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ECR 2019 / C-2493
Macroscopic fat containing lesions in the perirenal space
Congress: ECR 2019
Poster No.: C-2493
Type: Educational Exhibit
Keywords: Kidney, Retroperitoneum, CT, Diagnostic procedure, Tissue characterisation
Authors: M. Gredilla1, M. LETURIA ETXEBERRIA2, A. Serdio3, J. Elejondo Oddo4, F. J. Barba Tamargo5, E. Pardo Zudaire6; 1DONOSTIA/ES, 2San Sebastian, Gipuzkoa/ES, 3Donostia - San Sebastián/ES, 4San Sebastián/ES, 5San sebastian, Guipuzcoa/ES, 6San sebastian/ES

Findings and procedure details

Herein we report a variety of fat containing lesions appearing in the perirenal space:


1.      Renal angyomyolipoma and complicated angyomyolipoma with Wünderlich Syndrome:


Angyomyolipoma is the most frequent benign mesenchymal tumour of the kidney. It is composed of fat, smooth muscle and abnormal blood vessels. It can be a sporadic lesion or occur as a part of tuberous sclerosis. The vast majority will be sporadic and happen more frequently in middle-aged women.


On US angyomyolipomas appear as hyper-echogenic, well-defined lesions in the renal cortex with rear acoustic shadow.


On CT lesions will show fat attenuation values (less than -20 HU), demonstrating the presence of macroscopic fat.


It is important to emphasise that there are fat-poor angyomyolipomas and others that may only have microscopic fat, appearing hyper-dense on non-enhanced CT and hypo-intense on T2-weighted images. It is in these occasions in which MR fat-suppression sequences could be of use, showing the loss of signal intensity. Differential diagnosis must be made with the renal cell carcinoma (RCC).


Large lesions, > 4 cm, are at greater risk of spontaneous haemorrhage, which can be fatal. This is known as Wünderlich syndrome and depending on the number, size and clinic, different type of treatment will be needed (total or partial nephrectomy, arterial embolization, ablation therapies or medical treatment with mTOR inhibitors).


2.      Complicated cyst with fat:


Cysts are the most common focal kidney lesion in adults. Bosniak classification is used to grade cysts, from benign to malignant.


US is the chosen imaging technique used to distinguish cystic from solid renal lesions. CT and especially triple-phase CT will better characterise the lesions, always keeping in mind they may present atypical characteristics; morphological changes, fatty levels calcifications, septations, etc.


3.      Perirenal haemangioma:


Perirrenal haemangiomas are extremely uncommon benign mesenchymal neoplasms with variable and nonspecific imaging findings, sometimes mimicking those of exophytic renal cell carcinoma or other perirrenal tumours.


US usually shows a hyper-echoic, or less frequently hypo-echoic, mass. They will appear hypo-dense at non-enhanced CT with a globular and heterogeneous after contrast administration. High-signal intensity with flow-voids is a typical finding on T2-weighted images, allowing for diagnosis through MR.


4.      Extra-medullar haematopoiesis:


Process characterized by the development of haematopoietic tissue outside the primary medullar sites. It can be associated to hemolytic anemia, haemoglobinopathies, primary or secondary myelofibrosis, leukemia, lymphoma or metastases.


The most frequent sites of extra-medullar haematopoiesis are the liver, spleen and para-spinal regions of the thorax. However, it has also been described in other locations, including lymphatic nodes, mediastinum, central nervous system, pleura, lungs, heart, gastrointestinal tract and the kidneys.


The extra-medullar haematopoiesis affecting the perirrenal space is infrequent and it manifests as an infiltrative process that surrounds the kidneys or as a soft-tissue mass mixed with macroscopic fat.


This entity causes diagnostic difficulties as it can simulate neoplasms. Definitive diagnosis is usually established pathologically.


5.      Phaeochromocytoma with brown fat:


Phaeochromocytomas are uncommon neuroendocrine catecholamine-secreting tumours that arise from the chromaffin cells of the adrenal medulla. They are usually solitary and sporadic, although hereditary phaeochromocytomas associated to certain syndromes have also been described (Von Hippel-Lindau, Multiple Endocrine Neoplasia type 2 and Neurofibromatosis type 1).


The role of US is limited, with incidental adrenal masses being detected when they present a large size. 


