4.5 Special Techniques: TBI, TSI, SGRT & Gated Radiotherapy

4.5 Special Techniques: TBI, TSI, SGRT & Gated Radiotherapy

Specialized radiation delivery techniques extend linear accelerator capabilities beyond standard focal tumor treatments to address unique oncologic indications. Techniques such as Total Body Irradiation (TBI) for bone marrow transplant conditioning, Total Skin Electron Irradiation (TSI/TSEI) for cutaneous T-cell lymphoma, Surface-Guided Radiation Therapy (SGRT) for real-time tracking, and respiratory-gated radiotherapy demand specialized equipment, modified treatment geometries, dedicated quality assurance, and high clinical vigilance by radiation therapists.

Total Body Irradiation (TBI) Physics and Clinical Execution

Total Body Irradiation is delivered as part of a conditioning regimen prior to allogeneic or autologous hematopoietic stem cell transplantation. The primary clinical objectives of TBI are twofold: eradicating malignant hematopoietic cells (leukemia, lymphoma, myeloma) throughout the body and suppressing the host immune system to prevent donor stem cell graft rejection.

Physical and Geometrical Considerations

• Extended Source-to-Surface Distance (SSD): Standard linac fields (40x40 cm² at 100 cm SSD) cannot encompass an adult patient. TBI is delivered at extended SSDs ranging from 200 cm to 400 cm, expanding the radiation field to 150–200 cm across.
• Low Dose Rate Delivery: TBI is delivered at ultra-low dose rates (0.05 to 0.15 Gy/min, or 5–15 cGy/min) to maximize repair of normal tissues, dramatically reducing the incidence of severe acute toxicities, particularly radiation pneumonitis.
• Beam Modifiers & Compensators: Acrylic dose-spoiling screens are placed near the patient to increase superficial skin dose by generating secondary electron contamination. Lead or tungsten tissue compensators correct for anatomical thickness variations (e.g., neck vs. chest vs. pelvis).
• Organ Shielding: Custom lead or cerrobend lung blocks are mounted on shadow trays to limit cumulative lung dose to below the threshold for interstitial pneumonitis (typically <8.0–10.0 Gy).

Total Skin Electron Irradiation (TSI) Stanford Technique Protocols

Total Skin Electron Irradiation (TSI or TSEI) treats diffuse superficial cutaneous malignancies, most commonly Mycosis Fungoides (Cutaneous T-Cell Lymphoma). The goal is delivering a uniform dose of low-energy electron radiation to the entire skin surface (dermis and epidermis down to 5–10 mm depth) while completely sparing underlying bone marrow, viscera, and deep structures.

The Modified Stanford Technique

• Beam Energy & Degradation: High-energy electron beams (typically 6 MeV) are passed through an acrylic degrader plate to lower surface energy to 3–4 MeV, restricting 80% dose depth to the top 4–6 mm of tissue.
• Dual Angled Beams: To deliver uniform dose over a standing patient, two electron beams are directed at gantry angles of approximately 108° and 72° (angled ±15° to 20° above and below the horizontal plane), creating a composite uniform vertical beam profile.
• Six Patient Orientations: The patient stands on a rotating platform facing six dual-beam treatment positions spaced at 60° increments (Anterior, Posterior, Right Anterior Oblique, Left Anterior Oblique, Right Posterior Oblique, Left Posterior Oblique).
• Shielding Requirements: Internal lead eye shields protect the lenses, while custom lead cutouts shield nails, toes, and soles of the feet as clinically indicated.

Surface-Guided Radiation Therapy (SGRT) Principles & Calibration

Surface-Guided Radiation Therapy (SGRT) utilizes 3D optical camera systems (e.g., Vision RT AlignRT, C-RAD Catalyst) mounted on the vault ceiling to project structured light patterns onto the patient's skin surface. The system tracks thousands of surface points in real time, comparing the live 3D surface mesh to the CT simulation baseline reference image.

Clinical Advantages of SGRT

1. Non-Ionizing & Continuous: Operates continuously without exposing the patient to additional ionizing radiation dose.
2. Frameless Setup & Tattoo-Free Treatments: Eliminates reliance on permanent skin tattoos, improving patient psychological well-being and setup accuracy.
3. Real-Time Intra-Fraction Tracking & Beam Hold: Monitors patient movement during beam-on. If patient motion exceeds defined thresholds (e.g., >1.5 mm translation or >1.0° rotation), SGRT triggers an automated interlock that pauses radiation delivery instantaneously.

Motion Management and Respiratory Gating

Thoracic and upper abdominal tumors (breast, lung, liver, pancreas) undergo significant movement due to respiration. Uncontrolled respiratory motion requires large PTV margins, increasing normal tissue exposure. Respiratory gating restricts radiation delivery to a specific window (gate) of the patient's breathing cycle.

Respiratory Gating Modalities

• Amplitude-Based Gating: Beam turns on only when the respiratory waveform enters a specified displacement window (e.g., at end-expiration).
• Phase-Based Gating: Beam turns on during a pre-set percentage window of the respiratory phase (e.g., 40% to 60% phase).
• Deep Inspiration Breath Hold (DIBH): Widely used for left-sided breast cancer. The patient inhales deeply, expanding lung volume. This physically shifts the heart posteriorly and inferiorly away from the anterior chest wall treatment fields, reducing mean heart dose by up to 50–70% and lowering long-term cardiotoxicity risks.

Dosimetric Verification and In-Vivo Measurements for Special Techniques

Because special techniques operate outside standard treatment geometries (extended SSDs, non-standard gantry angles, complex surface profiles), in-vivo dosimetry is clinically mandatory. Thermoluminescent Dosimeters (TLDs), Optically Stimulated Luminescent Dosimeters (OSLDs), or semiconductor diodes are placed directly on the patient's skin surface (e.g., head, neck, lung block shadow regions, ankles) during TBI and TSI fractions to measure delivered surface dose directly and verify uniformity within ±5% to ±10%.

Technical Specifications Summary Table

The following table summarizes key technical parameters for specialized radiation delivery techniques: