Benign Mimics and Tumor-like Lesions in Urologic Pathology
Moderators: Dr. Mahul Amin and Dr. Liliane Boccon-Gibod
Section 1 -
Benign Mimics and Tumor-Like Lesions of the Kidney
Professor of Surgical Pathology, Hôpital Armand Trousseau
75571 PARIS Cédex (France)
Going through the Medline and looking for what has been published in the last decades
about renal pseudo-tumors tells you a lot about medical history and extraordinary progress of medical
Prior to the era of CT scan and MRI, a large amount of non tumoral or benign lesions could
be mistaken for a true renal tumor. In the literature from the 80's sonography proved to be a necessary
complement to excretion urography in assessing cysts, pseudo-tumors and variants of the structure of the
kidney, resembling space occupying lesions in the excretory urogram which was then the gold standard
One good example of Ultrasound importance is Bertin's columns hypertrophy, which could
easily be mistaken for intra-renal tumor. Renal tumors are either more echo producing or less echogenic
than adjacent normal renal parenchyma, whereas pseudo-tumors have the same echo characteristics as
surrounding and adjacent normal renal parenchyma. Most of these differential diagnoses have
nowadays disappeared, even prior to biopsy or surgery, and most of the present discussions take place
around the CT scan and/or the MRI.
Going back through our experience, we will focus on the issues which seem important to us,
in adults and also for some of them in children.
Xanthogranulomatous Pyelonephritis (XGPN) is a chronic suppurative inflammation of the renal
parenchyma that may extend to contiguous retroperitoneal and intra-abdominal structures and be suggestive
of renal malignancy.
Grossly, the kidney is destroyed by large yellowish masses distributed within the renal parenchyma
around the calices. This yellowish colour may be again misleading for a RCC. Histologically, large
sheets of lipid laden macrophages are encountered, sometimes surrounding a true abcess with neutrophils.
Surgery is curative, and most cases undergo nephrectomy, either with a presumed diagnosis of renal
tumor either with diagnosis of XGPN. XGPN can be focal and /or cystic therefore making the diagnosis
still more difficult with a renal tumor prior to surgery. Aspiration cytology or biopsy may be of help.
Malakoplakia is a rare chronic inflammatory disease affecting predominantly the bladder, occurring
occasionally in other organs from the genito-urinary tract (kidney, testis) and other organ systems.
Renal malakoplakia usually affects women over 40 years of age and is associated with recurrent urinary
tract infections. E. coli is the most common pathogen. Approximately 20%
of patients have an immunocompromised state. Clinical presentation (hematuria, palpable mass) and
radiological appearance often simulate a neoplasm, notably Renal Cell Carcinoma (RCC). Fever, flank
pain, pyuria may be associated and suggest an infectious mass. US and CT scan can reveal uni or
bilateral enlargement of the kidney. In the case of unilateral enlargement, nephrectomy has often been
performed, with a preliminary diagnosis of malignancy. In the more recent cases, MRI and early renal
biopsy have helped insure diagnosis; medical therapy has resulted in resolution of the lesions in some
cases, although in unifocal cases, surgical excision is still the treatment of choice. Grossly, affected
kidney present with soft tan yellow plaques and nodules, which can reach 3- 4 cm in diameter.
Histologically, renal parenchyma is infiltrated by conglomerates of foamy histiocytes, containing
abundant PAS positive granules (Von Hansemann cells) and diagnostic extra or intracytoplasmic basophilic
inclusions with surrounding clear halos (Michaelis-Gutman bodies). Important differential diagnoses are
post transplant lympho-proliferative disorders (PTLPD) and lymphoma, especially in the setting of
immunocompromised patients. Renal malakoplakia seems medically treatable when detected in early stages.
Inflammatory Myofibroblastic Tumor
Inflammatory Myofibroblastic Tumor (IMT) is a well known, although rare, entity. First described in
the lung, mainly in young adults and children, IMT has been now described at multiple extra pulmonary
sites. In the GU tract, IMT are more frequent in the bladder than in the kidney. A sery of 12 cases
have recently been reported by Kapusta et al: interestingly renal IMT occur later (mean age 57 y) in
life than other localisations. Tumours are mainly localised into the renal sinus (6/12) and present the
histological patterns described by Coffin et al in IMT: a) loosely organised spindle cells admixed with
inflammatory cells in a myxoïd background b) spindle cell proliferation admixed with collagen, with
lymphocytes and plasma cells c) hypo cellular fibrous tissue with fibrosis and few inflammatory cells.
