Unit
- Definition and purpose: Radiation therapy, also called radiotherapy, uses ionizing radiation to control or kill malignant cells. It is a localized treatment that targets tumors while sparing healthy tissues through precise delivery techniques. It can be used as a primary treatment, adjuvant therapy, or palliative care.
- Mechanism of action: Radiation damages the DNA of cancer cells, either directly by ionizing DNA molecules or indirectly through free radicals produced by ionization of water. Cancer cells are more vulnerable due to their rapid division and impaired repair mechanisms.
- Therapeutic ratio: The goal is to maximize tumor control probability while minimizing normal tissue complication probability. Treatment planning optimizes this ratio by adjusting dose, fractionation, and delivery technique.
- Clinical indications: Nearly 50% of cancer patients receive radiation therapy during their treatment. Common indications include breast, prostate, lung, head and neck, and brain cancers, as well as palliative treatment for bone metastases.
- Integration with other modalities: Radiation is often combined with surgery, chemotherapy, immunotherapy, or targeted therapy. Sequencing and timing are critical; for example, neoadjuvant chemoradiation can shrink tumors before surgery.
- Treatment workflow: The process includes consultation, simulation, treatment planning, quality assurance, daily treatments (fractions), and follow-up. Each step involves a multidisciplinary team.
- Fractionation schedules: Standard fractionation delivers 1.8-2.0 Gy per day over several weeks. Hypofractionation uses higher doses per fraction over fewer sessions, while hyperfractionation uses lower doses twice daily. Each schedule balances efficacy and toxicity.
- Types of radiation: Photons (X-rays, gamma rays) are most common; electrons treat superficial lesions; protons and heavy ions offer depth-dose advantages. The choice depends on tumor depth, location, and surrounding organs.
- Patient positioning and immobilization: Accurate and reproducible patient setup is essential for precise radiation delivery. Custom immobilization devices such as thermoplastic masks, vacuum bags, and bite blocks help minimize movement. Daily imaging, like cone-beam CT or portal imaging, verifies alignment before each treatment fraction.
- Image guided radiation therapy (IGRT): IGRT integrates imaging systems directly into treatment machines to track tumor and organ motion. Techniques include orthogonal X-rays, cone-beam CT, and fluoroscopy. Real-time adjustments account for respiratory motion, bladder filling, or tumor shrinkage, reducing margins and sparing healthy tissue.
- Stereotactic techniques: Stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) deliver high doses in few fractions with extreme precision, often using multiple beams or arcs. SRS treats intracranial lesions; SBRT treats extracranial tumors like early stage lung cancer or spinal metastases. These techniques require rigorous quality assurance and patient immobilization.
- Brachytherapy: Brachytherapy involves placing radioactive sources directly into or near the tumor. It delivers a high dose to the target while minimizing exposure to surrounding organs. Common applications include cervical, prostate, and breast cancer. Implants can be temporary (high dose rate) or permanent (low dose rate seeds).
Regulatory Note
Regulation: In the United States, radiation therapy facilities must comply with Nuclear Regulatory Commission (NRC) regulations for the use of radioactive materials and with state regulations for linear accelerators. These regulations cover licensing, safety procedures, personnel qualifications, and quality management programs.
10 CFR Part 35 (Medical Use of Byproduct Material) and state radiation control laws.
Quick Revision
- Radiation therapy uses ionizing radiation to damage DNA of cancer cells, exploiting their limited repair capacity.
- The therapeutic ratio balances tumor control against normal tissue toxicity through dose and fractionation optimization.
- Radiation is indicated for many cancers and can be integrated with surgery and systemic therapies.
- NRC and state regulations govern the safe use of radiation in medical settings.