Primary renal carcinoid tumor in a horseshoe kidney presenting with constipation: A case report

Abstract

BACKGROUND

Primary renal carcinoid tumors (PRCTs) are rare well-differentiated neuroendocrine neoplasms of the urogenital system, though neuroendocrine tumors predominantly arise within the gastrointestinal tract. PRCTs are particularly rare, with approximately 100 cases reported since the first documented case in 1966. Horseshoe kidney represents a well-recognized congenital anatomical risk factor for PRCT. Constipation as a predominant presenting symptom of PRCT is highly atypical and has seldom been described in previous literature.

CASE SUMMARY

A 30-year-old female patient presented with a one-year history of left-lower abdominal distension and chronic constipation. Incidental imaging revealed a left-sided renal mass in a horseshoe kidney with fused lower renal poles. Computed tomography urography further characterized it as a 5.6 cm × 4.7 cm soft-tissue mass in the mid-lower portion of the left kidney containing central necrotic changes. Preoperative staging work-up, including chest computed tomography and abdominal-pelvic imaging, excluded distant metastatic lesions. Laparoscopic left radical nephrectomy was performed. Postoperative pathological examination confirmed a PRCT, pT2aN0M0. Immunohistochemistry revealed focal chromogranin-A positivity and diffuse synaptophysin and vimentin positivity. The patient’s constipation and abdominal distension completely resolved at 4 weeks after surgery. Although serum neuroendocrine marker levels were not obtained before surgery, 12-month postoperative surveillance computed tomography revealed no evidence of tumor recurrence or metastasis.

CONCLUSION

PRCTs should be included in the differential diagnosis for renal masses occurring in patients with a horseshoe kidney. Atypical gastrointestinal symptoms such as refractory constipation may represent a clinical clue for this rare neoplasm. Definitive diagnosis relies on comprehensive pathological and immunohistochemical evaluation together with the exclusion of extrarenal neuroendocrine primary lesions.

Key Words: Primary renal carcinoid tumor; Neuroendocrine neoplasm; Horseshoe kidney; Laparoscopic radical nephrectomy; Constipation; Case report

Core Tip: Primary renal carcinoid tumor (PRCT) is a rare neuroendocrine neoplasm strongly associated with a horseshoe kidney. This case describes a patient with refractory constipation as the main complaint. Clinicians should be aware of unexplained constipation in patients with a horseshoe kidney, even in the absence of hematuria or lumbago. Exclusion of extrarenal neuroendocrine origin and adequate postoperative oncologic surveillance are essential for PRCT management.

INTRODUCTION

Carcinoid tumors are well-differentiated neuroendocrine neoplasms originating from enterochromaffin cells. Primary renal carcinoid tumors (PRCTs) are exceedingly uncommon given the absence of native enterochromaffin cells within the normal renal parenchyma. The cumulative literature suggests that patients with a horseshoe kidney have a substantially elevated risk of developing PRCT. Most published PRCT cases involve hematuria, flank pain or a palpable abdominal mass; presentations dominated by chronic constipation remain sparse. Here, we report one such unusual case and review the clinical, imaging, and pathological characteristics together with the diagnostic pitfalls of this rare entity. This case report follows the CARE-2016 reporting guidelines.

CASE PRESENTATION

Chief complaints

Persistent left-lower abdominal distension and refractory constipation for one year in a 30-year-old female.

History of present illness

The patient had a one-year history of chronic constipation without an obvious precipitating cause. She denied gross hematuria, lumbago, flushing, diarrhea, or significant weight loss. During a routine health-checkup, abdominal imaging identified a left renal mass and a concomitant horseshoe kidney anomaly. She was subsequently referred to our urology department for further evaluation and management.

History of past illness

The patient had no prior surgical history, no chronic gastrointestinal disorders, and no known drug allergies.

Personal and family history

Family history was negative for renal tumors and neuroendocrine neoplasms. The patient’s menstrual history was unremarkable.

Physical examination

Vital signs were within normal limits. Abdominal examination revealed no palpable mass and no tenderness or rebound tenderness. Additionally, there was no flank percussion pain.

