Renal Cell Carcinoma (RCC): Staging, CT, and MRI

Overview

• Clear cell RCC - 65% of tumors, accounts for 90% of metastases; chromosome 3 abnormalities; most virulent
• Papillary RCC - chromosome 7, 17, or Y aberrations; more indolent
• Chromophobe RCC - more indolent
• Collecting duct / medullary carcinoma - rare, aggressive
• Oncocytoma - benign (10% of all renal tumors)

TNM Staging (AJCC 2017)

T (Primary Tumor)

N (Regional Lymph Nodes)

Note: Some older texts (Grainger & Allison) retain N1/N2 distinction (single vs. multiple nodes); the current 8th edition AJCC collapses this to N1 = any regional node involvement.

M (Distant Metastasis)

Anatomic Stage Groups

Histologic Grading

Role of CT Scan

What CT Does Well:

• Detection: Contrast-enhanced CT identifies renal masses with high sensitivity. Enhancement of >20 HU after IV contrast is the hallmark of malignancy.
• Staging accuracy: Overall accuracy 72-90%.
• Perinephric/T3a disease: A discrete soft-tissue mass in the perinephric space is 98% specific for T3a disease (though sensitivity is only 46%). Perinephric fat stranding alone is nonspecific - present in T1/T2 disease due to edema or fibrosis.
• Vascular invasion (T3b/T3c): CT detects renal vein and IVC tumor thrombus with good accuracy, though IVC thrombus level (below vs. above diaphragm) is important for surgical planning.
• Lymph node staging: Based on size criterion (>1 cm = abnormal). Overall accuracy 83-89%. Micrometastases in normal-sized nodes are missed; reactive enlargement causes false positives.
• Distant metastases: CT of chest, abdomen, and pelvis detects lung, liver, adrenal, and nodal metastases. Chest CT is preferred over plain CXR when metastatic disease is clinically suspected.
• Calcification: CT is better than MRI at detecting small foci of calcification.
• Bosniak classification of cystic renal masses is based on CT appearances (Category I = simple cyst, 0% malignancy; Category IV = 70% malignancy risk).

CT Limitation:

• T2 vs. early T3a: CT is not accurate in distinguishing T2 from early T3a disease (though clinically significant mainly for nephron-sparing decisions).
• Less reliable for IVC thrombus level determination compared to MRI.

Role of MRI

When MRI is Preferred or Indicated:

1. IVC tumor thrombus assessment (T3b/T3c): MRI is superior to CT for determining the precise cranial extent of IVC thrombus - critical for surgical planning (determines whether cardiac bypass is needed). This is its most important indication.
2. Contraindication to CT contrast: When iodinated contrast is contraindicated (e.g., severe renal impairment, contrast allergy), gadolinium-enhanced MRI is the alternative. Note: gadolinium-based contrast should also be used with caution in severe CKD (NSF risk).
3. Problem-solving for indeterminate masses: MRI helps characterize masses that are equivocal on CT - particularly for distinguishing fat-poor angiomyolipoma from RCC, or evaluating complex cystic lesions.
4. Pregnancy: Avoidance of ionizing radiation.
5. Surveillance after ablation (RFA/cryotherapy): MRI can assess residual enhancement at ablation sites.

MRI Signal Characteristics of RCC:

• T1-weighted: Isointense or hypointense compared to renal cortex
• T2-weighted: Slightly hyperintense
• Post-gadolinium: Heterogeneous enhancement immediately after injection, decreasing on delayed images. Small, low-grade tumors show more homogeneous enhancement.

MRI Limitations:

• Respiratory misregistration - more prone than CT to motion artifact
• Calcification detection: Inferior to CT for small calcification foci
• Lymph node assessment: Not significantly better than CT for nodal disease
• Cost, availability, scan time: Less practical as a first-line tool

Summary: CT vs MRI in RCC

Initial Workup Protocol

1. Contrast-enhanced CT abdomen and pelvis (primary staging)
2. Chest radiograph (if negative for metastases, stop; if suspicious, add chest CT)
3. Urinalysis and labs (CBC, metabolic panel, LFTs, LDH, corrected calcium)
4. Bone scan / brain imaging - only if clinically indicated (bone pain, elevated alkaline phosphatase, neurologic symptoms)
5. MRI abdomen - if IVC thrombus extent unclear, or contrast contraindicated