Nuclear medicine diagnostics in oncology. Tumor imaging with nonspecific, specific and receptor tumorotropic radiopharmaceuticals. 99mTc MIBI, 99mTc Tektrotyd, 18F-FDG, 68Ga-PSMA, 68Ga-DOTATOC. Lymph node scintigraphy and visualization of sentinel lymph nodes. Application of SPECT-CT.
Nuclear medicine oncology is the branch of nuclear medicine that uses radiopharmaceuticals to detect, characterize, stage, monitor, and evaluate malignant tumors based on their physiological, metabolic, or receptor characteristics rather than only their anatomy.
Unlike CT or MRI, which mainly demonstrate the size and morphology of a tumor, nuclear medicine demonstrates:
- Tumor metabolism
- Cell viability
- Receptor expression
- Blood flow
- Functional activity
Principle of Tumor Imaging
After administration of a radiopharmaceutical,
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The tracer accumulates preferentially in tumor tissue because of:
- Increased metabolism
- Increased blood flow
- Increased mitochondrial activity
- Overexpression of receptors
- Increased glucose utilization
- Increased cell proliferation
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Gamma camera or PET scanner detects emitted radiation.
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Tumor appears as a hot lesion.
1. Nonspecific Tumorotropic Radiopharmaceuticals
These accumulate in tumors because of general tumor characteristics, not because they bind to a specific receptor.
Mechanisms include:
- Increased blood flow
- Increased metabolism
- Increased mitochondrial activity
- Altered cell membrane permeability
They are useful in many different tumors.
Example 1 – Tc-99m MIBI
Full Name
Technetium-99m Methoxy-Isobutyl-Isonitrile (Commonly called Sestamibi)
Mechanism
Tc-99m MIBI enters viable cells.
It accumulates inside: Mitochondria
Tumor cells contain increased mitochondrial activity.
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More tracer accumulates.
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Hot lesion.
Uses
- Breast tumors
- Thyroid tumors
- Parathyroid adenoma
- Brain tumors
- Lung tumors
- Some soft tissue tumors
Advantages
- Widely available
- Good image quality
- SPECT compatible
- Low radiation dose
Limitations
- Less specific
- Uptake also occurs in inflammatory lesions
- Physiological uptake may interfere with interpretation
2. Specific Tumorotropic Radiopharmaceuticals
These accumulate because they target particular tumor characteristics or tumor metabolism.
The most important is: 18F-FDG
Full Name
Fluorine-18 Fluorodeoxyglucose
Mechanism
FDG is a glucose analogue.
Tumor cells consume large amounts of glucose.
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FDG enters cells through glucose transporters (GLUT).
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FDG is phosphorylated.
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Cannot undergo further metabolism.
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Becomes trapped inside tumor cells.
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Tumor appears bright on PET.
Why do Tumors take up FDG?
Cancer cells demonstrate Warburg Effect
Tumor cells preferentially utilize glycolysis even in the presence of oxygen.
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Very high glucose consumption.
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High FDG uptake.
Normal Physiological FDG Uptake
Remember these organs because they are always bright:
- Brain
- Myocardium (variable)
- Liver
- Kidneys
- Urinary bladder
This is normal physiological uptake.
Clinical Applications of FDG PET/CT
Used in:
- Lung cancer
- Breast cancer
- Colorectal cancer
- Head and neck cancer
- Melanoma
- Lymphoma
- Esophageal cancer
- Ovarian cancer
Uses
FDG PET is used for:
- Diagnosis
- Initial staging
- Detection of metastases
- Therapy response
- Detection of recurrence
- Prognosis
Limitations
FDG also accumulates in:
- Infection
- Inflammation