10.2 Radiation Therapy and Radiobiology

Radiation therapy is a powerful tool in cancer treatment, using high-energy radiation to kill or damage cancer cells. This section explores various techniques, from external beam methods to internal radiation delivery, highlighting the precision and effectiveness of modern approaches.

Treatment planning and radiobiology are crucial aspects of radiation therapy. We'll examine how dosimetry, fractionation, and understanding cellular responses to radiation help maximize treatment effectiveness while minimizing damage to healthy tissues.

Radiation Therapy Techniques

Advanced External Beam Radiation Methods

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• Linear accelerators generate high-energy X-rays or electrons to target tumors precisely
• Proton therapy utilizes positively charged particles to deliver radiation with minimal damage to surrounding tissues
• Image-guided radiation therapy (IGRT) incorporates real-time imaging to improve accuracy of radiation delivery
• Intensity-modulated radiation therapy (IMRT) uses computer-controlled linear accelerators to deliver precise radiation doses to malignant tumors

Internal Radiation Delivery

• Brachytherapy involves placing radioactive sources directly inside or near the tumor
  • High-dose rate (HDR) brachytherapy delivers radiation in short, intense bursts
  • Low-dose rate (LDR) brachytherapy provides continuous low-dose radiation over a longer period
• Brachytherapy can be temporary or permanent depending on the type and stage of cancer

Treatment Planning and Dosimetry

Radiation Measurement and Planning

• Radiation dosimetry measures and calculates the absorbed dose in tissues exposed to radiation
  • Uses various units (Gray, Sievert) to quantify radiation exposure and effects
• Treatment planning involves creating a detailed strategy for delivering radiation therapy
  • Utilizes 3D imaging techniques (CT, MRI) to map the tumor and surrounding tissues
  • Determines optimal radiation beam angles, intensities, and delivery schedules

Dose Distribution and Timing

• Fractionation divides the total radiation dose into smaller doses given over a period of time
  • Standard fractionation typically involves daily treatments over several weeks
  • Hypofractionation uses larger doses over a shorter period
• Dose distribution aims to maximize tumor coverage while minimizing damage to healthy tissues
  • Utilizes isodose curves to visualize radiation distribution within the body

Radiobiology

Cellular Response to Radiation

• Radiosensitivity refers to the relative susceptibility of cells, tissues, or organs to the harmful effects of ionizing radiation
  • Varies among different cell types and tumor types
• DNA damage and repair mechanisms play a crucial role in radiation therapy effectiveness
  • Ionizing radiation causes single-strand breaks, double-strand breaks, and base modifications in DNA
  • Cells attempt to repair radiation-induced damage through various pathways (base excision repair, homologous recombination)

Tumor Microenvironment and Treatment Effects

• Tumor hypoxia occurs when tumor cells are deprived of oxygen
  • Reduces the effectiveness of radiation therapy as oxygen is required for the formation of DNA-damaging free radicals
  • Hypoxic cells can be up to three times more resistant to radiation than well-oxygenated cells
• Radiation-induced side effects can occur in both short-term and long-term timeframes
  • Acute effects (skin irritation, fatigue) typically occur during or shortly after treatment
  • Late effects (fibrosis, secondary cancers) may develop months or years after treatment completion