In-111 DTPA-D-Phe1-octreotide SPECT in a rare case of anorectal small-cell undifferentiated neuroendocrine carcinoma

In-111 DTPA-D-Phe1-octreotide (In-111 pentetreotide) is a widely used radiotracer for somatostatin receptor (SSTR) investigations in neuroendocrine (NE) pathology. Cellular SSTR density is directly related to the degree of NE cellular differentiation. Poorly differentiated cell lines show a surface reduction of SSTRs, which may lead to misinterpreting the In-111 pentetreotide scintigraphy results as negative. A case of anorectal small-cell undifferentiated neuroendocrine carcinoma is reported. This pathologic finding is rare, highly aggressive, and associated with a dramatic prognosis. The patient was investigated with In-111 pentetreotide SPECT and low-dose computed tomography (CT) for anatomic reference. Despite its low degree of differentiation, the tumor showed high tracer uptake characterized by a positive incremental trend as shown by comparison of the 4-hour and 24-hour SPECT images, suggesting specific receptorial binding of In-111 pentetreotide.