What does TBI stand for?
Total Body Irradiation
What does TSEI stand for?
Total Skin Electron Irradiation
When is TBI carried out?
Prior to a bone marrow transplant (mainly leukemia patients)
What is the purpose of TBI for bone marrow transplants?
Aim to give uniform dose to the whole body to suppress bone marrow before transplantation
TBI contributes to the eradication of the patient’s leukemic bone marrow, particularly from sanctuary sites (which chemo may miss) and also acts to cause severe immunosuppression to reduce risk the of host rejection of the transplanted stem cells from the donor marrow.
What energy photons are used for TBI? Why?
15 MV Highest available beam energy Higher dose to tissue within bone Lower dose to lung Greater homogeneity in bilateral irradiation
What is special about the procedure for TBI?
- Patient not on normal treatment couch.
- At extended distance so that a single field encompasses the whole patient
- Patient treated with opposed lateral fields to obtain lateral uniformity. Achieved by turning TBI couch through 180 degrees.
- Bolus used to achieve uniform thickness and therefore uniform dose
- Head/Neck compensator used to compensate for head thickness without using bolus round head
- Lung compensator used to account for lower density of lung compared to soft tissue
- Perspex side panel to increase superficial dose
Why are TLDs used for TBI?
MOSFETs have limited life
Diodes exhibit angular dependence
What appointments are required for a patient undergoing TBI?
CT scan for geometry of lung to determine compensator dimensions
Test dose to verify bolusing and compensation to ensure uniform dose
TBI treatment
What is the purpose of the compensator in TBI treatments?
To account for lack of tissue (such as lungs or where head is narrower than rest of body):
- Brass head compensators ranging in thickness from 1mm to 15mm are used to compensate for the thinner separation of the patient’s head/neck w.r.t. the widest part of the body. Head/Neck compensator is shaped so that the shadow fits around the shoulders
- Brass lung Compensators are used to compensate for the air in the lungs. The lung compensator is made individually for each patient using measurements taken from the CT scan.
What is the lung compensator in TBI chosen dependent on?
- Lung length
- Lung thickness
- Patient width (lateral separation)
All found from CT scan
What is the purpose of the test dose in TBI?
TLDs are used to check for uniformity and the correct dose. They are placed in 8 positions on either side of the body.
What is the purpose of the taking patient separations in TBI?
- Monitor Unit settings
- bolussing (to acheive lateral uniformity)
- head compensator thickness
What is used to keep the bolus firm?
A chiller cabinet.
Warmth makes the bolus soft.
What are the principles of TLD operation?
Electron trapped by impurities
Heating releases electrons – up to valence, then drop to vacancy, end up with glow curve.
Electrons out prop to dose.
What final checks are made prior to irradiation in TBI?
Callipers are used to check for uniform thickness for entire length of body.
Light field used to check patient is in correct position (collimator at 45 to ensure maximum dimensions laterally). Then compensators are taped onto the head using light field for guidance (shadows).
What positions can be used for TBI treatments?
Laying, standing, sitting, suspended (with jacket).
What are the commissioning steps for TBI?
- Uniformity (Ionisation chamber at various points within a water equivalent phantom on TBI couch):
- Sup-Inf (foot rest filled with tissue equivalent material to provide full scatter to ensure no underdosing of feet)
- Laterally (varies with patient width)
- Midline dose vs. Separation
- Midline/Surface Ratios
- Compensation:
- Head (Compensator thickness adjusted until Dhead = Dmidline)
- Lungs (Vary comp & separation until doses are equal.)
- Verification on Rando Phantom (5% variation limit)
How are TLDs calibrated for TBI treatment?
During TBI test dose a patient will receive approx 8cGy midline.
Obtain a TLD calibration curve use small doses. But as dose/MU is not linear or reproducible at low MU, the dose the TLD receives should be measured. TLD irradiated to same dose as ion chamber
TLD next to ion chamber. At depth of uniform beam – with big enough FS. Give similar dose as patient.
After measuring patient dose, correct the doses by the daily output factor: (standard output) / (output measured)
Describe the TBI technique.
- CT scan to establish lung thickness and length
- Make lung compensator
- TBI test:
- Bolus to shoulders (or hips if wider). If inadequate bolus ensure < 4cm difference between bolussed separations
- Lung and head compensators
- TLD measurements
- TLD calibration
- Adjustment of bolussing and/or compensators if required.
- Prescription
- MU determination
- Treatment
- Varients on patient position and shielding are used.
What is the purpose of TSEI?
To treat the cutaneous group of lymphomas which primarily involve the skin and affect other sites secondarily.
It is not a cure, but abates the disease for years. It is used as it is the only option.
(CUTANEOUS T-CELL LYMPHOMA (Mycosis Fungoides and Sézary Syndrome))
How many positions are used for the “Stanford Technique” (TSEI)?
6 (12 beams as split sup/inf)
Describe the TSEI technique.
- 6 field Stanford technique
- 2 Gantry angles, 6 positions,12 beams
- Schedule is as recommended by the UK Cutaneous Lymphoma Group :
- 30Gy in 20 fractions over 5 weeks, with 4 fractions/week of 1.5GY
- 6 patient positions per appointment.
- 12 fields
- Treat MTTF
How does the PDD for TSEI differ from regular electron PDDs?
Shallower gradient in fall off region. Peaks almost immediately. Appears as essentially a wobbly straight line plus xray tail.
What are the commissioning steps for TSEI?
- Lateral field uniformity
- Sup-Inf uniformity at various gantry angles (measured with farmer & buildup)
- Uniformity normalised to dose at centre (bottom measurements may overrespond due to scatter from floor)
- Rando phantom surface and midline doses per 1000MU x all fields.
- Rando PDDs at various sections with film
- TLD in centre of Rando to assess mid-body X-ray dose.
- Film in Rando mid-section:
- 10% uniformity
- 0.5% max midline dose
- Approx 85% at 4mm
- Standard output and energy at time of commissioning
