Evolving role of PET/CT with different tracers in the evaluation of pulmonary neuroendocrine tumours

Abstract

Neuroendocrine tumours (NETs) of the lung arise from bron-chial mucosal cells known as enterochromaffin cells orKulchitskycells,which are partofthe diffuseneuroendocrinesystem. These tumours account for 20–25 % of all primarylung neoplasms [1]. The pathological spectrum of pulmonaryNETsrangesfromlow-orintermediate-gradeneoplasms,alsoknown as typical or atypical bronchial carcinoids (BCs), re-spectively, to more aggressive tumours such as large cell andsmall cell lung cancer [1, 2]. In detail, the pathological dis-tinctionissubstantiallybasedonthemicroscopicmorphology,thegradeofdifferentiation,themitoticrateandthepresenceofnecrosis [2].The distinct features among the different pulmonary NETsinclude not only their pathological characteristics but also theclinicalbehaviour,epidemiology,treatmentandprognosis[3].From a clinical point of view, typical BCs are indolent neo-plasms with a good prognosis, whereas atypical BCs have aless indolent behaviour with a certain propensity for metasta-ticspread.Boththesewell-differentiatedpulmonaryNETsareoptimally treated with complete surgical excision, althoughsome divergent management strategies of care have beenrecently recommended for typical and atypical BCs [4, 5].Indeed, while surgical resection remains the gold standardin the treatment of BCs, the extension of parenchymalresection (lobar versus sublobar resection) and the needfor a radical lymph node dissection may be influenced by thehistologicalcharacterization[4,5].Conversely,moreaggressivepulmonary NETs, such as large cell and small cell lung cancer,often present with local invasion, thoracic lymph node metas-tasesanddistantspread;asaresult,affectedpatientsmaynotbecandidates for surgical resection, often being treated with che-motherapy with or without radiation therapy, showing a poorprognosis [3, 4].Therefore, a correct pathological identification of pulmo-naryNETsduringthepreoperativesettingisakeyelementforplanning the best strategy of care, considering the differentbiologicalbehaviourofthevariouspulmonaryNETsubtypes.Nevertheless, the preoperative pathology differentiationbetween typical and atypical BCs frequently appears notaccurate. Furthermore, the structural imaging findings areoften similar in typical and atypical BCs and, accordingly, aclear differentiation between these pulmonary NETs is notpossible through radiological findings only [1–4].Positron emission tomography (PET)/CT with differenttracers, such as