A collision of two primary renal neoplasms: A rare case report

Abstract

Renal collision tumors are rare neoplasms characterized by the coexistence of two histologically distinct tumor types within the same renal parenchyma, representing a diagnostic challenge due to their heterogeneous clinical and radiological features. We report a rare case of a renal collision tumor and discuss the diagnostic difficulties, pathological findings, and therapeutic management of this uncommon entity.

1. Introduction

Renal collision tumors are rare neoplasms characterized by the coexistence and juxtaposition of two or more distinct tumors of different histological origins within the same renal mass, without an intermingling or transitional zone between the tumor components.¹ Although renal cell carcinomas (RCCs) represent the most common primary renal malignancies, the occurrence of collision tumors involving different renal tumor subtypes or benign and malignant components remains an uncommon and challenging entity.¹ The pathogenesis of renal collision tumors remains poorly understood. Several mechanisms have been proposed, including the simultaneous development of independent neoplastic processes, a shared carcinogenic stimulus, or alterations in the renal microenvironment promoting tumor coexistence.² Due to their rarity and heterogeneous histological combinations, these tumors are frequently diagnosed only after surgical resection, as preoperative imaging findings are often nonspecific and may not reliably distinguish collision tumors from conventional renal neoplasms.^1,2

2. Case presentation

A 62-year-old african male patient, smoker, with multiple comorbidities, including diabetes, and hypertension, hospitalized for total hematuria evolving for 6 months, and right lumbar pain without fever. Clinical examination reveals a BMI of 28 kg/m², mild right lumbar tenderness, and a palpable mass. The general examination found a blood pressure of 120/70 mmHg and a pulse of 80 bpm. The physical examination found a palpable mass in the right lumbar fossa, no varicocele, and no low limb edema. The urine cytobacteriological examination was negative. He had a hemoglobin level of 8.6 g/dl, a calcium level of 2.33 mmol/l, and a creatinine level of 16 mg/l. The patient underwent a computed tomography scan, which revealed the presence of an upper polar left renal mass, heterogeneous in nature with tissue density, displacing the right adrenal gland, measuring 65*60*50 mm, and enhancing significantly after contrast injection (Fig. 1). Additionally, no secondary lesions were noted in the thoracic or skeletal regions. The case was discussed at the multidisciplinary team meeting. The patient underwent an open radical right nephrectomy by lombotomy. No post-surgical complications were observed in the aftermath of the surgery. The histological examination of the surgical specimen showed the presence of a clear cell carcinoma in the upper pole of the right kidney, measuring 6 cm, without invasion of the renal capsule or perirenal fat, and a type 2 papillary carcinoma, measuring 7 cm, infiltrating the perirenal fat (Fig. 2). Histological examination of the surgical specimen revealed two distinct renal cell carcinomas in the upper pole of the right kidney. The first tumor was a 6-cm clear cell renal cell carcinoma, confined to the kidney without invasion of the renal capsule or perirenal fat, corresponding to pT1b. The second tumor was a 7-cm papillary renal cell carcinoma, type 2, with invasion of the perirenal fat, corresponding to pT3a. The WHO/ISUP nuclear grades were grade II for the clear cell carcinoma and grade II for the papillary renal cell carcinoma. No tumor necrosis, sarcomatoid or rhabdoid differentiation was identified. Surgical margins were negative. The final pathological staging was therefore pT1b for the clear cell renal cell carcinoma and pT3a for the papillary renal cell carcinoma. No adjuvant therapy was administered after surgery. The patient was managed with close clinical and radiological surveillance following multidisciplinary discussion. After 18 months of clinical and radiological check-up, there was no functional complaint or any sign of reccurrence.

Fig. 1.

Computed tomography showed the presence of an upper polar renal mass, heterogeneous in nature with tissue density, displacing the right adrenal gland, measuring 65*60*50 mm, and enhancing significantly after contrast injection.

Fig. 2.

Primary renal collision tumor: Clear cell renal cell carcinoma and papillary renal cell carcinoma type 2. Sharp interface between the two distinct tumor components without transitional features.

3. Discussion

Primary renal collision tumors are rare neoplasms characterized by the coexistence of two or more histologically distinct tumor entities within the same renal mass, with no transitional zone or intermingling between the different components.¹ They should be distinguished from composite or hybrid tumors, in which the neoplastic components share a common histogenetic origin. Although renal cell carcinoma is the predominant malignant tumor encountered in these lesions, a wide spectrum of combinations involving benign, malignant, and metastatic tumors has been reported. Recent analyses suggest that renal collision tumors may be more frequent than previously recognized, although their diagnosis remains challenging due to their heterogeneous pathological features and limited preoperative characterization.^1,2 The pathogenesis of renal collision tumors remains unclear. Several hypotheses have been proposed, including the simultaneous occurrence of two independent neoplastic processes, a common carcinogenic stimulus affecting different renal cell populations, or local environmental factors within the renal parenchyma favoring tumor development.³ Molecular studies performed on selected cases have demonstrated distinct genetic profiles between the two tumor components, supporting the concept of independent tumorigenesis rather than a single clonal origin 2,3. However, overlapping molecular alterations in some cases suggest that a shared precursor event cannot be completely excluded.³ Preoperative diagnosis of renal collision tumors is particularly difficult because conventional imaging techniques often fail to identify the presence of multiple histological components.⁴ On computed tomography and magnetic resonance imaging, these lesions may appear as a single heterogeneous renal mass, mimicking conventional RCC.⁴ Although differences in enhancement patterns, areas of necrosis, or intratumoral heterogeneity may raise suspicion, definitive diagnosis usually requires meticulous histopathological examination after partial or radical nephrectomy.⁴ In recent years, advances in multiparametric imaging and molecular pathology have improved the understanding of these rare entities but have not yet allowed reliable non-invasive diagnosis.^4,5 From a pathological perspective, accurate identification of each tumor component is crucial because different histological subtypes may have distinct prognostic implications and therapeutic approaches. Immunohistochemistry and, in selected cases, molecular analyses play an essential role in differentiating collision tumors from mimickers such as sarcomatoid RCC, mixed epithelial and stromal tumors, or hybrid renal neoplasms.⁵ The largest recent literature review identified a broad range of renal collision tumor combinations, including RCC associated with oncocytoma, angiomyolipoma, lymphoma, and other rare renal neoplasms, highlighting the diagnostic complexity of these lesions. The optimal management of primary renal collision tumors remains similar to that of other localized renal masses, with surgical excision representing the mainstay of treatment.⁵ Partial nephrectomy should be considered whenever technically feasible, particularly for small and localized lesions, to preserve renal function.¹ Long-term follow-up is recommended because the prognostic significance of these tumors is not fully established and may depend mainly on the most aggressive component.^1,2 Several recent case reports have emphasized the importance of multidisciplinary evaluation combining radiological, surgical, and pathological expertise to achieve an accurate diagnosis and appropriate management.

4. Conclusion

In conclusion, primary renal collision tumors represent a rare and diagnostically challenging group of renal neoplasms. Awareness of this entity is essential for radiologists, urologists, and pathologists, as recognition of the coexistence of distinct tumor components has important implications for prognosis, treatment planning, and patient follow-up.

CRediT authorship contribution statement

Kays Chaker: Writing – original draft. Ayda Khadhar: Writing – original draft, Formal analysis. Boutheina Mosbahi: Writing – original draft. Afif Limem: Writing – original draft. Adel Ammous: Validation. Yassine Nouira: Validation.