Total Body Irradiation
Total Body Irradiation (TBI) involves giving a high uniform dose to the whole body using photon fields and is the subject of AAPM Task Group 29. TBI is most commonly prescribed as preparation for hematopoietic cell transplant (HCT) commonly referred to ask “bone marrow” or “stem cell” transplant. TBI improves HCT by suppression of the immune system, thereby reducing the likelihood of Graft Versus Host Disease. TBI is also able to eradicate the malignant hematopoietic cells or those effected by genetic disorders.
AAPM TG-29: The Physical Aspects of Total and Half Body Irradiation (External link)
Prescriptions are commonly specified as a dose to a point (typically patient center at level of umbilicus).
- Dose limits to other locations specified explicitly.
- Specify dose rate, especially for single and hypofractionated schemes.
- Clinical dose uniformity of ±10% at mid-line is acceptable.
Low Dose TBI
Prescription: 2Gy in single fraction
- Eliminating malignant cells
- Eliminating cells with genetic disorders
- Fanconi’s Anemia
- Thalessemia Major
High Dose TBI
- 4-10Gy in single fraction
- 10-14Gy in 8 fractions
- Typically delivered BID (twice daily)
Killing lymphocytes and destroy bone marrow reduces graft rejection in bone marrow transplant.
Associated Diseases: Aplastic Anemia
- Anterior-Posterior treatments allow better uniformity and ease of lung sparing.
- May be treated in standing position with support or decubitus (lying on side).
- Lateral field treatments with patient lying down more comfortable and require less specialized immobilization.
- Compensators will typically be used for the head and ankles.
Potential Clinical Complications
- Lung toxicity is a major concern with high dose TBI. This is the reason for reduced dose rate and lung blocking.
- Nausea, vomiting, and diarrhea are expected.
- Interstitial pneumanitis is a potentially lethal side effect the risk of which may be greatly reduced by fractionation.
Key Point: TBI involves delivering a potentially fatal dose of radiation. Even low dose TBI of 2Gy whole body dose is sufficient to induce Hematopoietic Syndrome while high dose TBI is sufficient to induce Gastrointestinal Syndrome.
- Dose rates kept low (5-10cGy/minute at depth of maximum dose).
- This reduces normal tissue toxicity especially for the lungs.
- Low dose rate becomes less critical for highly fractionated regimens.
- Energy: 10-18MV
- Higher energy reduces Tissue Lateral Effect thereby improving uniformity.
- Higher energy also reduces skin dose which may be undesirable.
- An extended SSD is typically used
- This improves homogeneity by reducing impact of inverse square law on percent depth dose.
- Gantry is positioned at 90 degree to produce a horizontal field.
- Collimator rotation (typically 45 degrees) may be needed to accommodate patient geometry.
An acrylic scattering plate, sometimes referred to as a spoiler, may be placed in front of the patient to increase skin dose. This is especially valuable for treatments using very high photon energy.
Compensating filters, commonly referred to as compensators, are attenuating material places in the beam path to compensate for differences in patient thickness.
Compensators are distinguished from compensating bolus because they are not placed in direct contact with the patient. As a result, compensators allow greater skin sparing than compensating bolus. Additionally, compensating filters are typically used in photon treatments.
- Compensators are typically made of lead, copper, or acrylic.
- Compensators will harden photon beams and increase scatter dose.
- Compensators can have finer resolution than an MLC.
The thickness of compensator material required may be estimated as in the below equation.
- t is the thickness of the compensator material.
- μ is the attenuation coefficient of the compensator material.
- TPR is the tissue-phantom ratio at the point of interest.
Key Point: The attenuation coefficient of a compensator used in TBI’s large field geometry will be different than the attenuation coefficient taken in standard narrow field geometry. Therefore, attenuation coefficients should be measured in the TBI setup.
Monitor Unit Calculations
Either percent depth dose (PDD) or tissue maximum ratio (TMR) may be used to compute monitor units.
- MU is the number of monitor units delivered per beam for parallel opposed TBI treatments.
- The 2 in the denominator divides up the total MU between each beam.
- O is the output factor. Unit: cGy/MU
- Sc is the collimator scatter factor.
- Sc, should be based on the jaw opening.
- Sp is the phantom scatter factor.
- Sp, should be based on the phantom size which may be determined by measuring the dimensions of the patient’s thorax.
- SCD is the source-to-calibration distance (i.e. the distance from the source to the point where the output factor was measured).
