Pancreatic neuroendocrine tumours are rare tumours that can either present with syndromes from excess hormonal production or from mass effect – from the primary or metastases. They vary widely in clinical course, with the main determinants of outcome being TNM staging and pathological grade. The available treatment options depend largely on the grade of the tumour – somatostatin analogues, targeted agents, chemotherapy and PRRT for lG1 and G2 PNETs, and chemotherapy as the mainstay of treatment for high grade NET/NECs. The paucity of randomised evidence in the treatment of this disease argues for ongoing research to understand the molecular genetics underlying PNETs, to develop possible future treatment options, as well as optimising use of existing ones.
Neuroendocrine tumours (NETs), formerly known as carcinoid tumours, arise from cells in the neuroendocrine system, a diverse population of hormone-producing cells distributed throughout the gastrointestinal tract as well as the bronchial system. Although NETs have been increasing in incidence, they still remain a rare tumour (being defined by the RARECARE group as incidence less than 6/100,000 per year).1
Pancreatic neuroendocrine tumours are generally abbreviated as PNETs. PNETs were previously named ‘islet cell tumour’ or ‘pancreatic carcinoid’, but the use of these terms is discouraged. PNETs comprise 5-10% of neuroendocrine tumours, and 1-2% of all pancreatic tumours.2 In contrast to other pancreatic tumours, PNETs are distinguished by marked heterogeneity – in histology, clinical course and prognosis. Neuroendocrine malignancies are named as neuroendocrine tumours (NET) if well or moderately well differentiated histologically, or neuroendocrine cancers (NEC) if poorly-differentiated.
The incidence of PNETs in Australia has been gradually increasing. Statistics from the South Australian Cancer Registry showed an incidence rate of 0.15/100,000 a year in 1980-2006, comprising 6.5% of all NETs diagnosed in that period.3 However, this incidence has been rising steadily over the last 20 years. The increasing survival rate from PNETs over that time likely reflects earlier diagnosis of disease and initiation of appropriate management.4
Histology can vary in PNETs according to the grade of tumour. Well-differentiated tumours express somatostatin receptors (SSTRs), have low mitotic counts (<2) and a Ki67 proliferative index of <2%. As tumours become less well differentiated, expression of SSTRs is lost and the mitotic count and Ki67 increase. Ultimately, a high-grade undifferentiated NEC can look identical to other undifferentiated carcinomas.
The initial World Health Organisation (WHO) classification in 1980 divided NETs by histological appearance into carcinoid tumours, mucocarcinoid tumours, mixed carcinoid adenocarcinoma and pseudotumour lesions. Since then, there has been a shift towards quantitative measures such as mitotic count and Ki67, culminating in the most recent European Neuroendocrine Tumour Society/World Health Organisation (ENETS/WHO) scheme of 2010 (table 1), which is the current standard for pathological reporting. However, routinely performing these counts can be time-consuming for pathologists, and samples can contain areas of both high-grade and low-grade disease. This poses challenges both in terms of accurate diagnosis and optimal management.
PNETs are classified as functional and non-functional, and these present in very different ways. The production of functional hormones can produce classical presentations, such as hypoglycaemia (insulinoma), gastric ulcers (gastrinomas), diarrhoea (vasoactive intestinal peptide), hyperglycaemia (glucagon) and even gallstones (somatostatinoma). In contrast, non-functioning tumours can grow slowly over months or even years and present with signs related to local mass effect – abdominal pain, anorexia, weight loss and nausea. Tumours in the head of the pancreas may present earlier with biliary obstruction. Unlike adenocarcinoma, differential outcomes have not been demonstrated for PNETs according to site within the pancreas.
Historically, diagnosis followed presentation with reported symptoms in most cases. However, the development of improved imaging modalities with increasing adoption has led to earlier diagnosis. Currently, PNETs present with functional syndromes in approximately 30% of cases. Approximately 10-15% of PNETs occur in the context of a genetic syndrome – either known from family history or as a new presentation. Germline mutations in a number of genes are associated with PNET including multiple endocrine neoplasia syndrome, type I (MEN1) and Von-Hippel Lindau syndrome (VHL). Clinicians should take a family history from all patients with PNETs and refer for evaluation by a cancer genetics service where needed.
Clinical workup for patients with PNETs consists of serum hormone levels, tissue biopsy, radiology and nuclear imaging. Serum Chromogranin A is the test of choice for PNETs, with good sensitivity, although false low-level elevations can occur in chronic kidney disease, congestive heart failure and medications such as proton pump inhibitors.5 Other markers such as serum serotonin, urinary 5-HIAA (requiring a 24 hour collection) and specific hormone tests in keeping with presenting symptoms should also be considered.
