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The science of internal dosimetry is a specialty within the general field of health physics. A working definition of health physics might be "the protection of people and their environment from the harmful effects of radiation while allowing its beneficial applications." In any application involving the use of ionizing radiation, the risks of its use must be balanced against its benefits. In medical uses of radiation, the benefits are immediately obvious and are directly received by the person who is exposed to the risk. This makes the balancing process considerably easier than, for example, in the use of nuclear power, where a small number of people incur a risk so that a broad region can receive a benefit.The evaluation of this balance, however, cannot occur without some quanti- fication of the risks. Internal dosimetry calculations provide estimates of the amount of radiation that is absorbed by different organs or organ systems. The assignment of a risk to a given radiation dose estimate is not a clear-cut procedure, but without the absorbed dose estimate it cannot be attempted. [ABSTRACT FROM AUTHOR]

The practitioner of pediatric nuclear medicine should have some knowledge of radiation effects and the potential hazards that may result from low-level radiation exposures. There are several reasons such information is essential. First, specialists should ensure that the exposure of patients to radiation from diagnostic or therapeutic procedures is not excessive. Although all current radiopharmaceuticals deliver radiation doses within a readily acceptable range, such was not the case 30 years ago when the radionuclides employed were generally longer-lived and emitted significant particulate radiation, e.g., iodine-131, strontium 87. As a result, before 1970 at Children's Hospital Boston, radionuclides were administered only to patients with advanced neoplastic diseases. Today, as new agents are introduced, it is imperative to understand the kinetics of their distribution and the resulting radiation doses delivered to various organs. Moreover, for those who participate in clinical trials, an estimation of the absorbed radiation dose is required by institutional review boards, as is some assessment of the potential hazard. [ABSTRACT FROM AUTHOR]

In the 1960s and 1970s, positron emission tomography (PET) was developed as a research tool, particularly for the investigation of neurophysiology.1-4 In the 1980s and 1990s, the clinical utility of PET in oncology, neurology, and cardiology was demonstrated.5-10 The approval of reimbursement by the U.S. Centers for Medicare and Medicaid Services for oncologic PET and the subsequent establishment of regional distribution centers of fluorine-18 (18F)-fluoro-2-deoxy-glucose (FDG) in the late 1990s contributed greatly to the expansion in the clinical use of PET. This expansion led to greater availability of PET for pediatric imaging as well as for adults.11,12 Pediatric PET has demonstrated utility in neurology as well as oncology, and the application of PET in pediatrics will continue to grow as its clinical potential is further realized and as new positron-emitting radiopharmaceuticals are developed. Several technologic factors, involving both physics and radiopharmaceutical chemistry, have contributed to the popularity of PET. It is substantially easier to develop a PET rather than a single photon emission computed tomography (SPECT) analogue to many naturally occurring, biologically relevant chemicals such as water, oxygen, carbon monoxide, ammonia, glucose, and a whole host of others. Thus PET is well suited to serve a very prominent role in this new and exciting era of molecular medicine. [ABSTRACT FROM AUTHOR]

Single photon emission computed tomography (SPECT) allows the user to obtain a threedimensional (3D) representation of the patient's in vivo radiopharmaceutical distribution. Planar nuclear imaging leads to a twodimensional (2D) image of a 3D object. In some cases, it can be difficult to detect or localize a certain feature due to the ambiguity introduced by background activity in the overlying and underlying tissue. Conversely, SPECT allows the 3D object to be represented as a series of thin, tomographic slices. This can lead to a substantial increase in image contrast that can greatly improve the ability to detect small features. Due to the 3D nature of SPECT, it can also greatly improve one's ability to localize these features. In addition, improved contrast can lead to an enhanced quantitative capability that can be of great value for both clinical and research purposes. For all of these reasons, SPECT has become an essential medical imaging modality over the past 30 years. [ABSTRACT FROM AUTHOR]

Despite dramatic advances in its prevention and treatment, infection remains a major cause of morbidity in children, accounting for approximately 30% of childhood deaths worldwide.1 The development of powerful antimicrobial agents has improved patient survival, but timely diagnosis is equally, if not more, important. In adults, most infections can be diagnosed with a thorough history, a complete physical examination, and appropriate laboratory tests. In the pediatric population, unfortunately, this is a difficult task. Children do not, or will not, verbalize their feelings, and the history is often little more than secondhand information obtained from a parent. The physical examination of an ailing child can be difficult, if not impossible. Further complicating matters is the fact that inflammatory conditions such as vasculitis and inflammatory disease may mimic infection. Consequently, empiric treatment with antibiotics, which may be neither appropriate nor effective, is often instituted. Imaging procedures are usually reserved for those patients in whom symptomatology or physical findings point to a specific region of the body. [ABSTRACT FROM AUTHOR]

The incidence of cancer is estimated to be 133.3 per million children in the United States.1 Although cancer is much less common in children than in adults (only about 2% of all cancers occur before 15 years of age), it is still an important cause of mortality in pediatrics. Approximately 10% of deaths during childhood are attributable to cancer, making it the leading cause of childhood death from disease.2 Nuclear imaging has played an increasingly important role in diagnosis, staging, treatment monitoring, surveillance, and prognostication in children with cancer. [ABSTRACT FROM AUTHOR]

Serial measurement of the glomerular filtration rate (GFR) is recognized as an important assessment of the status of kidney function for individuals in certain disease states and especially for those undergoing chemotherapy. To be clinically useful the method used to measure GFR must compare favorably to the GFR gold standard, which is the urinary clearance of inulin, and the variance between successive serial measurements must be kept small. [ABSTRACT FROM AUTHOR]

The definition of gastroesophageal reflux (GER) is the passage of gastric contents into the esophagus. Regurgitation is defined as passage of refluxed gastric contents into the mouth.Vomiting is the expulsion of gastric contents from the mouth. Many episodes of gastroesophageal reflux occur in healthy infants and children and are considered "sphysiologic.'s Such episodes are brief and either asymptomatic or cause mild regurgitation or occasional vomiting.1 Gastroesophageal reflux disease (GERD) occurs when episodes of GER produce symptoms and complications. [ABSTRACT FROM AUTHOR]

In some instances patients present with thyroid cancer that does not seem to fit the usual treatment guidelines as set forth in conventional texts. For example, there may be an unusual distribution of thyroid remnants or metastases or there sometimes are extensive metastases to the lung or other organs. In these cases knowledge about regional uptake of therapeutic iodine-131 (131I) is useful. [ABSTRACT FROM AUTHOR]

Thyroid scintigraphy plays an important role in the evaluation of the thyroid gland owing to the functional and anatomic information it provides. Ultrasound-guided fine-needle aspiration (FNA), as well as the increased availability of sensitive serum assays for thyrotropin (thyroid-stimulating hormone, TSH) and thyroglobulin (Tg), play an important role in the routine evaluation of thyroid disease. However, measurement of radioactive iodine uptake remains the only direct test of thyroid function and, when complemented by studies that permit anatomic correlation, scintigraphy is a powerful tool in the investigation of both benign and malignant thyroid disorders. This chapter focuses on the role of radioiodine in the diagnosis and therapy of hyperthyroidism and in the treatment and surveillance of children with differentiated thyroid cancer (DTC). The rarity of these diseases in children has precluded the generation of consensus guidelines, but approaches for the evaluation of thyroid nodules and for the preparation of patients prior to iodine-131 (131I) therapy are suggested. [ABSTRACT FROM AUTHOR]