- SMD is the source-to-midline distance (i.e. the distance from the source to the midline of the patient under TBI treatment).
Key Point: Output and scatter correction factors should ideally be measured using TBI geometry (i.e. at extended treatment distance) but may also be calculated from nominal beam data (i.e. at 100cm SCD). It has been shown that calculations based on nominal beam data vary by less than 1.5% from calculations based on beam data generated in TBI geometry. .
Dose monitoring is necessary to ensure proper prescriptive dosing. TLDs, OSLDs, or diode detectors are appropriate for treatment dose monitoring. Port films may also be used to assure alignment of compensators and blocks.
- Entrance and exit dosimeter pairs may be used to determine dose to mid-line
- May either take simple mean or, preferably, apply the known depth dose curve to determine mid-line dose.
- Placing dosimeters along mid-line is preferred when possible
- E.g. In mouth, between legs
Total Skin Electron Therapy
As presented in AAPM TG-30, the goal of total Total Skin Electron Therapy (TSET) is “to treat virtually the entire body surface to a limited depth and to a uniform dose using eletrons with a low X-ray background.” TSET is most commonly prescribed in the treatment of Mycosis Fungoides and is highly effective. Although the common treatment techniques were developed in the early 1950s, they see continued use today because of their safety and efficacy.
AAPM TG-25: Clinical Electron-Beam Dosimetry (External link)
Mycosis Fungoides (MF), sometimes called Sezary syndrome, is a low grade non-Hodgkin T-cell lymphoma of the skin. In advanced stages MF may involve the lymph nodes, blood, and visceral organs.
- Typical Age at diagnosis: 45-69 years.
- Incidence: Approximately 3 in 10.
- Disease progression:
- Begins as a scaly red rash in areas not exposed to the sun.
- Path Phase is marked by small areas of raised bumps and hardened lesions.
- In Tumor Phase MF presents itself as raised tumors on the skin.
Treatment options are dependent upon the extend of disease requiring either local control or treatment of extensive disease.
Local control options
- Topical Corticosteroids or Chemotherapy
- UV photon therapy
- Local Radiation: electron or orthovoltage X-ray
- Typical prescription: 20-30Gy in 1.2-2Gy/fraction
Extensive disease options
- Total skin electron therapy (TSET)
- Typical prescription: 30-36Gy surface dose over 6-10 weeks
- Highly effective treatment option with complete response rates of 95% for T1 to 75% for T4.
Total Skin Electron Therapy Treatments
Treatment Parameters Per Guidelines
- Standard course: Deliver at least 26Gy to a depth of 4mm over 6-10 weeks.2 This requires 31-36Gy surface dose.
- Shorter course: 12Gy in 3 weeks.
- Energy: 3-7MeV (at patient), 4-10MeV (at beam window)1
- SSD: 3-8 meters
- Field size at patient: 200cm by 80cm1
Field Inhomogeneity or Uniformity
- EORTC Consensus:
- <10% dose inhomogeneity to air at treatment distance2.
- AAPM TG-30:
- ± 8% vertically over central 160cm
- ± 4% horizontally over central 80cm
Penetration and dose fall off
- EORTC Consensus:
- 80% isodose line must be at least 4mm from skin surface
- 20% isodose line should be <20mm from skin surface
- AAPM TG-30:
- 50% isodose line should be 5-15mm
- 120%, may require shielding to achieve2
- EORTC Consensus
- Total dose to marrow from photons should be <0.7Gy
- AAPM TG-30
- <4% (1.6Gy)
1. AAPM Task Group 30. Total Skin Electron Therapy: Technique and Dosimetry. AAPM; 1987:58.
2. Total skin electron radiation in the management of mycosis fungoides: Consensus of the European Organization for Research and Treatment of Cancer (EORTC) Cutaneous Lymphoma Project Group. J Am Acad Dermatol. 2002 Sep;47(3):364-70.
Six Dual Field (Stanford) Technique
The stanford TSET technique uses 6 dual-fields. Each dual-field consists of one field angled up at about 20 degrees and another angled down at about 20 degrees such that the up and down fields match at the 50% isodose line at height of isocenter. A source-to-surface distance of 3-7 meters is usually sufficient to cover the patient with a uniform field size of 200cm by 80cm.
Important Field Points
- Angling the field’s central axis above the head and below the feet reduced Bremsstrahlung X-ray contamination.
- Bremsstrahlung X-rays are predominantly forward directed.
- Angles of approximately 20 degrees relative to horizontal.