Imaging modalities such as CT and MRI of the abdomen can often be helpful in defining the presence of a lesion and evaluating for liver metastases, and CT-guided biopsies (for example, of mesenteric masses)/endoscopic ultrasound (pancreatic masses) can often establish a tissue diagnosis. However, nuclear imaging has been increasingly used for staging and monitoring of response.
Nuclear imaging has been increasingly employed in establishing the extent and predicting behaviour of PNETs. DOTATATE/DOTATOC PET scans have replaced 111In-based octreoscan in Australia for G1 and G2 NET, giving much greater sensitivity and spatial resolution particularly with concurrent CT.6 FDG- PET, which relies on high cellular uptake of glucose, is useful in NEC. G2 NETs may be positive in both DOTATATE- PET and FDG-PET, reflecting varying degrees of cellular differentiation. While both FDG and DOTATATE PETs may be helpful in baseline staging of NETs (particularly G2 NETs), the FDG PET can be omitted in selected patients with low-grade NETs (e.g Ki67 <5%).
Patients diagnosed with NETs can vary greatly both in terms of their general status and aggressiveness of their disease. Therefore, benefit from a multidisciplinary approach may lead to optimal outcomes.11 Interested surgeons, nuclear medicine specialists, endocrinologists, histopathologists and medical oncologists are essential to formulate an accurate treatment plan in a complex disease.
Given the heterogeneity of NETs, the relatively low incidence and the multiple treatment modalities available, it has been hard to accrue patients to and complete clinical trials in NETs. However, global efforts and formation of international consortiums have allowed progress to be made based on randomised phase 3 trials. COMMNETS (detailed below) was formed in response to the need for an international NET consortium in the Asia-Pacific region.
Surgical resection with curative intent is the standard of care for localised disease. Control of hormone secretion around the time of surgery with SSAs is important. There is scant evidence for adjuvant therapy after NET resection, but adjuvant chemotherapy is sometimes offered for NEC.
Most patients present with disseminated disease (80% with liver metastases) at diagnosis. Therapeutic options vary significantly depending on the grade of the tumours, which are usually divided into options for G1-2 PNETs and G3 NECs.
Figure 1: NET management pathway
Grade 1-2 PNET
Somatostatin analogues (SSAs) octreotide, lanreotide, pasireotide) act on the somatostatin receptors and inhibit release of various pro-growth hormones such as GH, glucagon and insulin. They were initially used for symptomatic relief in NETs, but have recently been also shown to control tumour growth.12,13
Two long-acting SSAs, octreotide LAR and lanreotide, are available in Australia for routine clinical use. SSAs can be administered by the GP, in oncology centres or under currently available pharmaceutical company-sponsored drug administration programs. It is often preferable to give the initial dose in a clinic environment, but subsequent linkage to home injection services offers convenience and quality of life, particularly important in a disease which may have a long clinical course.
Dose escalation of SSAs for patients who progress on the standard dose is a therapeutic strategy under ongoing investigation.
Various biological or ‘targeted’ agents show activity in PNETs (table 3), with the most commonly used being sunitinib (a multitargeted tyrosine kinase inhibitor) and everolimus (a mTOR inhibitor). Treatment modalities recently investigated include the combination of everolimus and bevacizumab, octreotide and interferon, as well as octreotide and bevacizumab. The optimal use, sequencing and combination of biological agents is still an open question in the management of G1-2 PNETs.
Unfortunately, the use of PBS-funded sunitinib and everolimus are restricted in Australia. According to regulations, patients who have progressed on sunitinib are not eligible for funded everolimus and vice versa. While relatively less data exists to assess the efficacy of using these agents sequentially, this funding paradigm deprives clinicians of access to both drugs in an individual patient.
Interferon-A has been used for many years, particularly in northern Europe. There is good control in some cases of refractory diarrhoea and the drug can produce long-lasting tumour responses, but the chronic toxicity of marked fatigue and mood depression, as well as the lack of high level comparative evidence, have limited its widespread adoption in clinical practice.
The role of peptide receptor radionuclide therapy (PRRT) has been increasingly explored in the management of metastatic G1 and G2 PNETs over the last 20 years. Similar in concept to targeted radiation (131I) for thyroid cancer, PRRT utilises a peptide which attaches to the somatostatin receptor expressed on PNETs, linked to a radionuclide (most commonly 177Lutetium and 90Yttrium) which emits a beta particle. The recently reported NETTER-1 trial showed that PRRT was superior to increased doses of octreotide in controlling midgut NETs. Potential short-term adverse events from PRRT include nausea, vomiting and flare of symptoms; long-term adverse events include renal impairment and bone marrow toxicity, although these are only significant in <1% of treated patients.