Immunostaining for SMA and muscle specific actin was positive in most cases. Interestingly ALK was
negative in all the cases tested. This is again different form cases reported in the lung.
When facing a large cystic or multicystic renal lesion in an adult the main question is:
is it a true cystic lesion like a cystic nephroma (CN), a benign lesion with cysts like a cystic
hamartoma of the renal pelvis, or is it a real malignant tumor, like a multicystic renal cell carcinoma?
One can dispute the true nature of cystic nephroma. In children; this lesion has for a long time
been considered as a malformative cyst (syn: multilocular cyst of the kidney, or renal cystadenoma);
more recently the possibility of CN being the fully mature end of Cystic Partially Differentiated
Nephroblastoma (CPDN) has been raised. The differential diagnosis between these 2 entities relies on the
presence of immature metanephric elements (mainly tubules, rarely blastema) or heterologous components in
the septa of CPDN, whereas the thin septa of a Cystic Nephroma contain only connective mature tissue.
The treatment and prognosis issues for CN and CPDN are similar: surgery alone (nephrectomy) is followed
by total cure; no relapse have been described in children.
In the adult, when facing a putative CN, or others renal cysts, the multicystic variant of renal cell
carcinoma is an important issue to raise. Tumor component with clear cells can be obvious, but more
often, clear cancer cells are disposed in one layer and not present along most septa. A long search of
the different slides may be necessary to prove the neoplastic nature of the multilocular cyst. Prognosis
of this form of renal clear cell carcinoma is excellent, as surgery alone is curative, and can in some
instances be limited to partial nephrectomy. . Three cases of sarcomatous transformation of cystic
nephroma in adults have been reported on a poster at the 1997 USCAP meeting in Orlando , but these cases
have never been published and looking back to the images, they were more suggestive of other diagnoses.
Does Adult Mesoblastic Nephroma Exist ?
Description of Congenital Mesoblastic Nephroma (CMN)
CMN is the most frequent renal tumor in infants under 6 months of age and is not seen over 1 year of
age. Cases published in older children have been recently identified as Mesoblastic Stromal Tumor (MST);
Sex ratio M/F is 1:1; the renal mass is never encapsulated and is extremely infiltrative, invading sinus,
hilar and perirenal fat; it is composed predominantly of spindled mesenchymal cells of fibroblastic or
myofibroblastic lineage, admixed with a small number of tubules, seen at the periphery of the tumor
(entrapped normal renal tubules). The cellular variant is associated with t(12;15) translocation and
ETV6-NTRK3 gene fusion and is the renal counterpart of congenital (infantile) fibrosarcoma. The
classical variant of CMN is not associated with any gene fusion and only exceptional chromosomal
abnomalies have been associated with this lesion which is now considered by many as the renal counterpart
of of soft tissue fibromatosis. No epithelial component is associated in true CMN.
Tumors described in the adult as mesoblastic nephroma are totally different : Sex ratio M/F is 1:9,
lesions are encapsulated and a columnar epithelial component, sometimes presenting with cilia is
associated with the stromal component. Since Cystic Hamartoma of the renal pelvis has been described by Pawade in 1993 and Mixed epithelial and stromal tumor of the kidney by MIchal in 1998, these
diagnoses are more and more frequent in the urological community, and less and less adult MN have been
reported. It is our opinion that most of the previously published so-called "Adult" Mesoblastic
Nephroma" are either Cystic Hamartoma of the renal pelvis or Mixed Epithelial and stromal tumors of the
kidney, these last 2 entities being extremely close, not to say similar. Stromal estrogen and/or
progesterone receptor positivity in the majority of cases of cystic nephroma is a novel finding which
strongly support this view, which was already proposed by JB Beckwith in 1998 . The spectrum of Adult MN
is shrinking and we foresee it's disappearance in the next future.
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