title: Enhancement Threshold of Small (< 4 cm) Solid Renal Masses on CT.
abstract: OBJECTIVE
  The objective of our study was to determine whether small (< 4 cm) solid renal cell carcinomas (RCCs) enhance on CT below the published enhancement thresholds.
  
  
  METHODS
  For this retrospective study, we reviewed CT examinations of patients with pathologically characterized small solid renal masses. Patients with lipid-rich angiomyolipomas were excluded. The study included consecutive patients who had pathologically characterized small (< 4 cm) renal masses and had undergone CT during the un-enhanced phase and nephrographic phase (NP). Most CT examinations also included imaging during the corticomedullary phase (CMP); four patients did not have CMP images. The CT examinations were performed on a 64-MDCT scanner. Similar-sized ROIs were placed on each lesion on unenhanced and enhanced images and enhancement (in Hounsfield units) was calculated. Masses were considered solid with a threshold enhancement greater than either 15 or 20 HU.
  
  
  RESULTS
  There were 137 masses in 137 patients, 90 men and 47 women (age range, 21-91 years; median age, 65 years). The renal masses were 1.0-3.9 cm (median, 2.4 cm). Of the 137 masses, 117 (85.4%) were malignant and 20 (14.6%) were benign. One malignant mass and three benign masses did not have CMP images. Twenty of the 116 (17.2%) renal cell carcinomas (RCCs) did not reach the 15-HU threshold enhancement on CMP images, nine of 117 masses (7.7%) did not reach the 15-HU enhancement threshold on NP images, and four of 117 masses (3.4%) did not reach the 15-HU enhancement threshold on both CMP and NP images. The numbers of masses that did not reach the 20-HU enhancement threshold were 24 of 116 masses (20.7%) on CMP images, 14 of 117 masses (11.9%) on NP images, and 11 of 117 masses (9.4%) on both CMP and NP images. Depending on the phase or enhancement threshold, 13.6-59.1% of papillary RCCs did not reach the enhancement threshold.
  
  
  CONCLUSIONS
  A minority of small RCCs do not reach either a 15- or 20-HU enhancement threshold and might be misinterpreted as a hyperattenuating cyst. Most RCCs below these enhancement thresholds are papillary RCC.
authors.affiliation: 1 Department of Medical Imaging, University of Toronto, Toronto General Hospital, 585 University Ave, Toronto, ON M5G 2N2, Canada.|2 Department of Medical Imaging, Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada.
authors.forename: Fawaz
authors.initials: F
authors.lastname: Al Harbi
authors.affiliation: 1 Department of Medical Imaging, University of Toronto, Toronto General Hospital, 585 University Ave, Toronto, ON M5G 2N2, Canada.
authors.forename: Leila
authors.initials: L
authors.lastname: Tabatabaeefar
authors.affiliation: 3 Department of Surgical Oncology, Division of Urology, University of Toronto, Toronto, ON, Canada.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Jewett
authors.affiliation: 3 Department of Surgical Oncology, Division of Urology, University of Toronto, Toronto, ON, Canada.
authors.forename: Anthony
authors.initials: A
authors.lastname: Finelli
authors.affiliation: 2 Department of Medical Imaging, Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada.
authors.forename: Martin
authors.initials: M
authors.lastname: O'Malley
authors.affiliation: 1 Department of Medical Imaging, University of Toronto, Toronto General Hospital, 585 University Ave, Toronto, ON M5G 2N2, Canada.
authors.forename: Mostafa
authors.initials: M
authors.lastname: Atri
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019412:Anatomy, Cross-Sectional; D002292:Carcinoma, Renal Cell / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D007680:Kidney Neoplasms / Q000000981:diagnostic imaging*; D008297:Male; D008875:Middle Aged; D011857:Radiographic Image Interpretation, Computer-Assisted; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed; D047368:Tumor Burden; D055815:Young Adult
keywords: CT enhancement; renal mass