Currently, PRRT in Australia is available in a limited number of centres in each state in Australia. Each centre has slightly differing protocols, but the current protocol employed by Royal North Shore Hospital involves day treatment on one day every eight weeks, with four cycles in total.
Radiosensitising capecitabine is used by some centres before and after PRRT for each cycle. A multicentre phase 2 randomised control trial (CONTROL NETS) investigating the combination of CAPTEM (capecitabine/temozolomide chemotherapy) and PRRT for metastatic PNETs/midgut NETs (figure 2) is has opened in Australia under the auspices of the Australian Gastrointestinal Trials Group (http://agitg.org.au/clinical-trials/trials-open-to-recruitment/control-nets/).
The selection of PRRT compared to use of a targeted agent such as sunitinib or everolimus remains a difficult clinical dilemma in treatment of G1-2 PNETs, and the grade of tumour, potential side effects (e.g. hyperglycaemia for everolimus, fatigue for sunitinib) and biological aggressiveness all play a part in the individualised decision.
Figure 2: CONTROL NETS schema
Well-to-moderately differentiated tumours tend to be relatively chemoresistant. Different chemotherapeutic agents have been trialled in this setting including temozolomide, fluoropyrimidines and platinum agents, but all with disappointing results, with response rates for single agents in the 5-10% range and 20-30% with combination therapy.14
Surgical debulking, either for the primary or hepatic lesions, is occasionally considered in the treatment of metastatic PNET. This can be for symptomatic relief, prophylactically to prevent symptoms from a large tumour, or as part of an aggressive treatment strategy involving resection of oligometastatic disease. These options should be discussed in a multidisciplinary context with the input of experienced hepato-pancreatico-biliary surgeons.
Grade 3 NEC
Poorly differentiated NETs/NECs are treated using a different treatment paradigm to G1-2 NETs, reflecting that they behave in a much more aggressive way, akin to small cell lung cancer (another high grade tumour of neuroendocrine origin). They are usually FDG avid (conventional PET) and do not take up DOTATATE as they are too poorly differentiated to express the somatostatin receptor. Chemotherapy upfront is standard of care, with platinum/etoposide doublets the mainstay of treatment. There is some evidence that CAPTEM is a reasonable regimen for tumours with Ki67<55%.15 PRRT for NEC can result in suboptimal treatment due to areas of de-differentiated disease which do not express somatostatin receptors and hence do not take up administered PRRT.
PNETs are curable if resected early. However, the majority of cases present with liver metastases at diagnosis. This is reflected in the correlation between TNM staging and prognosis with five-year survival decreasing from 92% (stage I disease) 57% (stage IV disease).16 Other poor prognostic factors include G3 disease as well as features on FDG PET scan; in one retrospective series patients with FDG positive disease had a median survival of 15 months compared to 119.5 months for FDG negative patients.17
As with all cancers, patients diagnosed with rare tumours need much more than medical expertise. In Australia, NET patients have access to the Unicorn Foundation for support and educational resources. The Unicorn Foundation (unicornfoundation.org.au) was founded in 2009 with its main aim to promote awareness within the medical profession (both specialists and particularly GPs), and general public, and to provide patients facing a diagnosis of a NET with access to support groups throughout the country. They aim to reach the often disadvantaged rural patient population by funding a specialist NET nurse who is accessible via the telephone or email for advice on all aspects of living with a diagnosis of a NET. The Unicorn Foundation has many affiliations with international support networks and is active in raising funds and supporting research into NETs.
Given that patients with PNETs benefit from multidisciplinary care in a centre with expertise in NETs management,18 several organisations have attempted to formalise this process both to optimise patient care and collaborate in research. The Clinical Oncology Society of Australia has published guidelines (wiki.cancer.org.au/australia/COSA:NETs_guidelines) regarding NET management. Internationally, the European Neuroendocrine Tumour Society and the North American Neuroendocrine Tumour Society (ENETS and NANETS) have also published their guidelines. ENETS has an accreditation process for ‘Centres of excellence’.
In Australia, COMMNETS is a new initiative that aims to foster collaboration between the commonwealth countries in NETs. The inaugural COMMNETS conference was held in Hawaii in November 2015 involving delegates from Australia, New Zealand, Canada and Singapore (http://agitg.org.au/commnets-2015/). Barriers to care were identified and research priorities agreed upon using a modified Delphi process to generate a position document ‘Gaps in NET research’.
DLC is a Clinical Research Fellow funded by Sydney Vital as part of Cancer Institute NSW.