Ectopic insulin secreting neuroendocrine tumor of kidney with recurrent hypoglycemia: a diagnostic dilemma

The most common cause of endogenous hypoglycemia is hyperinsulinemia secondary to islet cell tumours of pancreas. Hypoglycemia due to non-pancreatic tumours is infrequently reported. Most of these non-pancreatic tumours secrete factors with insulin like activity. Ectopic insulin secretion has been reported in few cases but not convincingly proved. We report a case of ectopic insulin secretion by neuroendocrine tumour (NET) of kidney. The ectopic origin of the tumour is demonstrated by functional imaging (⁶⁸Ga-Dotanoc and ^99mTc-HYNICTOC), and by immunohistochemistry for insulin staining. We also extensively reviewed literature for similar cases of ectopic insulin secreting tumours published previously.

This 44-year- old female was admitted for evaluation of recurrent hypoglycemic episodes of 3 years duration. She had complains of forgetfulness, altered behaviour, headache, palpitation, sweating, and giddiness. Most of her symptoms occurred early in the morning and these symptoms improved spontaneously in 15 minutes. She had consulted various physicians including neurologist and psychiatrist and had been diagnosed as pseudo-seizure, panic attacks or conversion disorder. One year back, she consulted endocrinologist and was diagnosed to have hypoglycemia and hyperinsulinemia. For suspecting insulinoma, she underwent CT abdomen which showed a right renal mass lesion and possibility of renal cell carcinoma was entertained. She was advised surgery for renal tumour. However, patient did not agree for surgery. There were no complains of hematuria, urinary frequency, urgency, burning micturition, diarrhoea or flushing episodes. She was not on antidiabetic treatment and was not hypertensive. For last 2 months, patient was having recurrent hypoglycemic seizures and came to this hospital.

After admission, she had persisted hypoglycemia and required continuous infusion of 10% Dextrose at the rate of 75 – 100 ml/hr in addition to 1-2 hourly feeds. Her hemogram, electrolytes, renal and liver functions were normal. HbA1c was 4.5%. Hormonal assay showed thyroxine (T4) 5.67 (5.1 – 14.1 mcg/dl), TSH 4.42 (0.27 – 4.2 mIU/ml), Insulin like Growth Factor- I(IGF-1) - 165.90 (62 – 205 ng/ml), growth hormone 1.20 ng/ml and cortisol 15.78 mcg/dl. Samples collected during hypoglycemia showed a serum insulin of 134 (2.6 – 24.9 μU/ml) and C-peptide of 13.35 (1.1 – 4.4) ng/ml. During another symptomatic hypoglycemia, she had plasma glucose of 44 mg/dl while her plasma insulin and C-peptide were 49.81 miu/ml and 9.36 ng/ml respectively confirming endogenous hyperinsulinemic hypoglycemia. She underwent high resolution radiological and functional imaging for localisation of insulinoma.

Both multi-phase CT abdomen (Figure 1) and contrast enhanced MRI abdomen demonstrated a mass lesion arising from upper- mid pole of right kidney (Figure 1a, 1b, 1c). No lesion was identified in the pancreas (Figure 1d). Endoscopic ultrasound also did not demonstrate any pancreatic lesion. 68Gallium DOTANOC scan (Figure 2a-c) and 99mTc-HYNICTOC (Figure 2d,e) did not demonstrate any uptake in the pancreas but both demonstrated intense uptake in the mass lesion indicating a NET. She underwent abdominal exploration and right nephrectomy. Post-operative period was uneventful and she became euglycemic without need for dextrose infusion. Her glucose values were maintained between 120 – 160 mg/dl thus confirming the right sided mass near renal hilum to be the source of ectopic insulin secretion.

Islet cell tumours of pancreas produce insulin which cause hypoglycemia. Hypoglycemia due to non-pancreatic tumours is infrequently reported and poorly understood. Various mechanisms were proposed [1] are: (a) insulin or insulin-like activity produced by the tumour, (b) decreased gluconeogenesis, (c) disruption of glucagon metabolism, and (d) increased utilization of glucose by the tumour. The non-suppressible Insulin like Activity (NSILA) has been purposed to be secondary to Insulin like Growth Factor-2. The non-islet cell tumours which commonly cause hypoglycaemia are of mesenchymal, epithelial or hematopoietic cell lines. Fibrosarcomas, mesotheliomas, leiomyosarcomas, and hemangiopericytomas are the most frequent types of tumours which cause hypoglycemia. Hepatoma, gastric, pancreatic, exocrine gland and lung carcinomas are epithelial cancers with frequent hypoglycemic potential. Mesenchymal and epithelial tumors, which generally present as large masses, are located in the mediastinum or the abdomen. These tumours are known to cause hypoglycemia via secretion of IGF-2 that leads to stimulation of insulin receptors [2]. In a series of 78 cases of non-islet-cell tumour, hypoglycemia (NICTH) due to IGF-2 production, hepatocellular carcinoma and gastric carcinoma were the common causes [3].

