Insulinoma: a quarter century of dietary control

Insulinomas are rare pancreatic neuroendocrine neoplasms (NENs) that are typically sporadic and solitary, with the majority being <2 cm in diameter at diagnosis. The median duration of symptoms before diagnosis is variable; however, this is usually in the region of 12-18 months. We report on an insulinoma diagnosed some 25 years following initial symptoms, having by that stage attained a diameter of 4 cm. We present a 50-year-old man who was reported with hypoglycaemic symptoms on his wedding 25 years prior to eventual confirmation of an insulinoma. He had since learned to live with the symptoms by eating frequently to manage his hypoglycaemia. However, over recent months, he reported a substantial deterioration in his symptoms, and indeed, had collapsed on two occasions. He had a fasting glucose of 2.9 mmol/L with grossly inappropriate elevated insulin and C-peptide levels. MRI demonstrated a 4.1 cm lesion at the body of pancreas and an indeterminate 9-mm liver lesion with a negative 68Gallium-DOTATATE PET scan. Accordingly, he was initiated on diazoxide and referred to the surgical team for distal pancreatectomy: histology confirmed a 4.4-cm well-differentiated pancreatic NEN of intermediate grade (NEN G2, Grade 2, 2017 World Health Organization (WHO) pancreatic-NEN classification), with positive immunohistochemistry for insulin. His hypoglycaemia episodes have ceased, and he remains under active surveillance. Our case demonstrates the possibility of dietary control of insulinoma-induced hypoglycaemia, and the likelihood that such a prolonged delay in diagnosis has led to the uncommonly large size of the apparently benign tumour which is usually 'small and indolent'.

Most patients with insulinomas have lesions that are 1-2 cm in size, with 96% being less than 3 cm. The mean tumour size of insulinomas found in 3 of the largest reported series was 1.5 cm, with a range of 0.1-7.0 cm. It is not uncommon for patients to have symptoms for several months to years before diagnosis; however, no reported cases had the symptoms such long for 25 years, and the large size of the tumour in this case may reflect the very long history.