Laboratory examinations

The complete blood count and baseline renal function test results were within normal reference ranges. Urinalysis revealed no microscopic hematuria. Preoperative serum neuroendocrine markers, including chromogranin-A, 5-HIAA, NSE and peptide-YY, were not tested in this clinical setting, which constitutes a limitation for etiological inference. Gastroscopy and colonoscopy were performed to exclude a primary gastrointestinal neuroendocrine neoplasm.

Imaging examinations

Computed tomography urography demonstrated a horseshoe kidney with fusion of the bilateral lower renal poles (Figure 1). A 5.6 cm × 4.7 cm soft-tissue mass was located in the mid-lower segment of the left renal parenchyma adjacent to the renal isthmus. The mass displayed heterogeneous enhancement with central low-attenuation necrotic areas, and partially ill-defined borders toward the surrounding renal parenchyma. Preoperative differential diagnoses included renal cell carcinoma and fat-poor angiomyolipoma. A chest computed tomography (CT) excluded pulmonary metastatic lesions. The TNM classification was cT1bN0M0, stage I.

Figure 1 Computed tomography urography findings of primary renal carcinoid tumor within horseshoe kidney. A: Axial contrast-enhanced computed tomography (CT) image (nephrographic phase): Demonstrates a heterogeneous-density mass (orange arrow) within the left-sided moiety of the horseshoe kidney, with a central low-density necrotic area visible; B: Another axial CT slice shows mass boundary relative to fused renal parenchyma (orange arrow); C: Coronal reformatted computed tomography urography image shows tumor location adjacent to horseshoe-kidney renal isthmus (orange arrow); D: Three-dimensional volume-rendering reconstruction demonstrates horseshoe-kidney fused lower-pole anatomical configuration and spatial relationship of the mass.

FINAL DIAGNOSIS

Gross surgical specimen

The specimen is a solid soft tumor measuring 6 cm × 4.5 cm × 5 cm, exhibiting focal intratumoral necrosis (Figure 2). Histopathology: The histopathological examination revealed a well-differentiated neuroendocrine tumor morphology, characterized by a mitotic count of 1 per 10 high-power fields. The tumor has a World Health Organization grade of G2, and there was no lymphovascular invasion, with surgical resection margins being negative. No lymph node metastasis was identified in the resected regional lymph node specimens. Clinical TNM stage: T1bN0M0 (stage I). Pathological TNM stage: PT2aN0M0. Immunohistochemistry findings: The tumor showed focal positivity for chromogranin-A and diffuse positivity for synaptophysin and vimentin (Figure 3). It was negative for EMA, CK7, CK34βe, PAX-8, and p53, with a Ki-67 proliferation index of 3%. Final diagnosis: The final diagnosis is a PRCT arising within a horseshoe kidney, with no other lesions identified in the body.

Figure 2 Gross pathological specimen of resected primary renal carcinoid tumor. Gross specimen from laparoscopic radical nephrectomy (6 cm × 4.5 cm × 5 cm). A solid, soft neoplastic mass with focal intratumoral hemorrhage and central necrosis is present; residual normal renal parenchyma surrounds the tumor tissue. A centimeter scale bar is shown at the bottom of the photograph.

Figure 3 Immunohistochemical staining of primary renal carcinoid tumor (magnification ×200). A: Chromogranin-A staining: Focal positive cytoplasmic immunoreactivity within tumor cell nests; B: Synaptophysin staining: Diffuse strong cytoplasmic positive staining of neoplastic cells; C: Vimentin staining: Diffuse positive staining within tumor stroma and neoplastic cell population.

TREATMENT

Given the tumor dimension of 5.6 cm, proximity to the horseshoe-kidney isthmus, ill-defined tumor-parenchyma interface and malignant neuroendocrine tumor potential, laparoscopic left radical nephrectomy with regional lymph node sampling was performed instead of partial nephrectomy. The congenital renal vascular anatomy associated with the horseshoe kidney was identified intraoperatively, and the renal isthmus was preserved. The operation duration was 145 minutes; the estimated blood loss was 120 mL; and no intraoperative complications occurred.

OUTCOME AND FOLLOW-UP

The postoperative hospital stay was 6 days, and no surgical-site complications or adverse events occurred. The patient’s constipation and abdominal distension completely resolved at 4 weeks post-surgery. Surveillance abdominal-pelvic CT was performed at 6 months and 12 months postoperative, respectively. At the 12-month follow-up, no imaging evidence of local recurrence or distant metastasis was detected, and postoperative renal function remained stable.

