1- Developmental Renal Pseudotumors:
Prominent Columns of Bertin (PCB) Fig. 1
Prominent Columns of Bertin is a normal variant of the kidney formed by hypertrophied cortical tissue located between the pyramids that projects towards the renal sinus,
resulting in the expansion of the sinus.
It is due to the lack of reabsorption of the polar parenchyma of one or both kidneys that must be fused to form normal kidneys.
They are usually located at the junction between the upper third and the middle third of the kidney.
They usually measure less than 3 cm and contain kidney pyramids inside.
It may be difficult to differentiate from a small avascular tumor,
however,
the demonstration of the arcuate arteries in the US-Colour Doppler suggests a PCB rather than a tumor.
Patients with an atypical sonographic appearance or unusually large and prominent columns (septa) of Bertin are usually followed up with contrast-enhanced CT or MRI.
Prominent columns of Bertin can be confirmed by showing its enhancement to be similar to that of surrounding renal parenchyma on contrast-enhanced CT,
MRI,
or contrast-enhanced sonography.
Dromedary Hump Fig. 2
It corresponds to a focal protrusion on the lateral border of the left kidney,
due to the adpatation of the kidney to the adjacent spleen.
It is well defined,
with a vascularization similar to the surrounding renal parenchyma and without expansive effect.
Persistent Fetal Lobulation
The fetal kidneys are subdivided into lobes that may be separated by grooves.
Sometimes,
these lobulations persist into adulthood and can simulate a kidney tumor or scarred kidney.
Fig. 3
A persistent fetal lobulation is confirmed by demonstrating the presence of renal pyramids in the bulge bounded by septa of Bertin on either side.
Kidney Horseshoe Fig. 4
The incidence of horseshoe kidney in the general population is 0.01% to 0.25%.
In 95% of cases,
fusion occurs at the lower poles.
Typically,
the isthmus is composed of functioning renal tissue,
although in rare cases it is formed by fibrous tissue,
and in these cases it may simulate a renal tumor.
2- Infectious Renal Pseudotumors:
Focal Pyelonephritis
Focal pyelonephritis is seen on sonography as either a hypoechoic or hyperechoic lesion in the renal cortex extending from the renal medulla to the renal capsule,
with decreased perfusion on US-Colour Doppler imaging.
At CT or MR imaging,
pyelonephritis classically appears as a striated or heterogeneous nephrogram and may be unilateral or bilateral.
Fig. 5
In some cases of focal pyelonephritis,
a poorly defined interface between the infection and the renal parenchyma,
edema of the surrounding renal parenchyma,
or asymmetric perinephric stranding may be a hint to the proper diagnosis. Fig. 6
This alteration of the renal parenchyma associated with the classic symptoms of pyelonephritis (flank pain,
fever,
urinary symptoms and leukocytosis),
allows a correct diagnosis of the pathology.
However,
in patients in whom symptoms of infection are not clear,
focal pyelonephritis may mimic the appearance of a solid renal neoplasm.
Renal Abscess Fig. 7
Ultrasound and CT reveal a well-defined heterogeneous mass that may resemble a cystic renal neoplasm.
This differentiation can be very difficult when the typical clinical findings of the infection are not present.
In some cases,
the radiologist may be the first to consider the diagnosis based on imaging findings such as: irregular walls with increased through-transmission on sonography,
low attenuation lesion with enhancing walls on CT,
increased perirenal stranding and thickening of the Gerota fascia.
In these cases a remote history of fever,
leukocytosis,
or previously treated urinary tract infection should be sought.
3- Granulomatous Renal Pseudotumors:
Xanthogranulomatous Pyelonephritis (XGP) Fig. 8
Xanthogranulomatous Pyelonephritis is a rare inflammatory condition usually secondary to chronic obstruction caused by nephrolithiasis and resulting in infection and irreversible destruction of the renal parenchyma.
Focal or segmental XGP is more likely to mimic renal cell carcinoma on imaging because of its radiologic similarities,
which often result in its resection.
Sonographically,
XGP may appear as single or multiple hypoechoic areas in the parenchyma of an enlarged kidney,
with central echogenic foci representing calculi.
