From the Guidelines
The lesion detected in the retroperitoneal fat anterior to the psoas muscle and caudal to the lower kidney pole is likely a benign entity, such as a small lipoma, given its stable appearance over three years, and requires further evaluation with consistent imaging parameters and consideration for dedicated MRI with contrast to better characterize the lesion. The varying visibility across scans appears to be primarily related to changes in CT slice thickness rather than actual lesion changes, as thinner slices provide better spatial resolution and reduce partial volume averaging effects, allowing small lesions to be visualized more clearly 1. When evaluating retroperitoneal lesions, it is essential to consider the potential for soft tissue sarcomas, such as liposarcomas, leiomyosarcomas, or solitary fibrous tumors, which have varying risks for local recurrence and metastasis 1. However, the stable appearance of this lesion over three years suggests a low risk for malignancy.
Some key considerations for retroperitoneal lesions include:
- Retroperitoneal liposarcomas have poorly defined margins and an inherent higher risk for local recurrence, and may require extended surgical approaches to improve long-term local control 1
- Retroperitoneal leiomyosarcomas have more clearly defined borders, with a low risk for local recurrence after complete resection, but have a higher risk for systemic metastasis 1
- Retroperitoneal solitary fibrous tumors exhibit a low risk for local recurrence, and the aim of resection should be complete resection with negative margins while preserving uninvolved organs 1
Given the potential for various entities, including benign and malignant processes, I recommend obtaining a dedicated MRI with contrast of the retroperitoneum to better characterize this lesion, followed by consultation with a specialist (urologist or surgical oncologist) to determine if biopsy is warranted. Future CT scans should maintain consistent slice thickness (preferably the thinner 2.50 mm) to allow for accurate comparison and assessment of any changes over time.
From the Research
Lesion Characteristics
- The lesion is located in the fat anterior to the psoas muscle, caudal to the lower pole of the kidney
- It appeared foggy on computed tomography (CT) scans with varying slice thicknesses over a 3-year period
- The lesion initially decreased in visibility before becoming more visible at 3 years
- It measures 3mm
Diagnostic Considerations
- The lesion's appearance and behavior are not typical of a simple lipoma, as it changed in visibility over time 2
- The presence of a foggy appearance on CT scans may suggest a lipomatous lesion, but it is difficult to distinguish between benign and malignant lesions based on imaging alone 3, 4
- The small size of the lesion (3mm) makes it challenging to determine its nature, and biopsy may be necessary for a definitive diagnosis 4, 5
- The lesion's location and characteristics may also suggest other fat-containing lesions, such as liposarcoma, which can be differentiated from benign lipomas using imaging features and biopsy 6
Imaging Features
- CT scans with varying slice thicknesses showed a foggy appearance of the lesion over a 3-year period
- The lesion's visibility changed over time, initially decreasing before becoming more visible at 3 years
- The lesion measures 3mm, which is a small size that makes it difficult to determine its nature using imaging alone 5