DISCUSSION

PRCTs are a rare subtype of neuroendocrine neoplasms. Normal renal parenchyma lacks native enterochromaffin cells, and the cellular origin of PRCT remains incompletely elucidated. Multiple retrospective reviews have demonstrated a strong epidemiological association between horseshoe-kidney congenital malformation and PRCT; approximately 15%-20% of reported PRCT cases occur in patients with horseshoe kidney[1,2]. Two case‒control studies reported relative risks of 62 and 82, respectively, for PRCT among horseshoe kidney patients, and those risk estimates originated from retrospective single-institution case‒control cohorts[3,4]. Developmental maldifferentiation of misplaced endodermal progenitor cells within fused renal isthmus tissue has been hypothesized to underlie this elevated tumor risk.

Most PRCT patients remain asymptomatic in the early phases; symptoms appear when the tumor enlarges and compresses adjacent structures, including flank pain, hematuria, or a palpable abdominal mass. Atypical presentations such as predominant constipation are infrequently documented. Previous ovarian carcinoid case reports have proposed that excessive peptide-YY secretion may reduce intestinal peristalsis and induce chronic constipation[5]. In our patient, constipation fully resolved after tumor resection. However, serum peptide-YY levels were not measured preoperatively; therefore, the hormone-mediated constipation mechanism remains unclear and cannot be biochemically confirmed for this index case, representing an important study limitation. Clinicians should be aware of unexplained refractory constipation in patients with horseshoe kidney, even those without hematuria or lumbago.

Our case is comparable to the existing published PRCT-in-horseshoe-kidney case series: The patient was female and had a tumor size > 5 cm; unlike most prior reports, this patient’s dominant clinical manifestation was chronic constipation rather than hematuria or flank discomfort[6,7].

Preoperative differentiation between PRCT and other renal solid masses is challenging. On conventional CT imaging, PRCT may mimic renal cell carcinoma or fat-poor angiomyolipoma, with overlapping enhancement characteristics. Compared with clear-cell renal cell carcinoma, PRCT tends to show a relatively lower degree of heterogeneous enhancement; fat-poor AML lacks central necrosis, which is frequently observed in larger PRCT lesions. Somatostatin-receptor-targeted functional imaging, including octreotide scintigraphy and 68Ga-DOTATATE PET-CT, can assist in the diagnosis and metastatic surveillance of neuroendocrine neoplasms[8]. These functional examinations were not performed as part of the initial clinical work-up for our patient. We acknowledge that omission as one diagnostic limitation.

Surgical resection constitutes the mainstay curative therapy for localized PRCT. The selection between partial nephrectomy and radical nephrectomy depends on tumor size, tumor location, surgical margin feasibility and underlying renal anatomical variation[9]. Prognostic risk factors for PRCT include age > 40 years, tumor diameter > 4 cm, elevated mitotic activity, a Ki67 index > 10%, lymphovascular invasion and metastatic disease at diagnosis. The existing literature suggests that a PRCT arising within a horseshoe kidney may have a relatively favorable prognosis, although the evidence remains limited by the small sample[10-12].

The limitations of this case report should be emphasized. First, the lack of pre-operative circulating neuroendocrine hormone measurements prevents us from establishing a causal link between the tumor’s secretory activity and the symptoms of constipation. Second, somatostatin receptor functional imaging was not performed for diagnostic or staging purposes. Third, this is a single-case retrospective observation, which lacks population-level generalizability. To further validate the clinical features and prognostic factors of PRCT associated with horseshoe kidney, multi-institutional case series are needed.

CONCLUSION

PRCTs are rare neuroendocrine neoplasms that are strongly associated with congenital anomalies, particularly horseshoe kidney. Unexplained refractory constipation may serve as an atypical clinical warning sign in this patient population. A comprehensive preoperative work-up must include the exclusion of extra-renal neuroendocrine primary tumors. Definitive diagnosis relies on histopathology along with characteristic immunohistochemical markers. Individualized surgical strategies should take into account the complex vascular and parenchymal anatomy associated with the horseshoe kidney variant. Long-term oncologic surveillance is essential following curative resection.