• US: 

  • Useful for initial evaluation of a cystic renal mass

  • Color Doppler should be used to evaluate for internal BF

    • Also useful to distinguish a cystic mass from a renal artery aneurysm

• CT:

  • Useful for evaluation of both cystic and solid renal masses

  • Classification is based on morphology. Calcs play a limited role in classification.

  • Renal mass protocol

    • NECT: quantify baseline attenuation and evaluate for intralesional fat and calcs.

    • Nephrogenic phase: 100 sec delay - evaluate for enhancement 

    • Excretory phase: 8 min delay - helpful to show the relationship of renal mass to the collecting system for surgical planning and to diagnose mimics of cystic renal masses.

    • *Arterial phase can be performed for presurgical planning.

• MRI:

  • Useful for the evaluation of solid renal masses. 

    • Can provide info about internal fat and fluid content and enhancement.

  • Limited role in the evaluation of a cystic renal mass.  

  • Can provide more accurate characterization of enhancement and is most useful to distinguish a Bosniak IIF lesion from a Bosniak III lesion (which impacts management). 

  • MRI is not obscured by calcification which also permits easier lesion evaluation.

• Enhancement

  • Presence of enhancement is the most important characteristic to diagnose a solid renal mass.

    • CT: enhancement is quantified as the absolute increase in HU on postcon images compared to precon images

      • <10HU no enhancement

      • 10-19HU: equivocal enhancement

      • >=20HU: definite enhancement

    • MRI: enhancement is quantified as the absolute increase in SI as measured on postcontrast images

      • <15%: no enhancement

      • 15-19%: equivocal

      • >=20%: definite enhancement

  • Intralesional fat is almost always diagnostic of benign AML

  • Nonfat containing renal mass can still be benign, particularly when <1cm but the risk of malignancy increases with increasing mass size (>3cm).

  • Lesions are considered "too small to characterize" if the lesion diameter is less than twice the slice thickness.

    • E.g. using 3 mm slices, a lesion <6mm cannot be accurately characterized based on attenuation or enhancement.

• Cystic renal mass: <25% of mass is composed of enhancing tissue

• Bosniak I and II masses that are characterized as benign can be described as cysts.

• Bosniak IIF, III, and IV lesions should be termed cystic renal mass

WORK UP & CALCULATOR

CATEGORY I

CATEGORY II

6 proposed Bosniak subtypes can be distilled to these rules:

• NONenhancing (excludes thin septa/wall which can enhance)

• <4 septa with thickness <=2 mm

• Proteinaceous/hemorrhagic cysts <=3 cm in size

  • Nonenhancing, homogeneous, >=70HU or intrinsically T1 hyperintense

• Homogenous low-attenuation masses too small to characterize (lesion diameter is less than twice the slice thickness).

• ANY VIOLATION GET UPGRADED

Cysts in the renal sinus may be classified as parapelvic or peripelvic

• Parapelvic: renal cortical cyst that herniates into the renal sinus and is usually solitary. Usually large but solitary.

• Peripelvic: 2/2 lymphatic obstruction; often small and multiple

• When multiple renal sinus cysts are present, the appearance may mimic hydronephrosis. In contrast, renal sinus cysts are not contiguous with each other and will not contain excreted contrast on delayed imaging.

A hyperdense cyst cannot be diagnosed on postcontrast imaging alone bc it cannot be distinguished from an enhancing renal mass (unless dual energy CT is used).

BOSNIAK IIF

BOSNIAK III

Cystic RCC: manifest as a complex cystic mass

Multilocular cystic nephroma: 

• Benign cystic neoplasm w enhancing septa that occurs in a bimodal age distribution in baby boys and middle-aged women.

• Characteristic but nonspecific feature is the propensity to herniate into the renal pelvis and cause hydronephrosis

  • Adults: indistinguishable from cystic RCC

  • Children: indistinguishable from cystic Wilms tumor

Mixed epithelial and stromal tumor (MEST): benign neoplasm typically found in middle aged women. MEST may appear as either a solid or cystic mass.

BOSNIAK IV

ADPKD

• Bilaterally enlarged kidneys w/ multiple large renal cysts

• Present w progressive renal failure in their 3rd to 4th decades

• Kidneys are sometimes palpable from enlargement and patients may display secondary HTN and hematuria (nephrolithiasis vs rupture of a renal cyst into the collecting system)

• Imaging: kidneys are markedly enlarged and feature multiple cysts of varying attenuation or SI 2/2 hemorrhage

• ADPKD is not known to increase the risk of RCC. RCC assoc w/ ADPKD tends to occur at a younger age and is more often bilateral, multifocal and sarcomatoid.

ARPKD

• Enlarged kidneys w/ innumerable tiny renal cysts

• diagnosis of infancy and has a poor prognosis

• If child survives infancy, hepatic fibrosis usually develops

• Presents in utero as enlarged echogenic kidneys. Cysts are too small to be individually resolved by US.

Acquired cystic kidney disease

• Pts on longterm dialysis often develop many small renal cysts superimposed upon atrophic kidneys.

• Dialysis assoc cystic renal dz has an increased risk of RCC (2-3% prevalence)

Localized cystic renal dz

• Benign slowly progressive proliferation of cysts typically w/n a normal functioning kidney and asx

• Always unilateral and usually affects only part of the kidney

• No imaging FU or tx required.

LITHIUM NEPHROPATHY

• related to longterm lithium use and can present as nephrogenic diabetes insipidus or chronic renal insufficiency

• Imaging: numerous scattered uniform microcysts in bilateral normal sized kidneys

• US: small cysts usually appear as punctate echogenic foci. 

• CT may show calcification w/n the cysts.

• MRI: tiny nonenhancing fluid intensity cysts in the renal cortex and medulla. 

WORK UP

• The likelihood of malignancy in a solid renal mass increases with lesion size

  • 50% of solid renal masses <1cm are benign

  • 75% of solid renal masses >3cm are malignant

• In the presence of a solid renal mass, the renal veins must be carefully evaluated for tumor thrombus and extension.

RCC

• MC solid renal mass and arises from renal tubular epithelium

• Represents 2-3% of CA.

• RF: Smoking, acquired cystic kidney dz, von Hippel-Lindau, and TS

• RCC has propensity for venous invasion, which alters surgical approach.