CT is the most commonly used method for diagnosis. On un-enhanced CT they present as large and heterogeneous masses with areas of necrosis and cystic changes. When injecting contrast they show avid and heterogeneous enhancement more remarkable on portal venous phase. A small percentage may demonstrate calcification.


MR is the most sensitive modality for identification of phaeochromocytomas, and is useful in differentiating phaeochromocytomas from adenomas. Phaeochromocytomas manifest a high intensity signal on T2-weighted images compared to adenomas.


Functional imaging and PET/CT may be of use for better characterisation.


6.      Perirenal myelolipoma:


Myelolipomas are rare benign tumours localised in adrenal glands, being extra-adrenal myelolipomas, including perirenal ones, far rarer.


The imaging findings will depend on the major component of each mass. US echogenicity will be heterogeneous due to non-uniform structure.


Lesions will have negative HU values on CT with some high-attenuation regions corresponding to calcification or haemorrhage, causing diagnostic difficulties.


On T1-weighted images MR myelolipomas are seen hyper-intense with mild enhancement of the soft-tissue components after contrast injection. Fat-Suppressed sequenced will permit diagnosis by demonstrating signal loss.


CT findings are of utmost importance for pre-operative diagnosis and for establishing the differential diagnosis, which must be made with a wide variety of pathologies; benign (angyomyolipoma, lipoma) and malignant (liposarcoma).


7.      Perirenal lipomatosis:


Benign and unusual entity in which a marked atrophy and destruction of renal parenchyma and excessive fatty tissue proliferation occurs in the renal sinus and perirrenal space.


It is generally unilateral and associated to lithiasis, causing hydronephrosis and chronic inflammation.


US may suggest an echogenic mass that will posteriorly be characterised and diagnosed through CT, where renal parenchyma atrophy/destruction, perirrenal fat proliferation and calculi may be recognised. 


Differential diagnosis will be made with malignant neoplasms affecting the same region.


8.      Perirenal liposarcoma:


Retroperitoneal liposarcoma is a subtype of liposarcoma, a malignant mesenchymal tumour that may arise from any fat-containing region of the body. Liposarcoma is the most frequent primary retroperitoneal malignant neoplasm.


There are five histologic subtypes: well-differentiated, myxoid, round-cell, pleomorphic and dedifferentiated. Imaging findings reflect the histologic features, and therefore will depend on the subtype.


Well-differentiated liposarcomas, biologically the less aggressive ones, are the ones predominantly composed of macroscopic fat (lipoma-like component). On CT they will appear as hypo-dense masses of fatty tissue overgrowth, with similar attenuation values to that of subcutaneous fat, with fibrous, more irregular and nodular septa that will help distinguish liposarcoma from lipoma.


9.      Xhantogranulomatous pyelonephritis:


Chronic, granulomatous, destructive process of the kidneys thought to be related to an atypical and incomplete immune response to sub-acute, recurrent, bacterial infections. It usually affects middle-aged patients with female gender predilection.


Due to repeating bacterial infections, renal parenchyma is substituted by foamy macrophages, and the loss of renal function and destruction of renal parenchyma is caused by the severe and diffuse inflammation. The majority of the cases are associated to renal calculi, and therefore, hydronephrosis could be a contributing factor.


On the US kidney will appear enlarged, with central amorphous echogenicity and rear shadowing due to large size calculi (horn-shaped). In most cases a loss of normal renal architecture is seen.


TC is the imaging technique of choice for definitive diagnosis for two reasons; firstly, the majority of the cases show highly specific features that allow diagnosis of confidence. Secondly, the surgical plan depends on the correct description of the extra-renal extension.


10.  Subcapsular hematoma following partial nephrectomy, ureteroscope, radiofrequency and biopsy:


Percutaneous renal techniques with US guidance are helpful as both diagnostic and therapeutic methods. Peripheral and exophytic lesions can be more safely treated than central lesions that, as a general rule, get complicated more often, undergoing hematoma, infarction, urinoma, cutaneous fistula and ureteral stricture.


Despite the benefits of percutaneous procedures, as previously mentioned, complications must always be taken into consideration, as well as the necessity of follow-up imaging studies.


Hematomas are, fortunately, infrequent complications that seem worse at imaging than at clinical examination and that often resolve spontaneously. Here we present 4 post-non-invasive intervention cases of hematomas, which, additionally, exhibit fat degeneration as hypo-dense areas on CT.

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