Atrophic Kidney-Like Lesion – Case Report of A Provisional Entity with Brief Review of Literature

Atrophic kidney-like lesion is a recently recognized entity, post 2016 World Health Organization Classification of tumors of the urinary system. The behavior of this tumor is not fully known as only a handful of cases with limited follow-up are available. This entity closely mimics thyroid-like follicular carcinoma of the kidney, which has different prognosis. We report a case of incidentally detected atrophic kidney-like lesion in an elderly gentleman who had urothelial carcinoma of the urinary bladder with a brief review of literature. Atrophic kidney-like lesion and urothelial carcinoma of the urinary bladder association has not been reported in the literature.


INTRODUCTION
Atrophic kidney-like lesion (AKLL) is a recent entity described post 2016 World Health Organization (WHO) classification of tumors of the urinary system. With the available limited follow-up data, this is considered as a benign renal neoplasm with indolent behavior. Owing to its follicular architecture, it closely resembles thyroidlike follicular carcinoma of the kidney (TFRCC) (1). The distinction between the two is essential as the latter has chromosomal alterations in the form of gains and losses and an aggressive behavior with metastatic potential (2,3).

CASE REPORT
A 71-year-old gentleman was a known case of recurrent low-grade urothelial carcinoma for 9 months for which he had undergone transurethral bladder tumor resection thrice. On routine surveillance, contrast enhanced computed tomography (CECT) abdomen showed a wellcircumscribed tumour measuring 2.6x2.4cm in the midthird region of the right kidney with focal extension into the upper pole. The kidney measured 5.5cm in length with indistinct corticomedullary junction ( Figure 1A,B). The patient underwent simple nephrectomy for the lesion. On gross examination, the renal capsule was intact, and the cut surface showed a well-demarcated tan brown lesion measuring 3x2.8x2.5cm in the mid-third region. ( Figure  1C). The renal sinus and ureter were unremarkable. On microscopy, the lesion was well demarcated from the renal parenchyma by a thin fibrous capsule ( Figure 2A). The lesion was composed of compact tubules lined by flattened and atrophic epithelium interspersed by cyst-like follicles. Many of the follicles were filled with pale to dense eosinophilic material detached from the epithelium ( Figure  2B,C). The stroma between the follicles showed collagen deposition, few atrophic tubules, and capillaries. Focal amorphous calcific areas were also noted ( Figure 2D). No atypical features like mitosis, necrosis or high nuclear grade areas were noted.
Distinction of atrophic kidney-like lesion from the end stage renal disease induced by chronic pyelonephritis is important. Chronic pyelonephritis with thyroidization also shows atrophic tubules with hyaline casts mimicking the neoplasm. The important distinguishing point is the presence of a well-defined capsule, lack of inflammation, and absence of glomeruli in-between the lesion. In the present case, the background renal parenchyma did not show features of chronic pyelonephritis. The differential diagnosis of AKLL includes metastatic follicular carcinoma of the thyroid, metanephric adenoma, multilocular cystic renal neoplasm of low malignant potential, and International Society of Urological Pathology (WHO/ ISUP) Nuclear grade 1. There were no post-surgical complications. The patient has been on regular follow-up till date without any recurrence or distant metastasis.

DISCUSSION
The nomenclature "Atrophic kidney-like lesion (AKLL)" has been used interchangeably in the literature with "atrophic kidney-like tumour" and "atrophic kidney-like renal cell carcinoma" (4,5). This was originally described by Hes et al in 2014 as a separate entity and its clonal nature was proven in a series of 3 cases (1). Atrophic kidneylike lesion was initially considered under Thyroid-like follicular carcinoma of the kidney (TFRCC) (5). Following the original description, many cases were reviewed and described as reports and case series, of which few were initially diagnosed as TFRCC (6,7). The pathogenesis of AKLL is compared with that of glomerulocystic disease as the cells lining the cysts expressed WT-1, which is a marker of podocyte differentiation (8). At low power, both To the best of our knowledge, 13 cases of AKLL have been reported in the English literature (Table I). Many of the cases were reviewed in the previously published cases of TFRCC. This lesion is usually reported in young patients with no major gender predilection. Age distribution of AKLL is from 9 to 58 years with a median age of 30.7 years. No specific genetic alterations have been reported yet (8).
Classification of renal tumors is constantly evolving, and many new entities are being added and existing entities are being reclassified. Atrophic kidney-like lesion is a new entity with indolent behavior and described using different names. The Genitourinary Pathology Society (GUPS) has placed this lesion under "provisional entity" requiring more data for validation (4). The present case had thin well-defined capsule, occasional amorphous calcification with PAX8 and CK7 positivity. Features like high-grade nuclei, mitosis or necrosis were not seen. This index patient had completed treatment for recurrent low-grade papillary urothelial carcinoma of the urinary bladder and the renal lesion was incidentally detected on routine radiological surveillance. This association of AKLL with urothelial carcinoma has not been reported before.