Does a 3mm lesion anterior to the psoas, caudal to the lower pole of the left kidney, present since 1.5 months post-partial nephrectomy for stage T1a low-grade Chromophobe Renal Cell Carcinoma (RCC), represent a malignant or benign process?

From the Guidelines

This lesion most likely represents a benign process rather than a malignant one, given its small size, location, and stability over time, and therefore, no immediate intervention is needed. The key finding is that the lesion was present on CT imaging just 1.5 months after the partial nephrectomy for the stage T1a low-grade chromophobe renal cell carcinoma. The stability of the lesion over this extended period strongly suggests a benign etiology, as malignant lesions, particularly metastases, would typically show progressive growth over time 1.

Key Considerations

• The small size (3mm) and location within the fat anterior to the psoas muscle, caudal to the lower pole of the left kidney, is also more consistent with a benign process such as a small lymph node, fat necrosis, or post-surgical change.
• Given the history of recent surgery in the area, this could represent normal post-surgical scarring or inflammatory change that has remained stable.
• According to the American Society of Clinical Oncology clinical practice guideline, as many as 25% of small renal masses are benign renal cortical tumors, and another 25% are indolent in nature with limited metastatic potential 1.
• The guidelines recommend defining the growth rate of renal masses with serial imaging of the abdomen with CT or MRI within 6 months of the initiation of active surveillance for masses that are shown to be RCCs or oncocytic neoplasms and for those with indeterminate histology features 1.

Recommendations

• Continued surveillance imaging according to the standard protocol for monitoring after chromophobe RCC treatment would be appropriate to ensure ongoing stability.
• Imaging should be performed at least annually thereafter with CT, MRI, or US, and imaging surveillance of the chest on a yearly basis (or more frequently depending on clinical behavior) is recommended only in those patients with RCC or tumors with oncocytic features 1.

From the Research

Lesion Characteristics

• The lesion is 3mm in size, located anterior to the psoas, and caudal to the lower pole of the left kidney.
• It has been present since 1.5 months post-partial nephrectomy for stage T1a low-grade Chromophobe Renal Cell Carcinoma (RCC).

Differential Diagnosis

• The differential diagnosis for a cystic renal mass includes both benign and malignant entities, as noted in 2.
• Chromophobe RCC can present as a cystic mass, but it is a rare entity, accounting for only 4% of all cystic renal cell carcinomas 2.
• The imaging characteristics of chromophobe RCC can include a well-defined homogeneous mass and hypo-vascular soft tissue enhancement, as reported in 3.

Imaging Features

• The study in 3 analyzed the CT and MRI features of different histological types of RCC, including chromophobe RCC.
• The results showed that chromophobe RCC typically presents with equal density and more uniform enhancement on CT scans, and high signal on DWI.
• However, the small size of the lesion in question (3mm) makes it difficult to determine its imaging characteristics.

Prognosis and Treatment

• Chromophobe RCC generally has a good prognosis, as noted in 2 and 4.
• However, the prognosis for metastatic chromophobe RCC is worse than for advanced clear cell RCC, as reported in 4.
• The optimal treatment for metastatic chromophobe RCC is still unknown, and more studies are needed to determine the best course of treatment 4.

Histopathological Diagnosis

• The histopathological diagnosis of chromophobe RCC can be aided by immunohistochemical studies, as noted in 5.
• Chromophobe RCC is typically positive for epithelial membrane antigen and E-cadherin, and negative for vimentin and N-cadherin 5.
• However, the small size of the lesion in question makes it difficult to determine its histopathological characteristics.