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Introduction

Congenital anomalies of the urinary tract (CAKUT) represent an important chapter of malformative conditions because this is the most important cause of end-stage renal disease in children. Some of the most significant entities from CAKUT are renal fusion anomalies which include horseshoe kidney (HSK), sigmoid kidney, pie kidney or indeterminate kidney. Horseshoe kidney is the most frequent renal fusion anomaly, with an incidence of 0.25% in the general population^(2,4). Embryologically, the abnormality occurs between the fourth and the sixth week of gestation after the ureteric bud has entered in the renal blastema^(1,3,4).

Horseshoe kidney is rarely associated to tumoral lesions. In the medical literature, we find less than 200 cases published with this unusual association^(3,6).

Materials and method

We made a retrospective research for a period of 10 years (between January 2010 and January 2020) using as keywords “horseshoe kidney” and “renal tumor” and looking for cases that associated both pathologic conditions. In most of the cases, patients have been investigated by ultrasound (US) first. In all the cases, US revealed a morphologic anomaly of the kidney, suspected as tumoral lesion.

Even though ultrasound is the most accessible and reproducible investigation, this being very useful as a screening method as well as a follow-up exam, all the patients discovered with tumoral pathology have been investigated through sectional imaging methods for a complete diagnosis. Sectional imaging investigations were MDCT in all cases and MRI in selected cases, sometimes as a complementary exam after an inconclusive MDCT.
CT urography protocol has been represented by a classic CT urography.

CT examinations included one non-enhanced CT phase (NECT) in order to evaluate calcification/lithiasis, necrotic/hemorrhagic areas inside urinary tract tumoral lesions and three contrast-enhanced phases (corticomedullary, nephrographic and excretory phases) in order to evaluate associated anomalies. Before the CT exam, the allergic history and the serum creatinine level had been checked.

The injection ratio has been minimum 3 ml/sec and the iodinated product was injected in a quantity of 1-1.5 ml/kgc. The patients have been investigated on 16 or 64 MDCT scanners.

The physical parameters used for CT urography consists in axial thickness slices of 5 mm with 1.25 or 1.5 mm reconstruction interval. In order to improve the visualization of urinary pathways in excretory phase, we usually used a diuretic (furosemide) which we generally administrated after non-enhanced phase, except for cases that associated urinary pathways obstruction.

Contrast-enhanced CT (CECT) images have been acquired at 15 seconds after bolus-tracking detection or approximately 30-45 seconds after the injections’ start (corticomedullary phase), 45-50 seconds after corticomedullary phase (nephrographic phase) and 240-300 seconds after nephrographic phase (excretory phase). If renal tumor is confirmed, a delayed CT scan realized after minimum 30 minutes is useful for detecting pelvic invasion or the presence of other tumoral lesions inside urinary pathways (in transitional cell carcinomas – TCC).

Different planes reconstructions (coronal, sagittal, oblique views) have been performed in order to have an optimal visualization of the urinary tract lesions.

MRI is a helpful method for the patients with diagnosis uncertainties after CT scan. Our standard MR-urography protocol includes:

• T2 weighted FSE with fat saturation in axial plane gives information about renal parenchymal tumors and cystic areas (inside tumor or simple cystic lesions);

• T2 with long TE in coronal plane with fat saturation for a general mapping of the urinary pathways, being extremely useful for urinary pathways morphology;

• T2 weighted with short TE in coronal plane is useful for parenchymal lesions, tumoral invasion of renal pelvis or vascular invasion;

• DWI sequence (axial/coronal plane) is useful in tumoral/inflammatory pathology;

• Dynamic contrast injection of gadolinium (T1 sequences) in coronal-oblique plane including one non-enhanced phase, enhanced multiphase acquisitions and excretory phase 10-15 minutes after the paramagnetic contrast i.v. injection, which proves very useful for tumoral enhancement comparing to normal renal parenchyma and renal excretion, detecting perfusion abnormalities in early phase and the relationship between the tumor and the urinary tract;

• Delayed 3D T1 GRE images with fat saturation, preferable in coronal plane – useful for tumoral or malformative urinary conditions associating obstruction and secondary HUN (for example, PUJO/VUJO);

• Optional sequences can be T1 weighted GRE (2D in coronal/axial plane) or late postcontrast 3 DT1 weighted images, 30-120 minutes after contrast injection.

MR-urography can provide useful information in terms of both morphologic and functional details related to the urinary system, having an important role in complex malformative conditions associated to tumoral pathology, especially in associated genital abnormalities, urinary tract obstructions without a clear cause (for excluding a tumoral cause), inferior urinary tract malformations unclear to previous imaging (for example, aberrant implantation of the ureter) or vascular extrinsic obstruction (such as aberrant vessels) and in postoperative dilemmas.

Results

We selected five cases of tumoral lesions associating HSK, three males and two females, with ages between 48 and 65 years old. Unfortunately, malformative pathology was ubiquitary discovered in most of the cases during imaging investigation realized for neoplastic disease. These five cases have been investigated by MDCT, CT urography being able to detect both tumoral and malformative conditions; two cases have been investigated also by MRI for a precise diagnosis^(9,10).

The location of the tumour was at the level of the isthmus in one case, and the other tumors arose from the right half-kidney (three cases) and the left half-kidney (one case).
 

