Total Body Irradiation

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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

Total Body Irradiation: Bilateral Fields Setup

  • 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.

TBI lung shielding and dose distribution

⭐ 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.

Practice Questions: Total Body Irradiation

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Precisely directed high dose radiation is used in which of the following therapies?

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Flashcards: Total Body Irradiation

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_____ are the most common form of radiation used in radiotherapy

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_____ are the most common form of radiation used in radiotherapy

X-rays

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