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Oncology- The Role of Unsealed Radionuclides in Oncology
I. Nuclear Medicine in Oncology: The Big Picture
Nuclear medicine uses radioactive substances (radionuclides) to:
A. Bone Scintigraphy:
I. Nuclear Medicine in Oncology: The Big Picture
Nuclear medicine uses radioactive substances (radionuclides) to:
- Localize tumors: Pinpoint the location of a cancerous growth.
- Detect metastases: Identify cancer that has spread to other parts of the body.
- Monitor treatment response & detect recurrence: Track how well treatment is working and identify any cancer that comes back.
- Deliver targeted radiotherapy: Deliver radiation directly to cancerous cells.
- Radiopharmaceuticals: These are the workhorses. They consist of a radionuclide (the radioactive part) attached to a ligand (a molecule that targets specific cells or tissues). Gamma rays emitted by the radionuclide are detected to create images.
- Tumour detection mechanism: Radiopharmaceuticals concentrate in areas of abnormal biological activity, such as tumors, allowing visualization of these areas that are undetectable via cross-sectional imaging like CT or MRI.
- Scintigraphy limitations: While scintigraphy offers functional information that CT and MRI lack, it provides less anatomical detail. Often, multiple imaging modalities are used together.
- Common Radiopharmaceuticals:
- Iodine-123 (-131): Used for thyroid imaging and therapy.
- Thallium-201 (201Tl) & Gallium-67 (67Ga): Used for various imaging purposes.
- Technetium-99m (99mTc): An ideal radionuclide for imaging due to many favorable characteristics (details below).
- Gamma Cameras: Traditionally used for planar (2D) and whole-body scintigraphy.
- Single-Photon Emission Computed Tomography (SPECT): Uses computer processing to generate cross-sectional (3D) images, offering improved sensitivity and localization compared to planar scintigraphy.
- Positron Emission Tomography (PET): Uses positron-emitting radionuclides and provides quantitative tomographic images. Commonly used with 18F-fluorodeoxyglucose (FDG) to measure glucose metabolism in tumors, providing information about tumor vitality, cell turnover, and response to therapy.
A. Bone Scintigraphy:
- Radiopharmaceuticals: 99mTc-methylene diphosphonate (MDP) or 99mTc-hydroxymethylene diphosphonate (HDP).
- Procedure: Injected 2-4 hours before imaging.
- Sensitivity: High sensitivity (80-100%) for many cancers (breast, prostate, lung, etc.). Lower sensitivity (around 75%) for others (melanoma, small-cell lung cancer, etc.). "Cold" defects may indicate lesions lacking osteoblastic activity.
- Radiopharmaceuticals: 131I, 123I, and 99mTc (pertechnetate).
- 131I advantages & disadvantages: Cheap and readily available, but has a long half-life and emits alpha particles, leading to significant radiation exposure.
- 123I advantages & disadvantages: Excellent imaging properties and shorter half-life but expensive.
- 99mTc: Trapped in the thyroid temporarily but not permanently incorporated.
- Clinical indications: Evaluation of nodules and post-surgery follow-up for differentiated thyroid cancer.
- Principles: Uses tumor-seeking radiopharmaceuticals. Ideal agents have high tumor-to-background ratios, long retention times in tumors, and emit radiation energetic enough for therapy but with limited penetration to minimize damage to healthy tissues.
- Clinically useful radiopharmaceuticals: 131I, 89Sr, 32P, 186Re, 153Sm, 90Y.
- Uses: Treatment of thyrotoxicosis and differentiated thyroid carcinoma (ablation of remaining thyroid tissue after surgery, treatment of recurrent or metastatic disease).
- Uses: Treatment of neural crest tumors (pheochromocytoma, neuroblastoma, paraganglioma, medullary thyroid carcinoma). Variable success rates (e.g., >50% for malignant pheochromocytoma).
- Mechanism: Mimic calcium or phosphate to accumulate in areas of high bone turnover (near metastases).
- Examples: 89Sr (calcium analogue), 32P, 186Re, HEDP, and 153Sm (phosphate analogues).
- 89Sr: First systemic treatment for bone metastases in prostate cancer. Provides pain relief and delays disease progression in 75-80% of patients for 1-6 months. Myelosuppression (bone marrow suppression) is a side effect.
- 32P: Historically used for bone pain relief but limited by bone marrow toxicity.
- Mechanism: Direct injection of radiopharmaceuticals into body cavities (pleural, pericardial, peritoneal, bladder, CSF, cysts) to target tumors locally, minimizing systemic exposure.
- Radiopharmaceuticals: Colloids and monoclonal antibodies labeled with 32P, 90Y, or 131I.
- Concept: Targeted delivery of radiation using antibodies specific to tumor cells.
- Current status: Despite initial promise, radioimmunotherapy has faced numerous challenges and its future remains uncertain.
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