Special techniques

Challenging clinical setups

Step-by-step guidance and illustrative overviews for complex treatment setups, including why these techniques are used and how to simulate and deliver them.

Special technique · Transplant conditioning

Total Body Irradiation (TBI)

Delivers a uniform whole-body dose to ablate malignant bone marrow and provide immunosuppression before stem cell or bone marrow transplant.

Indications: AML, ALL (high-risk), CML, non-Hodgkin lymphoma, Hodgkin lymphoma, myelodysplastic syndrome.

Why therapists perform this treatment
  • Eradicates residual malignant cells in sanctuary sites (CNS, testes) that chemotherapy cannot reliably reach.
  • Provides profound immunosuppression to prevent graft rejection of the transplanted marrow.
  • Creates marrow space for engraftment of donor stem cells.
  • Typically combined with chemotherapy conditioning (cyclophosphamide, etoposide, or fludarabine).
  • Myeloablative regimens commonly prescribe 12 Gy in 6 twice-daily fractions; non-myeloablative uses lower single doses.
  1. 1

    Confirm prescription

    Verify diagnosis, transplant protocol, total dose, fractionation, dose rate (typically ≤10 cGy/min), and organ-at-risk limits (lungs, lenses, kidneys).

  2. 2

    Choose positioning

    Adults usually stand or sit at extended SSD in a TBI jig; pediatric or frail patients lie supine. Arms crossed over the chest help shield the lungs.

  3. 3

    Measure patient thickness

    Record AP and lateral thickness at head, chest, abdomen, pelvis, and legs to design tissue compensators for dose homogeneity (±10%).

  4. 4

    Define dose point

    Prescribe to midplane at the umbilicus; calculate monitor units at the extended treatment distance (commonly 4–6 m).

  5. 5

    Design lung shielding

    Create custom lung blocks to cap total lung dose below ~10–12 Gy and reduce the risk of interstitial pneumonitis.

  6. 6

    Reproducible setup

    Use a fixed jig with handles, head holder, and indexed supports; document SSD, patient height, and field size for daily reproducibility.

  7. 7

    Mark and image

    Place setup marks on stable landmarks; acquire simulation CT or 2D setup films at treatment distance to verify beam geometry and compensator fit.

  8. 8

    Calculate and verify

    Compute MUs accounting for extended-SSD output factors, inverse square, and compensator transmission; perform phantom verification before treatment.

  1. 1

    Pre-treatment check

    Independently verify prescription, plan, MUs, compensators, lung blocks, and dose rate before the first fraction.

  2. 2

    Daily setup

    Position patient in the jig, attach compensators, and place lung blocks; confirm alignment with lasers and setup marks.

  3. 3

    Beam-on

    Deliver AP/PA or bilateral large fields at the prescribed low dose rate to the entire body.

  4. 4

    In-vivo dosimetry

    Place diodes, OSL, or TLDs at multiple body sites (head, chest, abdomen, pelvis, legs) to confirm delivered dose and adjust compensators if out of tolerance.

  5. 5

    Fractionation

    Deliver twice-daily fractions separated by at least 6 hours over 2–4 days; keep dose rate low throughout.

  6. 6

    Organ shielding

    Add lens shielding for cataract risk and testicular/ovarian shielding when clinically appropriate.

  7. 7

    Supportive care

    Monitor for nausea, fever, and hypotension; administer antiemetics and hydration as needed.

  8. 8

    Document

    Record delivered dose, in-vivo readings, vital signs, and tolerance for each fraction.

Clinical key points
  • Critical organs at risk: lungs, lenses, gonads, kidneys, and liver.
  • Dose rate ≤10 cGy/min is essential to reduce pneumonitis and other toxicities.
  • Compensators and thickness-based dose correction keep whole-body dose within ±10%.
  • Acute effects: nausea, vomiting, fever, mucositis, and transient erythema.
  • Long-term risks: cataracts, infertility, growth delay in children, and secondary malignancy.