Ectopic insulin secreting tumours are rare, comprising only 1% to 2% of all insulinomas [4] and are commonly located in the peripancreatic or periduodenal region where most heterotopic pancreatic tissue is located. Ectopic insulin producing tumours located away from pancreatic beds are infrequently reported in literature. Only few cases were described in literature due to hyperinsulinemic hypoglycemia and non-pancreatic tumour and these tumors are described in Table 1.

The patient under discussion had presented with right renal mass lesion and hyperinsulinemic hypoglycemia. We initially considered the possibility of incidentally detected right renal cell carcinoma in patient with insulinoma. In our Institute, we have both PET and SPECT imaging agents for octreoscan available for the localisation of insulinoma. Somatostain receptor scintigraphy (SRS) is an established functional imaging method for patients with NETs. The sensitivity of SRS for insulinoma is 50 – 60% [12]. We did both ⁶⁸Ga-Dotanoc and ^99mTc-HYNICTOC scans for the localisation of insulinoma which unexpectedly revealed increased uptake in the right renal mass lesion itself confirming it to be a NET. To the best of our knowledge, this is the first case of ectopic insulin secretion confirmed in vivo by ⁶⁸Ga-Dotanoc and ^99mTc-HYNICTOC scan. Further, the neuroendocrine and insulin secreting nature of the tumour was confirmed ex vivo by histopathological examination, immunohistochemistry and ultra-structural examination. Paraganglioma and pheochromoctyoma were also reported to cause hyperinsulinemic hypoglycemia and also show tracer uptake by SRS and are immunoreactive for chromogranin A and syanptophysin. Though the mass lesion was arising from upper pole of right kidney, it was intra-renal and hisopathological examination was suggestive of NET.

NETs are neoplasms that arise from cells of dispersed neuroendocrine system. Although there are many kinds of NETs but they are treated as a group of tumours as these neoplasms share common features such as histology, immunoreactivity for neuroendocrine markers (chromogranin A, synaptophysin, neuron specific enolase), presence of neurosecretory granules, and secretion of biogenic amines and polypeptide hormones. Neuroendocrine tumours arising from pancreas are classified by the hormone most commonly secreted. Neuroendocrine tumours arising from the intestine, respiratory system and rest of the body were known as carcinoids but under the recent nomenclature they are all know as neuroendocrine neoplasm (NEN) of tumour. Primary carcinoid tumours of kidney are extremely rare and around 90 cases has been reported in literature [13]. None of the reported cases including two cases were reported from our institute previously [14], [15] were insulin secreting.

The cell of origin of renal carcinoid is unclear as neuroendocrine cells are not normally found in adult renal parenchyma. Carcinoid tumours of kidney are usually asymptomatic. In symptomatic cases, these tumours present with abdominal pain or abdominal mass with hematuria or fever. Evidence of carcinoid syndrome with serotonin-related flushing, generalized edema and diarrhoea, and occasional elevation of urine 5-hydroxyindoleaceticacid are uncommon and are seen in less than 10% of cases [14]. Our patient did not have these symptoms prior to presentation, although she had flushing, edema and blanching of the skin intraoperatively. Rarely, these tumours may present with neuroendocrine syndromes like cushing syndrome, vipoma, or glucagonoma [16]. Our case presented as insulinoma. Macroscopically, renal carcinoid tumours are usually solitary and unilateral. They are well circumscribed with a lobulated and bulging appearance. The cut surface the tumour is yellow-tan, or red brown as also observed in the case under discussion. Renal carcinoids exhibit histological features that are typical of carcinoids at other sites. Primary carcinoid tumors as well as metastasis possess high affinity receptors for somatostatin in 87% of cases [17]. Localization of gastrointestinal tract carcinoid tumours and pancreatic endocrine tumors has been achieved by the use of radiolabeled octreotide, a synthetic and slowly degraded somatostatin analogue that has a high affinity for somatostatin receptors. Use of indium-111 pentetreotide scanning in the diagnosis of carcinoid tumors had been reported [18]. In the present case, functional imaging with ⁶⁸Ga-Dotanoc and ^99mTc-HYNICTOC was done for the localisation of insulinoma. Both showed uptake in the right renal mass lesion and no uptake in pancreas (Figure 2a-e). All available modalities for diagnosis of carcinoid tumors were employed namely Electron microscopy, immunohistochemistry, both PET and SPECT octreotide scan along with conventional radiographic imaging techniques were used in preoperative diagnosis this tumour.

Prognosis of NET depends upon the grade and stage of the tumour. Although the grade of the tumour was low, however stage was high as there was metastasis in the regional lymph nodes. There was no recurrence of hypoglycaemia on till this time of reporting, however, long-term close follow up is needed for the malignant behaviour.

Ectopic insulin secreting extra-pancreatic tumours are rare. Confirmation of the source of hyperinsulinemia is often difficult. To our knowledge, this is the first case of extra-pancreatic insulin secreting neuroendocrine tumour fully characterised by biochemical, radiological & functional imaging, histopathology and immunohistochemistry. Our patient had successful post-operative outcome and maintained euglycaemia after three months of follow-up.

Written informed consent was obtained from the patient for publication of this Case report and any accompanying images. A copy of the written consent is available for review by the Editor of this journal.