The Physics of Radiation Therapy

Radiation therapy harnesses the power of ionizing radiation—high-energy waves or particles—to destroy cancer cells by damaging their genetic material. This prevents these cells from dividing and growing, eventually leading to their death.

Two primary types of radiation are used in cancer treatment: photon radiation (X-rays and gamma rays) and particle radiation (protons, neutrons, and other charged particles). Photon radiation passes through the body, depositing energy along its path, while particle radiation deposits most of its energy at a specific depth, potentially reducing damage to surrounding tissues.

Medical physicists work with radiation oncologists to calculate the exact radiation dose needed for each patient. These calculations take into account the cancer type, size, location, and the patient's overall health. Advanced imaging techniques like CT scans create three-dimensional maps of the treatment area, allowing for millimeter-precise targeting that maximizes tumor coverage while sparing healthy tissues.

How Radiation Damages Cancer Cells

At its core, radiation therapy works by damaging the DNA inside cancer cells. When radiation strikes cellular DNA, it creates breaks in the genetic strands that the cell cannot repair. While both healthy and cancerous cells experience this damage, cancer cells typically have reduced ability to repair DNA compared to normal cells.

This damage occurs through two main mechanisms: direct and indirect action. In direct action, radiation directly strikes and breaks DNA molecules. In indirect action—which accounts for approximately two-thirds of damage—radiation interacts with water molecules in the cell, creating free radicals that then damage nearby DNA.

The cell cycle plays an important role in radiation sensitivity. Cells are most vulnerable to radiation damage during mitosis (cell division) and least sensitive during the late synthesis phase. Since cancer cells often divide more rapidly than normal cells, they're generally more susceptible to radiation damage—a biological advantage that radiation therapy exploits.

External vs. Internal Radiation Methods

External beam radiation therapy (EBRT) delivers radiation from a machine outside the body. During treatment, a linear accelerator generates and aims radiation at the tumor from various angles. Modern EBRT techniques include:

  • 3D-conformal radiation therapy (3D-CRT): Uses imaging to shape radiation beams to the tumor's contours
  • Intensity-modulated radiation therapy (IMRT): Adjusts radiation intensity during treatment for even more precise targeting
  • Image-guided radiation therapy (IGRT): Uses real-time imaging to account for tumor movement
  • Stereotactic body radiation therapy (SBRT): Delivers higher doses in fewer sessions with extreme precision
  • Proton therapy: Uses protons instead of photons, potentially reducing side effects

Internal radiation (brachytherapy) places radioactive sources directly in or near the tumor. This approach allows for delivering higher doses to the tumor while reducing exposure to surrounding tissues. Depending on the cancer type, radioactive seeds, wires, capsules, or liquid may be used, either temporarily or permanently implanted. Brachytherapy is commonly used for prostate, cervical, and certain breast cancers.

Radiation Therapy Planning and Delivery

Creating an effective radiation treatment plan requires extensive preparation. The process begins with a simulation appointment where the patient is positioned exactly as they will be during treatment. Custom immobilization devices may be created to help maintain precise positioning. Detailed imaging scans map the tumor and surrounding anatomy.

The radiation oncology team then develops a treatment plan using specialized computer software. This plan specifies the radiation type, dose, beam angles, and treatment schedule. Multiple factors influence these decisions:

  • Tumor location, size, and type
  • Proximity to sensitive organs
  • Patient's overall health and age
  • Previous treatments received
  • Treatment goals (curative or palliative)

Quality assurance checks verify the plan's accuracy before treatment begins. During treatment sessions, which typically last 15-30 minutes, most of the time is spent on proper positioning. The actual radiation delivery often takes only a few minutes. Throughout the treatment course, regular imaging ensures the radiation continues to target the correct area, with adjustments made as needed to account for tumor shrinkage or patient weight changes.

Combining Radiation With Other Treatments

Radiation therapy rarely stands alone in cancer treatment. Multimodal approaches combine radiation with surgery, chemotherapy, immunotherapy, or targeted therapies to attack cancer through different mechanisms simultaneously.

When used before surgery (neoadjuvant), radiation can shrink tumors, making them easier to remove. After surgery (adjuvant), radiation targets any remaining cancer cells. Combined with chemotherapy (chemoradiation), radiation can enhance treatment effectiveness through radiosensitization—where chemotherapy makes cancer cells more vulnerable to radiation damage.

Emerging research focuses on combining radiation with immunotherapy. Radiation can sometimes create an abscopal effect, where treating one tumor triggers immune responses against cancer cells throughout the body. This phenomenon may help radiation and immunotherapy work synergistically, potentially improving outcomes for patients with metastatic disease.

Timing these combined treatments requires careful coordination among specialists. Side effects must be managed carefully, as combinations can sometimes increase treatment toxicity. The radiation oncologist works within a multidisciplinary team to determine the optimal sequence and intensity of these complementary approaches.