Neuroendocrine tumors (NETs) are rare tumours arising from the gastroenteropancreatic axis. They are slow growing and often metastatic. Diagnostic workup requires imaging; both anatomical (ultrasound, CT, MRI) and functional (somatostatin receptor scintigraphy, PET).
NETs strongly express somatostatin receptors (SSTRs) of five different subtypes (SSTRs15) with SSTR2 being the most common. Some NETs reflect hyperactivity of the guanethidine pathway. Imaging based on receptor expression helps to guide treatment in cases of inoperable or progressive NETs. Imaging with radiolabelled receptor binding somatostatin analogues and meta isobenzyl guanidine (MIBG) can be used to select patients for peptide receptor radionuclide therapy (PRRT).
We describe two cases with differential uptake patterns on 123I-MIBG, 111In octreotide and 111In DOTATOC scanning affecting treatment options.
Case 120 years old male referred for PRRT with metastatic gastric small cell NET.111In octreotide scan showed a focal hot spot in the skull but not on the MIBG scan. An 111In DOTATOC showed multiple hot spots in the skull confirming brain metastasis. The patient received cranial radiotherapy instead of PRRT.
Case 236 years old female with a mesenteric somatostatinoma referred for PRRT after an incomplete excision. An 111In octreotide scan showed significant uptake at the site of the known mesenteric tumour with normal uptake on MIBG scan.
An 111In DOTATOC scan confirmed presence of SSTRs bearing abnormal tissue at the same site. She received PRRT with 90Y DOTATOC with good tumour response.
As NETS vary in their expression of SSTRs along with difference in affinity of radiolabelled agents for SSTRs, tumoral uptake can vary with individual functional imaging. Our cases highlight this variation. Therefore we recommend imaging with at least two different radiolabelled somatostatin analogues, at the pre-treatment stage.
PRRT remains a promising option for inoperable NETs but requires a careful approach in patient selection.