What is a linear accelerator (LINAC) and how is it used in external‑beam radiation therapy?

Linear Accelerator (LINAC) in External Beam Radiation Therapy

Definition and Core Technology

A linear accelerator (LINAC) is a medical device that uses microwave technology to accelerate electrons to high energies, which are then directed at a metal target to produce high-energy X-rays (photons) or used directly as electron beams for cancer treatment. 1 The device delivers multiple intersecting beams of ionizing radiation designed to deposit high doses to the tumor while achieving rapid dose fall-off to surrounding normal tissues. 1

Technical Components and Function

The LINAC operates through several key mechanisms:

• Electron acceleration: Microwave-powered systems accelerate electrons through traveling wave accelerating structures, achieving energies typically ranging from 4-25 MeV. 2
• Radiation production: Accelerated electrons strike a high-atomic-number target (typically tungsten) to generate therapeutic X-rays through bremsstrahlung conversion, though the electron-to-photon conversion efficiency is only a few percent. 3
• Beam delivery: The treatment head contains beam-shaping devices including collimators and multi-leaf collimators that shape the radiation beam to conform to the tumor target. 1
• Gantry rotation: The LINAC gantry rotates around the patient, allowing beam delivery from multiple angles with submillimeter accuracy. 4

Clinical Applications in External Beam Radiotherapy

LINACs serve as the standard platform for delivering external beam radiation therapy across multiple cancer types:

• Hepatocellular carcinoma: LINAC-based photon therapy delivers stereotactic body radiation therapy (SBRT) in five or fewer highly focused treatments, with accurate pre-treatment planning of radiation distribution in tumor and non-tumor tissues. 1
• Non-small cell lung cancer: High-energy linear photon accelerators are the standard for thoracic radiotherapy, delivering minimum doses of 60 Gy with classical fractionation for locally advanced disease. 1
• Brain metastases: LINAC-based stereotactic radiosurgery (SRS) achieves local control rates of 75-95% for brain metastases, with the gantry moving in space to change delivery angles for precise targeting. 1, 4
• Prostate cancer: Modern LINACs with intensity-modulated radiation therapy (IMRT) and image guidance enable safe dose escalation to 75-79 Gy, improving local tumor elimination while reducing late complications. 1

Advanced LINAC-Based Techniques

Contemporary LINAC technology incorporates several sophisticated delivery methods:

• Stereotactic body radiation therapy (SBRT): Delivers ablative doses in 1-5 fractions using high-precision stereotactic targeting with accuracy within 1 mm. 1
• Intensity-modulated radiation therapy (IMRT): Modulates beam intensity across the treatment field to create highly conformal dose distributions that spare organs at risk. 1
• Image-guided radiation therapy (IGRT): Integrates real-time imaging (CT, MRI, or fiducial markers) to verify target position immediately before and during treatment. 1
• Hypofractionated stereotactic radiotherapy: Uses 3-5 fractions for larger tumors (>3 cm diameter) where single-fraction treatment would be unsafe. 4

Comparison with Alternative Technologies

No radiation delivery platform has demonstrated superiority over others; the primary factor in successful treatment is correct application by treating physicians. 4

Key distinctions between technologies:

• LINAC vs. Cobalt-60: LINACs offer higher beam energy, modulated dose rate, and smaller focal spot size, making it easier to create optimized conformal treatments. However, LINACs require more complex infrastructure, higher maintenance, more extensive staff training, and have higher life cycle costs. 5
• LINAC vs. Gamma Knife: Both achieve similar high rates of tumor control (95%) and clinical control (97%) for intracranial lesions. Gamma Knife excels for small-to-medium tumors (<3 cm) requiring single-fraction treatment, while LINAC-based systems are preferred for larger tumors requiring hypofractionation. 4
• LINAC vs. Proton therapy: Proton therapy provides superior dose sparing to non-tumor tissues due to the Bragg peak phenomenon, particularly valuable in hepatocellular carcinoma patients with limited hepatic reserve. 6 However, photon-based LINAC therapy remains more widely accessible and cost-effective. 1

Quality Standards and Requirements

Radiation oncology guidelines establish specific technical standards for LINAC-based therapy:

• Equipment specifications: Thoracic radiotherapy must use high-energy linear photon accelerators as the standard delivery platform. 1
• Dose delivery: Weekly doses with classical fractionation should not exceed 10 Gy to minimize toxicity. 1
• Treatment planning: Must conform to International Commission on Radiation Units (ICRU) reports 29,50, and 62 for dose specification and reporting. 1
• Safety margins: Treatment volumes should include all macroscopic tumor plus a safety margin of 1.5-2 cm. 1

Emerging LINAC Technologies

Recent innovations are expanding LINAC capabilities:

• MRI-LINAC integration: Combines on-table MRI imaging with radiation delivery, enabling real-time tumor visualization, organ motion management, and same-day adaptive replanning while the patient remains on the treatment table. 7
• FLASH radiation therapy: Experimental ultra-high dose rate delivery (≥40 Gy/s) using modified LINACs may reduce normal tissue toxicity while maintaining tumor control, though this remains investigational. 8, 3

Critical Implementation Considerations

Treatment with LINAC-based radiation therapy requires multidisciplinary team coordination and should be delivered at centers with appropriate expertise and quality assurance programs. 1, 6

Common contraindications and precautions:

• Inflammatory bowel disease: History of Crohn's disease or ulcerative colitis represents a contraindication to pelvic LINAC-based radiotherapy due to increased toxicity risk. 1
• Prior pelvic radiation: Previous pelvic radiotherapy precludes additional LINAC treatment to the same region. 1
• Respiratory function: For lung cancer, treatment planning must consider respiratory function to avoid severe radiation-induced complications when delivering doses >60 Gy. 1