Principles and technical equipment
The basis of radionuclide diagnostics is the detection of radiation from specific radiopharmaceuticals (RPs) that accumulate in target tissues. Specialized equipment is used to read this signal: gamma cameras, which are invariably coupled with computer systems for data reconstruction and processing.
Historically, planar (two-dimensional) scintigraphy was used in medicine. However, single-photon emission computed tomography (SPECT, or SPECT) has become the standard today. This method allows for cross-sectional imaging and precise determination of the spatial localization of isotope uptake.
Positron emission tomography (PET) has specific technical and logistical requirements. Due to the extremely short half-life of the radioisotopes used, a cyclotron—a facility for their continuous synthesis—must be located in the immediate vicinity of the PET scanner.
Selection of radiopharmaceuticals (RPs) in oncology
The choice of a specific RP strictly depends on the clinical task, tumor type, and the metabolic pathway under investigation. Agents can reflect the proliferative activity of tissues or selectively bind to specific receptors of neoplasms.
Main agents used in oncological practice:
- 123I-MIBG (meta-iodobenzylguanidine). This tracer is of critical importance, especially in pediatric oncology. It is used for initial diagnosis, precise staging, and subsequent assessment of therapeutic response in neuroblastoma.
- 18F-FDG (fluorodeoxyglucose). Combined with CT, this agent has become the modern "gold standard" for the management of patients with lymphoma. It has completely replaced the previously used gallium-67 isotope scanning for staging and treatment monitoring of this disease.
- $^{68}$Ga-somatostatin analogues. Used for targeted receptor imaging of specific tumors.
- Gallium isotopes. Traditionally used to identify areas with abnormally high proliferative or inflammatory activity.
Hybrid imaging: PET-CT and PET-MRI
A key vector in the development of modern nuclear medicine is the widespread adoption of hybrid technologies. Standalone radionuclide studies show metabolism excellently but do not always provide an accurate topographic picture. Therefore, scintigraphy or PET data are hardware-fused with anatomical imaging results.
- PET-CT. Combines metabolic assessment with X-ray computed tomography. CT provides a complete spatial representation of an organ based on differences in X-ray absorption by tissues (density coefficient assessment). This allows precise correlation of the RP accumulation site with a specific anatomical structure, which is especially important when bone structures are involved.
- PET-MRI (PET-MRI). Gaining immense popularity, particularly in pediatric oncology. MRI is based on the phenomenon of nuclear magnetic resonance of hydrogen protons and does not use ionizing radiation at all. Replacing the CT component with MRI leads to a significant reduction in the patient's total radiation burden. In addition, MRI provides higher soft-tissue detail, which perfectly complements the metabolic data of PET when assessing perineural invasion or soft-tissue involvement.
Differential diagnostics and related tasks
Radionuclide methods in oncology often overlap with the diagnosis of inflammatory and infectious processes. For example, 18F-FDG PET-CT is actively used not only for lymphoma staging but also in searching for the cause of fever of unknown origin, which frequently accompanies neoplastic diseases.
In addition, in the comprehensive management of cancer patients, other RPs may be used to assess concomitant pathology or therapy complications:
- To assess renal function and detect parenchymal scarring, 99mTc-DMSA (dimercaptosuccinic acid) and 99mTc-MAG3 (merCAcetyltriglycine) are used.
- When occult gastrointestinal bleeding is suspected, 99mTc-pertechnetate is used.
- To assess biliary tract patency — 99mTc-mebrofenin.
It is important to consider that radionuclide diagnostics has a well-defined temporal window of informativeness. It serves as the method of choice for complex diagnostic tasks, supplementing rather than replacing routine primary screening methods.
Class and exam questions
Why is a cyclotron needed for PET?
It is necessary for the synthesis of radioisotopes. Due to their very short half-life, the cyclotron must be located in the immediate vicinity of the PET scanner; otherwise, the agents will lose their activity before administration to the patient.
Which method replaced gallium-67 scanning in lymphomas?
Currently, the "gold standard" for staging and assessing treatment response in lymphoma is hybrid PET-CT imaging using 18F-FDG (fluorodeoxyglucose).
What is the main advantage of PET-MRI over PET-CT in pediatric oncology?
The main advantage of PET-MRI is a significant reduction in the dose of ionizing radiation due to the absence of the X-ray CT component. This is critically important for children's safety while maintaining high diagnostic accuracy.
What else to study on this topic
- Radioisotope diagnostics in nephrology and urology (99mTc-DMSA, 99mTc-MAG3)
- Myocardial scintigraphy: perfusion assessment and diagnostic time windows
- Basic MRI protocols of the abdominal cavity for space-occupying lesions
- Comparison of diagnostic capabilities of CT and MRI in oncology
- Physical principles of nuclear magnetic resonance (NMR)
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