Primary Leiomyosarcoma of Kidney-an uncommon entity.

A 51-year-old female presented in emergency department with worsening pain right flank and diarrhea. No history of fever, hematuria or weight loss. No significant abnormality on general or systemic examination. Lab investigations showed elevated C-reactive protein, white blood cells and erythrocyte sedimentation rate.

Contrast enhanced CT of the abdomen revealed a large well demarcated, low attenuating lesion in right renal fossa replacing the right kidney. No discernible renal parenchymal tissue was evident. Lesion margins were lobulated with areas of ring like calcification inferiorly. It measured 9x8.8x7.5 cms and had Hounsfield Unit value of 22. Posteriorly, the Gerota’s fascia was infiltrated with indistinct fat planes with liver superiorly. Thin peripheral rim of enhancement was seen (Fig.1a, b). No locoregional lymphadenopathy or venous thrombosis. Delayed 15 minutes urography series demonstrated no excretion of contrast into right renal pelvis and ureter (Fig. 2a, b). Based on above CT findings and increased inflammatory markers, the reporting radiologist raised a possibility of chronic non-functioning right kidney with focal collection in right renal fossa. Patient subsequently underwent right nephrectomy.

On histopathology, gross examination showed right kidney completely replaced by a tumor, 9 cm in largest diameter. It was light brown with areas of necrosis and was firmly adherent to the perinephric fat.  On microscopy, it was composed of spindle cells with pleomorphic nuclei and brisk mitotic activity(Fig.3). Areas of necrosis were also evident. Immunohistochemical studies revealed tumor cells negative for keratinin (AE1/AE3, CAM5.2) and epithelial membrane antigen (EMA) and positive for vimentin and smooth muscle actin (Fig.4a). A minority of tumor cells were positive for desmin(Fig.4b). In some areas up to 30% of tumor cells were positive for Ki-67, a proliferative marker. 

Sarcomas of the kidneys are extremely uncommon tumors with leiomyosarcomas being most frequent amongst these, constituting 0.12% of renal malignancies [1,2]. Leiomyosarcomas in kidneys can predominantly arise from renal pelvis smooth muscles, vessels, parenchyma or capsule. These lesions are mainly seen in females in their 5-6^th decade (as in our case), more commonly involve the right kidney [2,3]. Presenting symptoms are nonspecific and often overlap with other renal pathologies.

From imaging perspective, making a correct diagnosis is challenging due to overlap of imaging features with renal cell carcinoma and inflammatory lesions like renal abscess or xanthogranulomatous pyelonephritis [4,5]. On CT, sarcomas appearance can vary from low attenuating, cystic appearing to solid masses [1]. Though not specific, certain imaging features favoring sarcomas are: large size lesions without regional lymphadenopathy, relatively fast-growing mass and a pseudocapsule [6]. Solid lesions closely mimic renal cell carcinomas whereas cystic or complex cystic lesions can be confused with infective etiology especially xanthogranulomatous pyelonephritis. In our case, absence of renal pelvis staghorn calculus with typical peripheral low attenuation /cystic areas giving bear paw sign excludes xanthogranulomatous pyelonephritis. On histopathology, these sarcomas can be confused with sarcomatoid variant of renal cell carcinoma and require specific immunohistochemical studies to reach at the diagnosis [7].

Possibility of renal tumor was not raised on CT in our case due to elevated inflammatory parameters. Final diagnosis was made on histopathology after surgical excision. However, retrospectively reviewing the CT imaging revealed some pertinent features, in form of bulky low attenuation mass, patient demographics and right renal involvement warranting its inclusion in differential diagnosis on imaging.

Treatment of renal leiomyosarcomas is by radical nephrectomy with or without adjuvant radiotherapy and/or chemotherapy [7]. Our patient recovered well after nephrectomy and long term follow up CTs of chest and abdomen are negative for metastasis.

In conclusion, our purpose of presenting this case is not to suggest leiomyosarcoma as a sole diagnosis on imaging but to rather include it in the list of CT differentials of renal masses especially large low attenuating/ necrotic masses.

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