- Because many electrons enter the body obliquely, electrons penetrate less deeply than the 2cm per MeV in water approximation.
- Field energy at accelerator must be 1-2MeV higher than at the patient surface because electrons loose approximately 1MeV of energy every 4 meters in air.
The Stanford technique utilizes 6 dual field patient positions which are delivered in sets of 3 fields on sequential days. Use of 6 positions improves uniformity over a 4 field technique and is less complex than an 8 field or continuous rotation technique.
Important Setup Points
- Patient is elevated off floor to reduce floor scatter dose.
- A scatterer is a Lucite (acrylic) panel about 1cm thick placed 20cm in front of the patient during treatment.
- The scatterer induces large angle scattering near the patient which improves dose uniformity but also reduces penetration and leads to less steep dose fall off.
Use of Boosts
- Boost are used to cover areas underdosed by the TSET delivery. Such areas include:
- Inframammary region
- Perianal region
- Inner buttocks and thighs
- Thick cutaneous tumors may also require a boost due to their depth.
- Boost may be delivered by electron or orthovoltage (~120kV) photon fields.
- Boost fields should assure a surface dose of at least 50% of prescription dose.
- Boosts improve progression-free survival by approximately 10-20%.
- Areas to be shielded
- Eyes must be shielded to <15% of prescription dose at surface.
- To prevent back scatter from increasing eyelid dose, eye shields may be coated with paraffin.
- Tops of feet
- Eyes must be shielded to <15% of prescription dose at surface.
- A common shield thickness of lead for 4 MeV electrons is 2mm.
- Eye shields are likely to be thinner as they are placed under the eye lid.
Potential Adverse Effects
|Adverse Effect||Incidence Rate|
|Complete temporary alopecia (hair loss)||100%1|
|Temporary Nail Statis||100%1|
|Edema of hands and feet||<50%1|
|Anhidrosis (reduced ability to perspire), Parotiditis, and gynecomastia (in males)||<3% each1|
|Chronic nail dystrophy, chronic xerosis, permanent alopecia||<1% each1|
|Mortality due to TSET||0%1|
|Immunosupression is common making infection an important cause of marbidity and mortality2||Common in advanced MF patients.2|
1. Total skin electron radiation in the management of mycosis fungoides: Consensus of the European Organization for Research and Treatment of Cancer (EORTC) Cutaneous Lymphoma Project Group. J Am Acad Dermatol. 2002 Sep;47(3):364-70.
2. L. Gunderson L, E. Tepper J. Clinical Radiation Oncology. Elservier Health Sciences; 2007.
Alternatives to the Stanford Technique
Several alternative treatment methods exist including the 4 dual-field and 8 dual-field techniques. Continuous rotation, in which the patient is placed on a rotating platform, is also sometimes used. Pendulum Arc, in which the treatment head is continually rotated in a 50 degree arc, is sometimes used to avoid the match line inherent in all dual-field techniques.
Calibration point is positioned along the center of the treatment plane at the height of linac isocenter (sees figure above) at depth of maximum dose.
Calibration point dose is the dose measured at the calibration point for a single dual-field (i.e. two fields angled at approximately +20° and -20°).
Calibration point dose output factor (not defined in TG-30) is the calibration point dose per MU of each treatment beam in the dual field. Unit: cGy/MU. (E.g. If the superior and inferior field each deliver 100MU and the calibration point dose is measured as 4cGy, the calibration point dose output factor is 4cGy/100MU = 0.04cGy/MU.)
Treatment skin dose is defined as mean dose along a circle near the surface of a cylindrical polystyrene phantom 30cm in diameter and 30cm high which has been irradiated by all six dual fields.
B factor is a factor that relates the calibration point dose to the treatment skin dose. B factors are typically between 2.5 and 3.1 because the calibration point receives dose contribution from approximately 3 fields.
Monitor Unit Calculation
Monitor units for a TSET deliver may be calculated as in the below equation. The total MU delivered for all fields will be 6 times the MUsingle field.
Common TSET Dosimeters
- Parallel-Plate Ion Chambers
- Required by TG-30 and TG-51 for calibration of absorbed dose.
- Measurement of depth dose.
- Measures ionization which must be converted to dose.
- Diode detectors
- Provide good spatial resolution and do not require significant correction between ionization measurement and absorbed dose.
- Useful for measuring treatment skin dose across entire plane
- May be useful in depth dose
- Thermoluminescent dosimeters
- Useful for patient dose monitoring
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