Incidental Hyperdense Masses On Nonfocused Ct

Occasionally a patient is scanned for a nonurologic reason and an incidental renal lesion is detected that has all the characteristic features of a simple cyst, except that its attenuation is above water density. On contrast-enhanced CT examinations, both hypovascular solid renal masses and hyperdense cysts will appear less dense than the adjacent highly enhancing renal parenchyma. Comparison to other water-density structures, such as the gallbladder or urinary bladder, is a simple method to make sure the high attenuation number is not a machine calibration problem. Routine, frequent water phantom checks of the scanners should prevent drifting of the attenuation numbers. If the high attenuation is not due to artifact and if only postcontrast images are available, the critical issue of whether renal mass enhancement has occurred cannot be assessed.

If not spurious, these "hyperdense" cysts may have achieved their high internal HU values by virtue of increased protein content related to prior hemorrhage or

Scan Urinary System

Figure 9 Bosniak Category IV cystic neoplasm. (A) A 67-year-old male with gross hematuria and history of prostate carcinoma. Precontrast scan shows interrupted calcifications at the cyst/parenchymal interface. (B) After intravenous contrast, a nodule within the cyst enhances from 27 to 154 HU. (C) The center of the mass remains at low attenuation (11-17 HU). (D) Delayed image shows heterogeneity of the periphery of the mass. An encapsulated renal cell carcinoma was found at surgery. Abbreviation: HU, Hounsfield unit.

Figure 9 Bosniak Category IV cystic neoplasm. (A) A 67-year-old male with gross hematuria and history of prostate carcinoma. Precontrast scan shows interrupted calcifications at the cyst/parenchymal interface. (B) After intravenous contrast, a nodule within the cyst enhances from 27 to 154 HU. (C) The center of the mass remains at low attenuation (11-17 HU). (D) Delayed image shows heterogeneity of the periphery of the mass. An encapsulated renal cell carcinoma was found at surgery. Abbreviation: HU, Hounsfield unit.

another undetermined process (14-20). New and Aronow found that blood with a hematocrit of 45% measures 58 HU on CT, rising to 60 to 80 HU after blood clot retraction with the protein component being responsible for the high attenuation (21). This appears to be consistent with the frequent finding of hemorrhagic cysts in the range of 70 to 90 HU (19,20).

The importance of hemorrhagic cysts lies in distinguishing them from high attenuation solid renal tumors (Fig. 13) (22). Ultrasound may not be helpful because the contents (blood or debris) that make the cyst high in attenuation may also generate internal echoes indistinguishable from a solid tumor. Generally speaking, cysts

Bosniak Classification

Figure 10 Bosniak Category IV cystic neoplasm. A 65-year-old female with a left neck mass and a 3 cm mass in the right lung. CT shows a thick-walled cystic renal mass with enhancing mural nodules. Core biopsies consistent with renal cell carcinoma. Patient expired five months after diagnosis. Abbreviation: CT, computed tomography.

Figure 10 Bosniak Category IV cystic neoplasm. A 65-year-old female with a left neck mass and a 3 cm mass in the right lung. CT shows a thick-walled cystic renal mass with enhancing mural nodules. Core biopsies consistent with renal cell carcinoma. Patient expired five months after diagnosis. Abbreviation: CT, computed tomography.

detected on enhanced CT examinations that are only mildly hyperdense, with CT attenuation numbers in the range of 20 to 40 HU, often are anechoic on ultrasound, and, with the help of subsequent ultrasonography, can safely be assigned to Category II or IIF, depending on location, size, and the presence or absence of other

Subcentimeter Cysts Breast
Figure 11 Bosniak Category IIF cyst. Nearly completely calcified mass with no obvious enhancing elements. A benign chronic inflammatory mass was resected. Source: Case courtesy of Dr. Frank Muto, Charleston, WV.
Bifid Collecting System Kidney Image
Figure 12 (A) A 51-year-old female with breast carcinoma and a subcentimeter rim calcified renal cyst, Bosniak Category II. (B) Five years later the lesion has not grown but shows some increase in extent of calcification.

minor complexities (Fig. 14). In comparison, the more highly attenuating masses are more likely to have internal echogenic material that may convincingly mimic carcinoma.

Another way of handling this situation is to obtain delayed CT images at the time of discovery (Fig. 15). The added scan demonstrates whether the high attenuation is a fixed feature of the mass due to its non-neoplastic content or is a temporary

Figure 13 Renal cell carcinoma. (A) A 42-year-old female with back pain, hematuria, and a renal mass discovered by lumbar spine MR. CT without intravenous contrast shows mildly hyperdense (55 HU) 3 cm mass. (B) After intravenous contrast, the masses enhance to 88 HU, consistent with solid, not cystic character. Abbreviations: CT, computed tomography; HU, Hounsfield unit; MR, magnetic resonance.

Figure 13 Renal cell carcinoma. (A) A 42-year-old female with back pain, hematuria, and a renal mass discovered by lumbar spine MR. CT without intravenous contrast shows mildly hyperdense (55 HU) 3 cm mass. (B) After intravenous contrast, the masses enhance to 88 HU, consistent with solid, not cystic character. Abbreviations: CT, computed tomography; HU, Hounsfield unit; MR, magnetic resonance.

Shorthand For With Contrast

Figure 14 Bosniak Category II hyperdense cyst. (A) A 47-year-old male with a pancreatic pseudocyst and incidental high attenuation (72 HU), homogeneous 1.8 cm mass in the posterior upper pole. Bosniak Category II hyperdense cyst versus solid, enhancing renal neoplasm. (B) Ultrasound examination shows no internal echoes. Abbreviation: HU, Hounsfield unit.

Figure 14 Bosniak Category II hyperdense cyst. (A) A 47-year-old male with a pancreatic pseudocyst and incidental high attenuation (72 HU), homogeneous 1.8 cm mass in the posterior upper pole. Bosniak Category II hyperdense cyst versus solid, enhancing renal neoplasm. (B) Ultrasound examination shows no internal echoes. Abbreviation: HU, Hounsfield unit.

manifestation of its internal vascularity, implying a tumor. This concept of "de-enhancement" was suggested by Macari and Bosniak in 1999 (23). They investigated patients with both known and incidentally discovered high attenuation masses with 15-minute-delayed CT scans. Seventeen hyperdense lesions showed a mean de-enhancement of 32 HU (range 15-67), from an initial mean attenuation of about 82 HU on nephrographic phase images down to 50 HU on the delayed scans.

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