Is a 3 mm nodule appearing 1 month after partial nephrectomy for stage 1 Chromophobe Renal Cell Carcinoma (RCC) located in the fat anterior to the psoas muscle below the lower pole of the left kidney concerning for a metastatic lymph node or a benign finding?

Differential Diagnosis for a 3 mm Nodular Appearance after Partial Nephrectomy

Single Most Likely Diagnosis

• Benign reactive lymph node: Given the small size (3 mm) and the recent surgical history (1 month post-partial nephrectomy), a benign reactive lymph node is the most plausible explanation. The body's response to surgery can include the enlargement of lymph nodes due to inflammation or reaction to the surgical procedure itself.

Other Likely Diagnoses

• Surgical clip or artifact: The appearance could be due to a surgical clip or an artifact from the surgical procedure, especially given its small size and location near the surgical site.
• Benign fatty lesion: A small benign fatty lesion, such as a lipoma or a fatty infiltration, could also present as a small nodule in the fat anterior to the psoas muscle.
• Hematoma or seroma: Although less likely a month after surgery, a small hematoma or seroma could still be present and appear as a nodule on imaging.

Do Not Miss Diagnoses

• Metastatic lymph node: Although the primary concern, metastasis to a lymph node is less likely given the small size of the nodule and the early stage of the primary tumor (stage 1 Chromophobe RCC). However, missing a metastatic lymph node could have significant implications for treatment and prognosis.
• Local recurrence: Early local recurrence of the RCC, although unlikely given the short timeframe and the stage of the primary tumor, is a critical diagnosis not to miss due to its implications for further treatment.

Rare Diagnoses

• Lymphoma: A new onset of lymphoma presenting as a single small lymph node in this location would be rare but could be considered in the differential diagnosis.
• Other rare tumors: Other rare tumors, such as a soft tissue sarcoma or a metastasis from an unknown primary, are extremely unlikely but cannot be entirely ruled out without further investigation.
• Infectious or inflammatory process: A localized infectious or inflammatory process could theoretically present as a small nodule, although this would be uncommon in the context provided.