuloma could present as a 3 mm nodule, although this would be less likely given the patient's history and the nodule's appearance.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Metastatic disease: Although the primary tumor was small and low-stage, there is always a risk of metastatic disease, especially in the context of a new nodule. However, the nodule's stable size and benign appearance make this less likely.
  • Local recurrence: Recurrence of the chromophobe RCC could present as a small nodule, although the fact that the nodule was present on early scans and has remained stable in size makes this less likely.
• Rare diagnoses:
  • Sarcoma: A primary sarcoma or a sarcomatoid transformation of the RCC could present as a new nodule, although this would be extremely rare.
  • Other rare tumors: Other rare tumors, such as a paraganglioma or a neurogenic tumor, could also present as a small nodule, although these would be unlikely given the patient's history and the nodule's appearance.