Endocrine Abstracts

Pheochromocytomas (PHEO) and paragangliomas (PGL) are neuroendocrine tumors that were born to be imaged using specific radiopharmaceuticals. This is based on the expression of the cell membrane norepinephrine transporter system and somatostatin receptors, as well as GLUT and amino acid transporters found on PHEO/PGL cells.

Anatomical imaging modalities (e.g. CT or MRI) may define the size, shape, structure and enhancement of tumors well, but they lack the ability to explore their functional characteristics. Various functional imaging methods exist, of which positron emission tomography (PET) has become the most popular, in part due to its availability and decreasing cost. PET scanning has the ability to measure tumor characteristics in vivo, including such processes as glucose and energy metabolism, DNA and protein synthesis, angiogenesis, apoptosis, hypoxia, and blood flow, among others. Thus, PET in endocrine oncology provides specific tumor detection (‘in vivo’ histology); identifies specific tumor function and targets for therapy; predicts and assesses responses to therapy regardless of tumor size changes (‘early’ tumor response); may determine the prognosis even before metastatic lesions occur; and eliminates unnecessary treatments, if, for example, a target molecule is not found.

The recent introduction of [¹⁸F]fluorodopamine and [¹⁸F]fluorodopa as well as [⁶⁸Ga]DOTATOC and Gluc-Lys[¹⁸F]TOCA, together with the previously used [¹⁸F]fluorodeoxyglucose, has revolutionized current imaging approaches to PHEO and PGL. The most important results from these studies support the use of [¹⁸F]fluorodopamine as the first line of imaging in newly diagnosed PHEO and PGL patients (except in those with head and neck tumors), the use of [¹⁸F]fluorodopa as the first line of imaging for SDHB/D- and non-SDHB/D-related head and neck PGLs, and the use of [¹⁸F]fluorodeoxyglucose in the assessment of metastatic SDHB-related PHEO or PGL. The newest data also suggests the use of [¹⁸F]fluorodeoxyglucose in all metastatic PHEOs and PGLs. [¹²³I]MIBG scintigraphy was found to be suboptimal in PGLs and to perform suboptimally or even poorly, especially in head and neck PGLs. This imaging method performs well in non-metastatic PHEO and is also recommended in those patients in whom [¹³¹I]MIBG therapy is planned. Octreoscan has a good value in patients with metastatic PHEO or PGL and in the detection of SDHB-related PHEOs/PGLs in SDHB carriers. PET/CT, rather than PET, has become almost a necessity to perform these studies, and PET/MRI is on the horizon, with some preliminary results already available. The recent use of microCT, MRI, and microPET in an animal model of metastatic PHEO contributed to the introduction of novel therapeutic options awaiting testing in future clinical trials.

Finally, future trends in functional imaging, including its use in pharmacodynamics and molecular imaging, tightly linked to individualized medicine to ‘secure’ the optimal therapeutic plan, will also be outlined.