What is the clinical significance of a 3mm nodule in the fat anterior to the psoas muscle, which appeared one month after partial nephrectomy for a 1.3cm chromophobe renal cell carcinoma (RCC) and has become more defined over three years, in a patient with a history of T1A stage RCC with no evidence of disease (NED) on previous surveillance scans?

From the Guidelines

The 3mm nodule in the fat anterior to the psoas muscle is likely a post-surgical change rather than a concerning new finding, and continued routine surveillance is appropriate rather than any intervention. Given the small size of the nodule, its stability over time, and its appearance immediately post-surgery, it is reasonable to conclude that this finding represents normal post-surgical scarring or fibrosis 1. The fact that the patient's chromophobe renal cell carcinoma was small (1.3 cm), completely removed with adequate margins, and classified as stage T1A, which carries an excellent prognosis, further supports this conclusion. The absence of aggressive features like sarcomatoid differentiation or necrosis is also reassuring.

Some key points to consider in this case include:

• The patient's history of T1A stage RCC with no evidence of disease (NED) on previous surveillance scans suggests a low risk of recurrence 1.
• The use of multiphase contrast-enhanced CT of the abdomen and chest for the diagnosis and staging of renal tumours is recommended, but in this case, the nodule's stability and small size suggest that it is not a concerning finding 1.
• The European Association of Urology guidelines on renal cell carcinoma recommend offering surgery to achieve cure in localized RCC, and the patient has already undergone partial nephrectomy 1.
• The guidelines also recommend intensifying follow-up for patients with a positive surgical margin, but in this case, the patient's margin was adequate, and the nodule is likely a post-surgical change rather than a recurrence 1.

Overall, based on the patient's history, the characteristics of the nodule, and the current guidelines, continued routine surveillance is the most appropriate course of action, rather than any intervention 1.

From the Research

Clinical Significance of a 3mm Nodule

• The clinical significance of a 3mm nodule in the fat anterior to the psoas muscle, which appeared one month after partial nephrectomy for a 1.3cm chromophobe renal cell carcinoma (RCC) and has become more defined over three years, is uncertain and requires further evaluation 2, 3, 4, 5.
• Soft-tissue masses, including those in the fat anterior to the psoas muscle, can be benign or malignant, and it may be difficult to differentiate between them based on imaging alone 2, 3, 4, 5.
• Magnetic Resonance Imaging (MRI) is a useful tool for evaluating soft-tissue masses, but its accuracy in distinguishing between benign and malignant lesions is limited, especially for small lesions like the 3mm nodule in question 3, 4, 5.

Imaging Features and Diagnostic Accuracy

• Several studies have investigated the imaging features that can help differentiate between benign and malignant soft-tissue masses, including size, signal homogeneity, perilesional edema, hemorrhage, necrosis, and bone and neurovascular involvement 3, 4, 5.
• A study published in the AJR American Journal of Roentgenology in 1995 found that no single imaging feature or combination of features could reliably distinguish between benign and malignant lesions, but that a subjective analysis of MRI findings could correctly diagnose benign lesions in 44% of cases 3.
• Another study published in the Journal of the Chinese Medical Association in 2009 found that a combination of imaging features, including necrosis, maximal mass diameter, peritumoral edema, and absent fibrosis, calcification, and fat rim, could predict malignancy with a sensitivity of 84.2% and specificity of 64.0% 4.
• A study published in the Journal of the Medical Association of Thailand in 2008 found that heterogeneous signal on T2-weighted images, perilesional edema or invasion, and necrosis were statistically significant features for differentiating between benign and malignant soft-tissue masses 5.

Surveillance and Follow-up

• The patient's history of T1A stage RCC with no evidence of disease (NED) on previous surveillance scans is important to consider when evaluating the clinical significance of the 3mm nodule 6.
• Surveillance imaging strategies for patients with a history of RCC are not well established, and the optimal approach is likely to be individualized based on clinical presentation, history, and histopathologic characteristics 6.
• Further imaging and follow-up may be necessary to determine the nature and significance of the 3mm nodule, and to assess for any changes in size or characteristics over time 2, 3, 4, 5.