CT evaluation can be considered helpful in the presence of features such as abscess replacing the renal parenchyma,
with low-attenuation areas (lipid-rich xanthogranulomatous tissue) and calcification in the mass.
If calculi are not present,
focal XGP with a low-attenuation area in the renal parenchyma may suggest a diagnosis of renal tumor.
Renal Tuberculosis Fig. 9
Renal tuberculosis occurs due to hematogenous seeding that affects the glomerulus and peritubular capillaries from a pulmonary site of primary infection.
Renal involvement begins with the formation of small granulomas in the renal cortex,
adjacent to the glomeruli (patient may be asymptomatic).
Upon reactivation of host defense mechanisms,
cortical granulomas increase in size.
Granuloma formation,
caseous necrosis and cavitation are stages of progressive infection,
which can eventually destroy the entire kidney (Patients may have symptoms of urinary tract infection,
low weight,
anemia and hematuria).
Communication of the granulomas with the collecting system can lead to regional spread of the bacilli into the renal pelvis,
ureters and urinary bladder.
The host response induces fibrosis,
calcium deposition,
and stricture formation which may contribute significantly to obstruction and progressive renal dysfunction.
It is common to form granulomatous masses with necrosis and calcifications inside,
and great renal destructuration.
If the history of tuberculosis is not known,
these masses may not be differentiated from renal neoplasms.
4- Inflammatory Renal Pseudotumors:
IgG4 Renal Pseudotumors Fig. 10
Hyper-IgG4 disease is a systemic autoimmune disorders in which there is a patchy or diffuse infiltration of the organs with IgG4 positive plasma cells along with fibrosis.
It affects more frequently middle-aged males.
The main characteristic features of this disease are the elevation of serum IgG4 and/or infiltration of different organ by IgG4 positive plasma cell along with tissue fibrosis and obliterative phlebitis.
The most common organs involved include the pancreas,
salivary glands,
and ocular adnexa.
Renal involvement is very uncommon.
Most reported cases of IgG4-related disease involving the kidney have a history of prior pancreatic involvement.
On imaging studies,
the most common findings are low attenuation cortical nodules,
wedge shape lesions,
or diffuse patchy areas. These lesions usually do not present areas of hemorrhage or necrosis inside.The size is variable,
reporting cases of lesions of more than 8 cm in maximal dimension.
At US,
the lesion can present as a hypoechoic or hyperechoic mass with associated vascularity at color Doppler ultrasound; and at CT,
these lesions may be low in attenuation.
Most reported IgG4 kidney patients have been diagnosed with serologic studies showing a marked elevation of IgG4 and supportive radiographic imaging.
The treatment of choice is corticosteroids.
In the few case reports and case series the renal masses have been shown to decrease in size on therapy,
with some disappearing and others recurring after cessation of therapy.
The diagnosis requires a biopsy,
however,
in patients with a history of IgG4-related disease in other organs,
they allow a control of the evolution of the lesion through imaging studies after beginning corticosteroid therapy.
5- Vascular Renal Pseudotumors:
Renal Haematoma: Fig. 11
Renal hematoma and,
in particular,
subepithelial pelvic hematoma is a rare entity that clinically simulates a renal malignancy.
The diagnosis of this entity is based on imaging tests.
The knowledge of this rare benign pseudotoumoral lesion can prevent an unnecessary surgery.
However,
in the published literature,
the difficulty of this preoperative diagnosis is pointed out,
which in most cases has led to nephrectomy,
establishing the definitive diagnosis by means of the pathological examination.
Ultrasound usually shows an enlarged kidney,
associated with an oval image in the renal sinus,
without vascularization in US-colour doppler.
CT showed a high-attenuation round lesion in the left renal sinus that did not enhance after contrast.
The excretory phase can show an extrinsic compression on the renal pelvis,
with or without obstructive effect.
It is often difficult to rule out an underlying lesion,
and it may be useful to complete an MRI study after a few days.
The history of anticoagulant therapy or vasculitis and the absence of contrast enhancement of the lesion are indicative of the existence of a hematoma.