A Critical Assessment of the Linear No-Threshold Hypothesis: Its Validity and Applicability for Use in Risk Assessment and Radiation Protection.

The Society of Nuclear Medicine and Molecular Imaging convened a task group to examine the evidence for the risk of carcinogenesis from low-dose radiation exposure and to assess evidence in the scientific literature related to the overall validity of the linear no-threshold (LNT) hypothesis and its applicability for use in risk assessment and radiation protection. In the low-dose and dose-rate region, the group concluded that the LNT hypothesis is invalid as it is not supported by the available scientific evidence and, instead, is actually refuted by published epidemiology and radiation biology. The task group concluded that the evidence does not support the use of LNT either for risk assessment or radiation protection in the low-dose and dose-rate region.

Normal Variants and Pitfalls Encountered in PET Assessment of Gynecologic Malignancies.

Combined PET/computed tomography is used for oncological indications. PET/computed tomography benefits from the metabolic information of PET and the anatomic localization of computed tomography. The integrated scanner provides data with accurate registration of anatomy and molecular information. Many physiologic conditions, normal variants, and benign lesions within the pelvis and the body can cause confusion and uncertainty. False-negative results owing to low 18F-fluorodeoxyglucose uptake from the tumor can produce diagnostic challenges and inaccurate conclusions. This article reviews normal variants and potential pitfalls encountered in PET assessment of gynecologic malignancies to provide useful information for the referring and reporting physicians.

Future of Radiation Protection Regulations.

THERE IS considerable disagreement in the scientific community regarding the carcinogenicity of low-dose radiation (LDR), with publications supporting opposing points of view. However, major flaws have been identified in many of the publications claiming increased cancer risk from LDR. The data generally recognized as the most important for assessing radiation effects in humans, the atomic bomb survivor data, are often cited to raise LDR cancer concerns. However, these data no longer support the linear no-threshold (LNT) model after the 2012 update but are consistent with radiation hormesis. Thus, a resolution of the controversy regarding the carcinogenicity of LDR appears to be imminent, with the rejection of the LNT model and acceptance of radiation hormesis. Hence, for setting radiation protection regulations, an alternative approach to the present one based on the LNT model is needed. One approach would be to determine the threshold dose for the carcinogenic effect of radiation from existing data and establish regulations to ensure radiation doses are kept well below the threshold dose. This can be done by setting dose guidelines specifying safe levels of radiation doses, with the requirement that these safe levels, referred to as guidance levels, not be exceeded significantly. Using this approach, a dose guidance level of 10 cGy for acute radiation exposures and 10 cGy y for exposures over extended periods of time are recommended. The concept of keeping doses as low as reasonably achievable, known as ALARA, would no longer be required for low-level radiation exposures not expected to exceed the dose guidance levels significantly. These regulations would facilitate studies using LDR for prevention and treatment of diseases. Results from such studies would be helpful in refining dose guidance levels. The dose guidance levels would be the same for the public and radiation workers to ensure everyone's safety.