Radiotherapy Principles and Techniques
Radiotherapy uses ionizing radiation to destroy cancer cells by damaging their DNA. Because cancer cells divide rapidly and have weaker DNA repair mechanisms than normal cells, they're more vulnerable to this kind of damage. Understanding how radiation is delivered and dosed is central to nuclear medicine and shows up frequently in introductory physics problems involving energy absorption and radioactive decay.
Principles of Radiotherapy Dosage
The radiation dose absorbed by tissue is measured in gray (Gy), defined as the absorption of 1 joule of radiation energy per kilogram of tissue:
Total doses for cancer treatment range from about 20 to 80 Gy, depending on cancer type and stage. Early-stage breast cancer, for example, typically requires 40–50 Gy.
Rather than delivering the full dose at once, doctors use fractionation: splitting the total dose into smaller amounts given over several weeks. This works because healthy cells repair DNA damage between sessions more effectively than cancer cells do. A typical schedule might be 2 Gy per session, five days a week, for several weeks.
Dose planning uses imaging (CT, MRI) to map the tumor's shape and location so that radiation is concentrated on the tumor while sparing as much healthy tissue as possible.
External Beam vs. Implant Techniques
These are the two main approaches to delivering therapeutic radiation. Each has distinct physics and clinical applications.
External beam radiation therapy (EBRT) delivers radiation from outside the body, usually from a linear accelerator that produces high-energy X-rays or gamma rays aimed at the tumor.
- 3D conformal radiotherapy (3DCRT) shapes the radiation beam to match the tumor's three-dimensional geometry, reducing exposure to surrounding tissue.
- Intensity-modulated radiotherapy (IMRT) goes a step further by varying the beam's intensity across the treatment field. This allows higher doses to reach the tumor while delivering lower doses to adjacent healthy structures.
Brachytherapy (radioactive implants) places sealed radioactive sources directly inside or very near the tumor. Because radiation intensity drops off sharply with distance (following an inverse-square relationship), this delivers a high dose to the tumor with much less exposure to surrounding tissue.
- Interstitial brachytherapy implants radioactive seeds or pellets directly into the tumor tissue. This is commonly used for prostate cancer.
- Intracavitary brachytherapy places radioactive sources inside a body cavity adjacent to the tumor, as in cervical cancer treatment.
The key trade-off: EBRT is non-invasive and treats a wider range of tumor locations, while brachytherapy delivers more concentrated doses but requires a procedure to place the sources.
Radiopharmaceuticals in Cancer Treatment
Radiopharmaceuticals are drugs containing radioactive isotopes that target specific cancer cells from inside the body. They can be given orally, intravenously, or by direct injection.
These drugs reach cancer cells through two main mechanisms:
- Receptor binding: The drug attaches to receptors that are overexpressed on cancer cell surfaces, concentrating the radioactive isotope at the tumor.
- Metabolic uptake: Cancer cells consume certain compounds (like glucose or amino acids) at higher rates than normal cells, so radioactive versions of those compounds accumulate preferentially in tumors.
Once localized, the isotope emits radiation that damages cancer cell DNA and triggers cell death. Radiopharmaceuticals can be used alone or combined with other treatments like chemotherapy or EBRT.
Two important examples:
- Iodine-131 () for thyroid cancer. Thyroid cells naturally absorb iodine, so radioactive iodine concentrates in thyroid tissue and thyroid cancer cells, delivering targeted radiation with minimal effect on other organs.
- Radium-223 dichloride () for prostate cancer that has spread to bone. Radium mimics calcium chemically, so it gets incorporated into bone at sites of metastasis, irradiating the cancer cells there.
Advanced Radiotherapy Techniques and Considerations
Particle therapy, such as proton therapy, offers a physics advantage over traditional X-ray or gamma-ray treatments. Protons deposit most of their energy at a specific depth in tissue (the Bragg peak), then stop. This means less radiation reaches healthy tissue beyond the tumor compared to photon beams, which continue depositing energy as they pass through the body.
Managing side effects is a major part of treatment planning. Radiation can damage healthy tissue near the tumor, causing effects like skin irritation, fatigue, or organ-specific complications depending on the treatment site. Fractionation and precise targeting techniques both help reduce these effects.
Radiation protection measures safeguard patients, medical staff, and the public. The three basic principles are minimizing exposure time, maximizing distance from the source, and using appropriate shielding. These follow directly from the physics of how radiation intensity decreases with distance and how materials absorb radiation.