A sheep in wolf's clothing: Castleman disease masquerading as a neuroendocrine tumour

A healthy 40 year old female presented with acute onset right iliac fossa pain. CT abdomen (Fig. 1a, b) showed a 42 × 38 × 50 mm enhancing mass anterior to the left kidney within the small bowel mesentery, containing flecks of calcification with a central region of low attenuation, suggestive of a small bowel carcinoid tumour. Subsequent F18-FDG PET/CT showed a mildly avid mass with central photopaenia, and Ga68-Dotatate PET/CT (Fig. 1c) demonstrated moderate dotatate uptake, consistent with a low grade neuroendocrine tumour. All other biochemistry and circulating tissue marker levels were normal Following MDT discussion, the patient was commenced on a monthly somatostatin analogue (lanreotide) with the aim of preventing a carcinoid crisis peri-operatively. Seven months after presentation, the patient underwent a laparoscopic small bowel resection. The pathological diangosis of the surgically excised mass was unicentric Castleman disease (UCD) of hyaline vascular dsubtype (Figs. 2 and 3). The patient remains well 6 months post operatively, with no residual symptoms. The term Castleman Disease (CD) describes a group of at least four distinct diseases with different aetiologies and natural histories comprising of unicentric (UCD) and multicentric (MCD) subytpes. MCD is further divided into idiopathic MCD (iMCD), human herpes virus-8 (HHV8)-associatedMCD (HHV8-MCD), and polyneuropathy, organomegaly, endocrinopathy, monoclonal plasma cell disorder, skin changes (POEMS)-associated MCD (POEMS-MCD).1 Initially thought to be reactive/inflammatory, there is now overwhelming evidence that UCD represents a clonal neoplasm which probably arises from follicular dendritic cells.1 UCD typically presents as either as an incidental finding found on imaging undertaken for other reasons, or as non-tender lymphadenopathy with symptoms dependent on the site and size of the mass. The diagnosis of UCD may be difficult, if not impossilbe, to make on minimally invasive biopsies such as fine needle aspiration (FNA) cytology. Fortunately complete surgical excision of UCD is usually curative, and recurrence after excision is extremely rare.1 UCD is frequently positive on both 68Ga-DOTATATE and 18F-fluorodeoxyglucose positron emission tomography/computed tomography, and as in this case, the diagnosis of gastrointestinal neuroendocirine tumour (NET) is commonly considered before surgical exicision is undertaken.2, 3 The clinical diagnosis of small bowel NET is often challenging due to the clinical manifestation of non-specific symptoms such as abdominal pain.4 For patients who are suspected to have NETs, a range of biochemical tests and imaging are recommended as part of the diagnostic workup,5 which in this patient was negative. Although 68Ga-Dotatate is thought to have very high specificity and sensitivity in identifying gastrointestinal NETs, particularly when utilized in conjunction with the F18-FDG PET/CT,6, 7 this case highlights how the radiological featues of UCD can mimic a NET on somatostatin receptor imaging - an increasingly recognized pitfall.2, 3 The gold-standard diagnostic tool for both CD and neuroendocrine tumours is biopsy and histological analysis. UCD of hyaline vascular subtype is characterized by follicular centre atrophy, mantle cell proliferation and the presence of radially penetrating hyalinized vessels (Fig. 3)1, 8 and on excision speciemens submitted for pathological analysis it is easily distinguished from NET.4-6 Based on the patients abdominal pain and provisional diagnosis of an NET, surgical resection was planned. This is the first-line management for intermediate-grade small bowelNET as it reduces mechanical obstruction from the mass and decreases symptoms associated with the production and release of vasoactive substances.9 Although surgical resection is also the mainstay of treatment of UCD and is almost always required for diagnosis, this case highlights that Castleman disease should be considered in the differential diagnosis of suspected small bowel NETs. Research ethics approved by Northern Sydney Local Health District (NSLHD) Research Ethics and Governance committee. Natasha Umrigar: Software; writing – original draft; writing – review and editing. Anthony J Gill: Conceptualization; data curation; funding acquisition; project administration; writing – review and editing. Anubhav Mittal: Funding acquisition; investigation; project administration; supervision; writing – review and editing. Jaswinder S Samra: Conceptualization; funding acquisition; project administration; resources; validation; writing – review and editing. Open access publishing facilitated by The University of Sydney, as part of the Wiley - The University of Sydney agreement via the Council of Australian University Librarians.