Renal leiomyomas are exceptionally rare, benign, mesenchymal tumours originating from smooth muscle in the kidney. Renal smooth muscle can be found within the renal capsule, calyces, pelvis, and blood vessels; hence, any of these areas can be a site of origin for leiomyomas.
Steiner and colleagues found that of symptomatic leiomyomas, 53% of tumours were localised subcapsular, 37% capsular, and 10% within the renal pelvis .
Historically, because of their small size, most renal leiomyoma cases were discovered incidentally based on autopsy findings . However, since the advent and improved access to imaging modalities such as ultrasound and CT, renal leiomyomas are being discovered more frequently.
Although usually incidental discoveries, clinical presenting signs and symptoms comprise abdominal or flank pain, a palpable flank mass, and haematuria in 20% of those with symptoms. Cases suggest a higher prevalence in adult females, with an average age of 42 at presentation [1, 3]. Leiomyomas of the kidney are even rarer in the paediatric population, and there have been very few reported cases of these tumours in the literature. Some cases of interest included a six-year-old boy with an initially suspected Wilms tumour; a leiomyoma occurring in a transplanted kidney in association with the Epstein–Barr virus (EBV), five years post renal transplant; and a vascular renal leiomyoma in an 11-year-old with bilateral RCC .
Over ten years, the Brady Urological Institute reviewed several consecutive nephrectomies as a result of renal tumours. Of a total of 1030 nephrectomies, renal leiomyomas comprised 0.3% of the total, and 1.5% of all benign tumours removed .
In terms of etiological factors leading to the development of a renal leiomyoma, there are various theories and suggested causative factors. Krishnan and colleagues showed a possible causal relationship between Epstein-Barr virus and renal leiomyoma in immunocompromised patients, while Tsujimura and colleagues suggested a possible link with tuberous sclerosis; a condition more commonly associated with renal angiomyolipoma (AML) [6, 7].
When considering the relevant differential diagnosis concerning clinical presentation and imaging studies, three renal tumours that are more commonly found are AML, oncocytoma, and the diagnostic importance that lies in distinguishing them from the malignant RCC . Renal cell carcinomas make up the majority of contrast-enhancing small renal masses, the most prevalent histological type being the clear cell RCC. Frank and colleagues demonstrated a clear cell RCC incidence of 55% after 2770 surgeries (radical nephrectomies or nephron-sparing) for solid, unilateral, non-metastatic tumours .
AMLs are benign tumours in the perivascular epithelioid cell tumour family, comprising variable mixtures of thick-walled blood vessels, mature fat, and smooth muscle. It may sometimes be represented by dominance of the smooth muscle component, hence its likeness to a leiomyoma. Furthermore, they are characterised by smooth muscle and melanocytic marker (HMB-45) co-expression . In contrast to this, renal leiomyoma tumour cells typically test negative for HMB-45, but express smooth muscle actin and desmin. Angiomyolipoma is also relatively well-distinguished radiologically as an enhancing mass with macroscopic fat, devoid of calcification. Small, fat-poor tumours can be more challenging to distinguish from RCC .
Oncocytoma often displays the appearance of a well-circumscribed, homogeneous, and solid lesion, and may often demonstrate central scarring—features that are not mutually exclusive with the presentation of RCC . Postoperatively the pathologist also needs to differentiate leiomyoma from leiomyosarcoma . Histological features associated with leiomyosarcoma are tumour necrosis, nuclear pleomorphism, and increased mitotic activity .