Total Body Irradiation

Total Body Irradiation - Body Under Siege

• Definition: Radiation therapy delivered uniformly to the entire body.
• Primary Goals:
  • Eradicate malignant cells (leukemia, lymphoma).
  • Immunosuppression for HSCT (prevent graft rejection).
  • Bone marrow ablation.
• Key Indications:
  • HSCT conditioning (most common).
  • Palliation (widespread low-grade lymphoma, rare).
• Dose: Typically 10-15 Gy fractionated (e.g., 1.5-2 Gy/fraction). Single dose ~7.5 Gy (higher toxicity).
  • Lung shielding vital; mean lung dose often <8-10 Gy.

⭐ TBI is a cornerstone of myeloablative conditioning for allogeneic HSCT, especially in acute leukemias.

Total Body Irradiation - Zap Strategy

• Goal: Eradicate malignant cells (leukemia, lymphoma), immunosuppression for BMT/SCT.
• Techniques:
  • AP/PA (Anterior-Posterior/Posterior-Anterior): Patient standing or lying, large SSD.
  • Bilateral: Two opposing large fields, patient lying on side.
  • Rotational or translational methods for dose uniformity.
• Beam: High energy photons (e.g., 6-10 MV LINACs) to ensure penetration.
• Dose Rate:
  • Low Dose Rate (LDR): 5-15 cGy/min (classic).
  • High Dose Rate (HDR): >15-20 cGy/min (more common now with fractionation).
• Fractionation:
  • Single dose: e.g., 10 Gy (historically, higher toxicity).
  • Fractionated: e.g., 12-15 Gy in 6-8 fractions over 3-4 days (improves tolerance, reduces late effects).
• Organ Shielding: Lungs (critical), kidneys, lenses (sometimes).

  ⭐ Lung dose is a major limiting factor; typically kept < 8-10 Gy with shielding to reduce risk of pneumonitis.

• Immobilization: Essential for reproducibility.
• In vivo dosimetry: TLDs or diodes on skin to verify dose delivery. 📌 Total Body Irradiation = Treat Big Intervals (fractionation).

Total Body Irradiation - Target Lock

• Aim: Uniform dose (typically ±5% to ±10%) across entire body for marrow ablation/immunosuppression pre-HSCT.
• Techniques: AP/PA or bilateral fields, extended SSD; compensators/bolus for homogeneity.
• Dose: 10-14 Gy total; single or fractionated (e.g., 1.5-2 Gy/fx, BID).
• Lung Shielding: Critical to prevent pneumonitis.
  • Custom blocks (Cerrobend) or MLCs.
  • Mean Lung Dose (MLD) target: < 8-10 Gy.
• Other OARs: Kidneys (limit < 12-14 Gy), Lens (limit < 5 Gy if possible).
• Verification: In vivo dosimetry (TLDs/diodes) at key anatomical points.

⭐ Lungs are the primary dose-limiting organs in TBI; mean lung dose is crucial, often kept < 8 Gy to minimize radiation pneumonitis risk.

Total Body Irradiation - Radiation's Toll

• Acute Toxicities (Days to Weeks):
  • Gastrointestinal Syndrome: Nausea, vomiting, diarrhea, mucositis, parotitis.
  • Hematopoietic Syndrome: Myelosuppression leading to pancytopenia; nadir at ~2-3 weeks. Risk of infection, bleeding.
  • Pulmonary: Acute radiation pneumonitis (RP); typically 1-3 months post-TBI. Can be dose-limiting.
  • Skin: Erythema, desquamation.
  • Constitutional: Fever, fatigue.
• Late Toxicities (Months to Years):
  • Ocular: Cataracts (most common non-fatal late effect, dose-dependent).
  • Endocrine: Hypothyroidism, hypogonadism (infertility).
  • Pulmonary: Chronic fibrosis, restrictive disease.
  • Renal: Radiation nephropathy (proteinuria, hypertension, renal failure).
  • Skeletal: Growth retardation (children), osteonecrosis.
  • Carcinogenesis: Increased risk of secondary malignancies (e.g., AML, MDS, solid tumors).
  • Hepatic: Veno-occlusive disease (VOD), especially with prior chemotherapy.
  • Neurologic: Cognitive dysfunction (rare).

⭐ Interstitial pneumonitis is a major dose-limiting acute toxicity, often occurring 1-3 months post-TBI, and can be fatal if severe.

High‑Yield Points - ⚡ Biggest Takeaways

• TBI is crucial for pre-HSCT conditioning, mainly in leukemias and lymphomas.
• Aims: Malignant cell kill, immunosuppression for graft acceptance, and marrow space creation.
• Dose: 10-15 Gy in fractions; low dose rate (e.g., 5-15 cGy/min) is vital to reduce toxicity.
• Interstitial pneumonitis is the primary dose-limiting toxicity; lung shielding is often employed.
• Other major risks: Veno-occlusive disease (VOD), cataracts, endocrine dysfunction, and secondary malignancies.