Assessing renal capsule invasion with arterial spin labeling MRI

The staging of renal cell carcinoma (RCC) and assessing the prognosis of a patient rely on the tumor size and perinephric fat invasion seen on magnetic resonance imaging (MRI). But for tumors with a diameter less than 7 centimeters (cm), visualization and assessment of perinephric fat invasion (PFI) may not be diagnostically feasible.

Renal capsule invasion occurs before PFI. An intact pseudocapsule surrounding a renal tumor is a sign that indicates lack of PFI, whereas when a pseudocapsule has a visible defect, this indicates aggressive biological behavior. Because arterial spin labeling (ASL) is considered to be a promising technology to assess tumor perfusion in renal masses, a Chinese research team conducted a study to prospectively evaluate the performance of combining morphological and ASL MRI to detect pseudocapsule defects in RCC and to predict renal capsule invasion.

In Clinical Radiology, the researchers reported that ASL MRI could be used successfully to assess pseudocapsule defects in lesions smaller than 7 cm and to accurately predict renal capsule invasion. The study showed that the combined T2-weighted and ASL images had 100% specificity compared to 71.4% for the T2 images alone with respect to identifying renal capsule invasion. A high diagnostic specificity could potentially help physicians determine if a patient requires a radical nephrectomy or if nephron-sparing surgery instead would be an appropriate clinical treatment.

The study conducted by clinicians at the West China Hospital of Sichuan University in Chengdu and the Chengdu University of Traditional Chinese medicine included 20 consecutive patients with clear cell RCCs who received treatment at West China Hospital over a six-month period. Lead author and radiologist H. Zhang, MD, and co-authors stated that all patients had a renal lesion between 1.9 to 6.4 cm on an abdominal computed tomography (CT) scan and/or an ultrasound examination. All of the tumors were histopathologically confirmed as stage pT1 with perinephric fat invasion.

All patients had had a presurgical MRI. Three radiologists independently evaluated the T2-weighted MRI images alone, and subsequently the T2-weighted images combined with the ASL images. The two groups of images were assessed with respect to diagnostic specificity, sensitivity, positive predictive value (PPV) and negative predictive value (NPV). A pseudocapsule was defined as the hypointense signal rim surrounding the tumor on T2-weighted images. If this signal rim was discontinuous, the pseudocapsule was defined as having a suspicious defect.

Pseudocapsule defects were identified on T2-weighted images in 15 cases. Twelve of these showed existing blood flow in defect areas on perfusion images. The T2-weighted images combined with ASL had a 100% specificity and PPV compared to T2-weighted images alone at 71.4% and 86.7% respectively. T2-weighted images alone outperformed the combined images with respect to sensitivity (at 100% vs. 92.3% respectively) and NPV (100% vs. 87.5% respectively.

The authors believe strongly in the diagnostic potential of combined image assessment. But they cautioned that the small size of the study may have overrepresented the diagnostic accuracy for detecting renal capsule invasion and recommended that larger studies be conducted to confirm this capability. They also pointed out that because all of the patients had pseudocapsule defects adjacent to the renal capsule, their findings are applicable only to this tumor configuration. A larger sample size with a variety of locations of pseudocapsule defects could have produced different results. Again, they recommend additional, larger studies to validate or modify their findings.

REFERENCE

1. Zhang H, Wu Y, Xue W, et al. Arterial spin labelling MRI for detecting pseudocapsule defects and predicting renal capsule invasion in renal cell carcinoma. Clin Radiol. 2017 72;11: 936-943.