IMAGING

Significance of the pseudocapsule on MRI of renal neoplasms and its potential application for local staging: a retrospective study
Roy C Sr, El Ghali S, Buy X, Lindner V, Lang H, Saussine C, Jacqmin D
University Hospital of Strasbourg, Radiology B Chirugie A, Hopital Civil CHU, Strasbourg, France
AJR Am J Roentgenol. 2005; 184: 113-20

• Objective: The purpose of our study was to evaluate the role of MRI in showing a pseudocapsule for local staging of renal tumors, and its potential application to select patients for partial surgery.
• Materials and Methods: Eighty tumors (73 renal cell carcinomas [RCCs] and seven oncocytomas) were preoperatively evaluated by MRI. MRI findings were assessed with a special focus on perinephric fat and pseudocapsule. Correlations were performed with pathologic staging after surgery.
• Results: At pathology, a pseudocapsule was recognized in 79 cases. Twenty-three RCC were staged pT3a (21 clear cell; two papillary). MR images exhibited a pseudocapsule in 90% of cases as a hypointense rim surrounding the tumor on T2-weighted images. MRI findings concerning isolated analysis of the pseudocapsule for differentiating stage T1/T2 from T3a were sensitivity: 86%, 50%; specificity: 95%, 92%; positive predictive value: 95%, 33%; negative predictive value: 88%, 92%; and accuracy: 93%, 89%, for clear cell and papillary types, respectively. For stage T3a, with both abnormalities of the pseudocapsule and perirenal fat, results were, for overall RCC sensitivity: 84%; specificity: 95%; positive predictive value: 91%; negative predictive value: 91%; and accuracy: 91%.
• Conclusion: The identification of the pseudocapsule offers an additional value for local staging by MRI. The presence of an intact pseudocapsule is a sign of lack of perinephric fat invasion. It is more likely to predict that the tumor can be removed by partial surgery.
  
  
• Editorial Comment
  The identification of a well defined pseudocapsule around a small tumor has recently been considered a very important finding for the local staging of a possible renal cell carcinoma. As we know this pseudocapsule represents a narrow area of fibrosis and normal renal parenchyma, which is displaced by the slow growing tumor. The demonstration of a well defined pseudocapsule by helical CT indicates that in the majority of cases the renal tumor is confined to the kidney (stage T1), thus amenable to conservative surgery: open partial nephrectomy or laparoscopic partial nephrectomy (1). The identification of this pseudocapsule by CT has 95% accuracy for predicting that the tumor is confined to the kidney. The authors used in this study MR imaging findings for identification of the pseudocapsule and their results confirm the conclusion of the study using multidector CT. The authors shows that the presence of pseudocapsule (thin, regular, linear band with hypo- intensity signal in both T1 and T2 –weighted images), is a reliable imaging sign to predict lack of perinephric fat invasion. Consequently, this sign is useful for differentiating stage T1/T2 from stage T3a, with a 92% positive predictive value .When there is disruption of this pseudocapsule and consequent opacification of the perirenal fat manifested as spiculation around the mass, the overall accuracy for predicting stage T3a is 91% .This finding is very useful for adequate surgical management of renal tumors since in many centers stage T3a tumor is considered a limitation for conservative renal surgery.

Reference
1. Catalano C, Fraioli F, Laghi A, Napoli A, Pediconi F, Danti M, Nardis P, Passariello R: High-resolution multidetector CT in the preoperative evaluation of patients with renal cell carcinoma. AJR 2003; 180: 1271-7

Dr. Adilson Prando
Chief, Department of Radiology
Vera Cruz Hospital
Campinas, São Paulo, Brazil