PET Scanning in Thymic Neuroendocrine Tumors

Abstract

To the Editor: We read with interest the recent report by Tiffet and colleagues (July 2003)1Tiffet O Nicholson AG Ladas G et al.A clinicopathologic study of 12 neuroendocrine tumors arising in the thymus.Chest. 2003; 124: 141-146Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar of their series of 12 cases of thymic neuroendocrine tumor. They note that although octreotide scintography has been reported as useful in detecting thymic neuroendocrine tumors,2Nilsson O Kolby L Wangberg B et al.Comparative studies on the expression of somatostatin receptor subtypes, outcome of octreotide scintigraphy and response to octreotide treatment in patients with carcinoid tumours.Br J Cancer. 1998; 77: 632-637Crossref PubMed Scopus (102) Google Scholar, 3Silva F Vazquez-Selles J Aguilo F et al.Recurrent ectopic adrenocorticotropic hormone producing thymic carcinoid detected with octreotide imaging.Clin Nucl Med. 1999; 24: 109-110Crossref PubMed Scopus (18) Google Scholar scanning for somatostatin receptors was not of help in their series. In addition, only one patient in the series of Tiffet and colleagues1Tiffet O Nicholson AG Ladas G et al.A clinicopathologic study of 12 neuroendocrine tumors arising in the thymus.Chest. 2003; 124: 141-146Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar was noted to be meta-iodobenzylguanidine (MIBG) positive. The accurate localization of the primary tumor site and possible metastases is crucial for optimum treatment of these tumors. We have recently treated two patients with this rare disease, both of whom presented with Cushing syndrome caused by ectopic adrenocorticotropic hormone secretion. In both cases, we found that positron emission tomography (PET) scanning with fluorine-18 fluorodeoxyglucose was useful either to localize the tumor or identify metastases. In our first case (patient 1), the tumor, a typical carcinoid (well-differentiated neuroendocrine carcinoma), was not localized with CT and an octreotide scan result was negative. However, the tumor did show up as a focal “hot spot” on subsequent PET scanning (Fig 1). A gated MRI of the hot spot gave further, accurate localization. PET scanning therefore localized the tumor when other modalities had failed. In the second case (patient 2), an atypical carcinoid (moderately differentiated neuroendocrine tumor), the tumor was localized with a CT scan of the chest but subsequent PET scan revealed an unsuspected metastasis in the lumbar spine (Fig 2). The lumbar metastasis required orthopedic fixation before the surgery to remove the mediastinal tumor. Octreotide scanning was not performed in patient 2, but MIBG scanning revealed the same distribution of metastases as the PET scan, and the patient received radioactive (I131) MIBG treatment.FIGURE 2PET scan in patient 2 showing the thymic neuroendocrine carcinoma and lumbar spine metastasis.View Large Image Figure ViewerDownload (PPT) As far as we are aware, PET scanning has not previously been described as a useful investigative tool for neuroendocrine tumors of the thymus. However, on the basis of our limited experience with this condition, we would suggest that in addition to CT, MRI, octreotide, and MIBG scanning, PET may be a valuable tool. Where available, it could be used and evaluated further in patients presenting with thymic neuroendocrine tumors. The sensitivity and specificity of PET compared to octreotide and MIBG scanning is unknown and requires further investigation.