Non-PET Neoplasm Imaging and Radionuclide Therapy

In this chapter, tumor imaging using conventional gamma camera techniques including single-photon emission computed tomography (SPECT) and SPECT/computed tomography (SPECT/CT) as well as less frequently employed or emerging radionuclide tumor therapies are addressed. The more commonly encountered entities of thyroid cancer and bone tumors and metastases are discussed in detail in Chapter 4 and Chapter 8 , respectively. Positron emission tomography (PET) imaging of neoplasms is discussed in Chapter 11 . The affinity of various tumors for specific radiopharmaceuticals is shown in Box 10.1 , and the relative value of various imaging procedures for different tumors is shown in Chapter 11 , Table 11.1 . Although some of these techniques have been largely supplanted by fluorine-18 fluorodeoxyglucose ( ¹⁸ F-FDG) PET/CT imaging, some are still useful in special settings.

During the past decade, new biotechnologic advances have spurred the development of increasingly sensitive and specific tumor imaging agents for use in both single-photon and positron imaging. As these agents have become available, they have spurred the concept of developing a drug or closely related group of drugs with high affinity to a particular tumor that can be labeled with a diagnostic imaging radionuclide to assess the location and extent of disease as well as separately labeled with a therapeutic radionuclide (such as a beta-emitter) to treat the neoplasm in a dosage commensurate with the tumor burden revealed by imaging. The success of treatment can be subsequently assessed using the diagnostic imaging version. This has been called “theranostics” (therapy + diagnosis). The simplest example is sodium iodide. Labeled with iodine-123, it is used diagnostically in differentiated thyroid cancer and its metastases. However, labeled with iodine-131, it becomes a therapeutic drug. More sophisticated compounds and their analogs are emerging for use with other tumors, such as somatostatin receptor radiopharmaceuticals for neuroendocrine malignancies. Tumor-imaging radiopharmaceuticals may be divided into two broad groups:

• Those designed to target specific tumor antigens, receptors, or metabolic processes, including monoclonal antibodies, peptides such as somatostatin (octreotide), and metaiodobenzylguanidine (MIBG).

• Those with nonspecific affinity for neoplastic tissue, including gallium-67 citrate ( ⁶⁷ Ga), thallium-201 chloride ( ²⁰¹ Tl), technetium-99m ( ^99m Tc) sestamibi, and ¹⁸ F-FDG. These may be used to image a range of tumors in various organs.

Both categories of radiopharmaceuticals are used in clinical nuclear medicine practice. Typical administered activities and radiation doses for tumor-seeking radiopharmaceuticals in current use are given as sample techniques in Appendix E .