Simultaneous occurrence of renal cell carcinoma and angiomyolipoma in the same kidney is rare and has been reported more frequently in patients with tuberous sclerosis complex (TSC).

We present a case of a 77-year-old female with no history of tuberous sclerosis complex, who presented to our outpatient department due to bilateral renal tumors that had been previously monitored at other medical institutions. However, recent imaging revealed enlargement of one of the tumors in the right kidney. A sono-guided core needle biopsy of the growing tumor was performed, with the initial pathology report indicating an ambiguous diagnosis between clear cell renal cell carcinoma and angiomyolipoma. A robot-assisted retroperitoneal laparoscopic partial nephrectomy was carried out. The final pathology confirmed a collision tumor composed of clear cell renal cell carcinoma (pT1a) and angiomyolipoma. A follow-up CT urography performed three months after surgery revealed only a small amount of fluid collection at the surgical bed.

Renal collision tumors (RCTs) are rare entities in which two or more histologically distinct tumors coexist in the same kidney without intermixing. Our case contributes to a growing body of literature that highlights the diverse and sometimes novel combinations of tumor types involved in RCTs. A recent review identified 48 such cases. The most common components remain clear cell RCC, papillary RCC, and oncocytoma, although benign, borderline, and hematopoietic malignancies have also been reported. Angiomyolipoma (AML), while benign and the most common fat-containing renal tumor, is rarely associated with RCC, especially in patients without tuberous sclerosis. Fewer than 15 such cases have been reported. AMLs larger than 4 cm are at increased risk for hemorrhage due to abnormal, fragile blood vessels, as seen in our patient. In contrast, RCCs rarely present with spontaneous retroperitoneal bleeding. RCTs may be under-recognized due to limited tumor sampling or misclassification as unclassified RCC. Thorough gross sampling, especially in tumors with variable morphology, is essential for detecting collision tumors. This is critical since prognosis and management may differ depending on which component is identified in biopsy. Our case underscores the importance of considering RCTs in patients presenting with complex renal masses, even in the absence of classic risk factors like tuberous sclerosis.

Renal collision tumors are rare, with no established management guidelines. Prognosis depends on the most aggressive tumor component. In our case of a 77-year-old woman without tuberous sclerosis, radical nephrectomy revealed a collision tumor of renal cell carcinoma and angiomyolipoma. Multidisciplinary care and active surveillance are essential, as follow-up protocols remain undefined. Further research is needed to clarify the pathogenesis and guide targeted therapy.