Referring to the dimensions of the tumors, four of five tumors were voluminous, each one having dimensions over 10 cm, one of them being huge (large axis of almost 20 cm).
Three of the cases had associated thrombosis, two of them had tumoral thrombosis secondary to tumoral vascular invasion and one only cruoric thrombosis that affected the proximal segment of IVC and iliac common veins, secondary to tumoral compression of IVC. In two cases, thrombosis had extended into the ipsilateral renal vein. One case associated tumoral thrombosis and extended above diaphragm till the right atrium (RA). In one case, the thrombosis extended into the contralateral renal vein.

Three cases had metastatic disease. In one case we found an end-stage neoplastic disease with pulmonary, hepatic, adrenals,  bone metastasis and adenopathies. The second case had possible bone metastasis. Another case had multiple adenopathies and distant metastasis located in the lungs.

Three of the cases had an imaging aspect on the enhancement curve suggestive for clear-cell renal cell carcinoma (ccRCC).  The enhancement curve was based on corticomedullary, nephrographic and excretory phase⁽¹⁰⁾. The enhancement curve of different histopathological subtypes of RCC is represented in Figure 1^(11,12). The enhancement curves from our patients are represented in Figure 2.

We did not find any transitional cell carcinoma (TCC) or benign lesions.

Most of the patients have been considered surgically non-resectable, only one having a partial nephrectomy⁽³⁾.

The most significant two cases are presented below.

Case 1: Female, 52 years old, HSK and left unit renal tumor

Case 2: Male, 65 years old, HSK and voluminous right renal tumor

Discussion

Most of the horseshoe kidneys are asymptomatic, being discovered as incidentalomas during CT or MRI investigations performed for suspected tumoral lesions⁽¹⁾. The bridge between two renal units can be formed by parenchymal or fibrotic tissue and almost all cases are located at the lower pole of the kidney. The double association of minimal ectopia and malrotation is a predisposing factor for obstruction, infections, trauma or tumors⁽¹⁾.

There are another malformative conditions associated to HSK such as vesico-ureteric reflux (VUR) and genital malformative conditions.

Even though tumoral lesions developed on the horseshoe kidney are rare, the frequency of tumoral lesions on horseshoe kidney is higher than on normal kidney, this frequency being explained by the particular anatomy and the supplementary vascular supply⁽¹⁾.

The radiologist has an important role in detecting and characterizing both pathologic conditions. The clinician’s questions refer to location of the lesion, relations with neighboring organs and especially depicting vessels that feed the tumoral lesion or the remaining part of the kidneys after heminefrectomy (if possible).

The large dimension of the tumors (four tumors bigger than 10 cm) is highly suggestive for malignancy.

In children we can rarely find renal tumors, most of them being histologically proved as Wilms tumors, but the frequency of this association (HKS and WT) is higher^(1,3,5). In adults, tumors like RCC, TCC, carcinoid tumors, oncocytomas or sarcomas^(1,7,8) can appear. The most frequent tumor on HSK is RCC, but the frequency of RCC is similar to normal kidneys, so there is no increased risk in HSK for developing RCC^(1,3,11).

In our cases, all renal tumors have an imaging aspect of malignant tumor, meaning intense “wash-in” in the parenchymal areas of the tumors and rapid “wash-out”. From the histological subtypes of RCC, the enhancement curve on CT scan of three of our patients, measured in Hounsfield units (HU), is similar to clear-cells RCC found in the literature rather than papillary or chromofobe RCC, even though only one case had a histopathologic confirmation.

The imaging differences between ccRCC and chromophobe type of RCC are not so obvious, so we cannot certainly exclude this histopathological type (Figures 1 and 2).

The other two cases are uncertain, even though one of them (Case 4) has an enhancement curve which can suggest papillary RCC.

The frequency of carcinoid tumor is higher than of normal kidneys, but we did not find any case. The bridge is more frequent affected by the tumoral lesions because of the teratogenic injuries which are at the origin of bridge development⁽¹⁾. Also, the frequency of TCC on horseshoe kidney is higher because of urinary stasis which is a favorable factor for lithiasis and infections^(1,3,11).

The most frequent sites for metastatic disease are: lungs, adrenals, abdominal organs (like liver) and brain⁽¹³⁾. Lungs are the most common site of distant metastasis.

The cases that associate thrombosis (renal vein, VCI or common iliac vein thrombosis) are considered surgically non-resectable or in a terminal stage, that’s why these cases don’t have biopsies. Tumoral thrombosis is a severe but rare complication of neoplastic disease, its frequency in the literature being appreciated at 10% of tumoral lesions⁽¹³⁾.

CT is the most convenient imaging method, giving us complete information regarding the malformation (isthmus type, degree of malrotation, associated vascular anomalies – aberrant/multiple vascular variants if present) and also about the tumoral lesions (tumor location, tumoral components – cystic/necrotic areas or calcifications, degree of vascularity, renal pelvis invasion, another tumoral lesions inside the urinary tract in TCC type, tumoral extension, distant metastasis including pulmonary metastases), giving us the possibility of a correct staging in a single examination.

The imaging investigation algorithm for this pathological association is presented in Figure 6.

Conclusions

CT urography is an excellent diagnostic tool for tumo­ral and malformative lesions of the urinary tract, remaining the main imaging method for the reno-urinary pathology.

MR-urography represents an additional imaging tool in such cases, providing useful information in terms of both morphologic and functional details related to the urinary system, having an important role in complex malformative conditions (especially pelvic anomalies) associated to tumoral pathology. 

Conflicts of interests: The authors declare no conflict of interests.