PET/CT in diagnosis and monitoring the effect of treatment in children with malignant tumors
Authors: Divošević Sunčana 
Published: July 2024

PET/CT is the most sensitive and highly specific imaging technique for determining the location of tumors, primary tumors of unknown location, determining the extent and activity of malignant disease, monitoring treatment effects, detecting local recurrence and distant malignancy, precise planning of various treatment modalities and planning/determining the radiation field. PET uses biologically active molecules labeled with short-lived radionuclides that emit positrons and are the product of nuclear reactions in a cyclotron. Fluorine-18 (18F) is the most commonly used radionuclide in nuclear medicine, and its relatively long half-life of 110 minutes allows adequate synthesis of radiopharmaceuticals and monitoring of biological processes, as well as delivery to PET machines in remote facilities. The most commonly used radiopharmaceutical, which is now used in clinical practice in more than 90% of PET/CT examinations, is 2-(18F)-fluoro-2-deoxy-D-glucose (18F FDG), a glucose derivative that reflects the accumulation and consumption of glucose in cells, i.e. glucose metabolism. PET/CT has an advantage over other diagnostic imaging techniques because by combining PET, which is the evaluation of the intensity of the metabolic activity of a specific radiopharmaceutical in the cells, and CT, which shows the anatomy and morphology of the organs, we simultaneously obtain information about pathological abnormalities in both the function and morphology of the lesions or organ and the presence of viable tumor or inflammatory tissue. It is important to mention that with the latest PET/CT equipment we have the possibility to visualize metabolic activity even in very small lesions (2 mm). The advantage of this diagnostic method is that the findings are not only analyzed visually, but also quantitatively by measuring the intensity of radiopharmaceutical accumulation in the lesions, thus achieving objectification of the findings and the possibility of adequately monitoring the effect of different forms of treatment in control imaging. PET/CT allows us to identify prognostically risky patients and to select the most appropriate forms of treatment for each individual patient. PET/CT is the most sensitive method for distinguishing treatment-related changes from residual or recurrent disease. A continuous decrease in metabolic activity in the tumor area indicates a positive effect of the treatment. PET/CT enables precise monitoring of the effect of all forms of treatment (surgical, chemotherapeutic, radiotherapeutic and radiosurgical procedures) and has the advantage that the entire body is captured and analyzed with one image in the field of view and not just a single segment. The radiation exposure in PET/CT results from the use of radiopharmaceuticals and CT imaging, and the ALARA principle (as low as reasonably achievable) is followed, whereby the dose is adjusted taking into account the weight and age of the child and the type of device, taking into account the longer life and radiation sensitivity of the child's organism.To assess the treatment effect, the RECIST criteria (Response Evaluation Criteria in Solid Tumors) are used to evaluate the size of the lesions and the PERCIST criteria (PET Response Criteria in Solid Tumors) to evaluate the metabolic activity of the lesions. Inclusion should also take into account the characteristics of the pediatric population, such as the need for parental or guardian consent for the examination, the period of starvation before the examination, etc., difficulties in establishing venous access, sedation/anesthesia of young children due to the need for rest during imaging, and marked physiological accumulation of radiopharmaceuticals in different parts of the body.Compared to adults, there is a slightly higher percentage of total accumulation of radiopharmaceuticals in the brain, lower excretion of activity in the urine, greater accumulation in the bone marrow due to more pronounced hematopoiesis, greater accumulation in regions with lymphatic tissue, pronounced physiological accumulation in growth zones, greater accumulation in brown adipose tissue, greater accumulation in the thymus (physiological, after chemotherapy), while the use of hematopoietic growth-promoting factors can lead to a pronounced diffuse accumulation of activity in the spleen and bone marrow. PET/MR offers a safer, more specific and more efficient assessment of the extent of disease in children than any other diagnostic imaging technique. The advantage over other hybrid diagnostic procedures is that a high resolution and a high contrast between diseased and healthy tissue is achieved with significantly less exposure to ionizing radiation. Artificial intelligence tools are now used to process the image data and analyze the findings, which improve image quality and perform the most accurate data reconstruction and support the physician in the search for suspicious lesions, measuring and determining the etiology of pathological processes in comparison with earlier diagnostic methods, and reducing the possibility of overlooking suspicious lesion.