Chapter 18: Total Body Irradiation

TBI with megavoltage photon beams is most commonly used as part of the conditioning regimen for bone marrow transplantation. The role of TBI is to destroy the recipient's bone marrow and tumor cells, and to immunosuppress the patient sufficiently to avoid rejection of the donor bone marrow transplant.

The choice of a particular technique depends on the available equipment, photon beam energy, maximum possible field size, treatment distance, dose rate, patient dimensions, an the need to selectively shield certain body structures.

The anteroposterior (AP) / posteroanterior (PA) technique generally provides a better dose uniformity along the longitudinal body axis but the patient positioning, other than standing upright, may pose problems.

The bilateral TBI (treating from left and right) can be more comfortable to the patient if seated or lying down supine on a TBI couch, but presents greater variation in body thickness along the path of the beam. Compensators are required to achieve dose uniformity along the body axis to within plus or minus 10%, although extremitites and some noncritical structures may exceed this specification.

The choice of photon beam energy is dictated by 

• 1.) Patient thickness.
• 2.) Specification of dose homogeneity. 
• 3.) The patient diameter along the path of the beam also affects dose uniformity, depending on beam energy.

The term tissue lateral effect has been used to describe the situation in which lower energy or a thicker patient treated with parallel opposed beams can give rise to an excessively higher dose to the subcutaneous tissues compared with the midpoint dose.

If the maximum thickness of the patient parallel to the beam central axis is less than 35 cm and the source to surface dose (SSD) is at least 300 cm, a 6MV beam can be used for parallel opposed TBI fields without increasing the peripheral dose to greater than 110% of the midline dose.

For patients of thickness greater than 35 cm, energies higher than 6MV should be used to minimize the tissue lateral effect.

Megavoltage range. As the photon beam energy decreases, the point of maximum dose gradually moves toward the surface or skin.

• 1.) Energy
• 2.) Field Size.
• 3.) SSD.
• 4.) Beam angle relative to the surface.

Most TBI protocols do not require skin sparing. Instead, a bolus or a beam spoiler is specified to bring the surface dose to at least 90% of the prescribed TBI dose.

Patient support and positioning devices are designed to implement a given treatment technique. 

Important criteria include...

• 1.) Patient comfort.
• 2.) Patient stability.
• 3.) Reproducibility of setup and treatment geometry that allows accurate calculation and delivery of dose in accordance with the TBI protocol.

The source to body axis distance is measured by a sagittal laser light installed in the ceiling to mark the TBI distance.

To achieve dose uniformity within approximately plus or minus 10% along the sagittal axis of the body, compensators are designed for head and neck, lungs (if needed), and legs.

The reference thickness for compensation is the lateral diameter of the body at the level of the umbilicus (not including the arms), assuming that the protocol specifies dose prescription to be at the midpoint at the level of the umbilicus.

The principle of the technique is that the standing TBI allows shielding of certain critical organs from photons and boosting of superficial tissues in the shadow of the blocks with electrons. For example, dose to the lungs can be reduced using lung blocks of about one half-value thickness and the chest wall under the blocks can be boosted with electrons of appropriate energy.

The AP/PA technique can also be adopted for treating small children in the reclining position. The patient is treated in the supine and posterior positions while lying down on a low-height couch, with the couch top only a few inches off the floor. The shielding blocks are placed on top of an acrylic box tray at a short distance from the patient's surface. The tray, which is about 1 cm thick, also acts as a beam spoiler to build up the skin dose to at least 90% of the prescription dose when treated with parallel opposed TBI fields.

A direct output calibration of the machine for TBI may be performed by measuring dose per monitor unit using 0.6-cm-cubed Farmer-type ionization chamber placed in a water phantom of dimensions approximately 40 x 40 x 40 cm-cubed. The position of the chamber is fixed at the TBI distance (source to body axis distance). The collimator is opened to its maximum size and the chamber depth is varied by moving the chamber and the phantom while keeping the source to chamber distance constant (equal to TBI distance).

It is assumed in this case that the patient is dosimetrically equivalent to the phantom, which is not a bad approximation considering the fact that the tissue maximal ratios (TMRs) for large fields (e.g., > 30 x 30 cm) are not very sensitive to field dimensions.

The equivalent field at the point of calculation means that it is dosimetrically equivalent to the patient in terms of scatter.

Although patient dimensions vary, scatter factors are not too sensitive to field size variation for large fields. Therefore, it is reasonable to use a fixed equivalent field size for TBI. A 40 x 40 cm field for large patients and a 30 x 30 cm field for pediatric patients seem to be reasonable approximations (within approximately plus or minus 2% of dose accuracy.

Most TBI protocols require dose homogeneity along the body axis to be within plus or minus 10%. This requirement cannot be met without the use of compensators.

General principles of compensator design have been discussed in Chapter 12. Compensator design for TBI is complicated because of...

• 1.) large variation in body thickness
• 2.) lack of complete body immobilization
• 3.) internal tissue heterogeneities. 

Considering only the lung inhomogeneity and change in body thickness, compensators can be designed to deliver the dose within acceptable uniformity.

The thickness of compensator require along a ray line depends on...

• 1.) tissue deficit compared to the reference depth at the prescription point
• 2.) material of the compensator (e.g., its density)
• 3.) distance of the compensator from the point of dose compensation
• 4.) depth of the point of dose compensation
• 5.) field size
• 6.) beam energy

Because the compensator is designed to be dosimetrically equivalent to a bolus (of thickness equall to the tissue deficit) but placed at a distance from the skin surface, the bolus-equivalent thickness of the compensator is reduce to compensate for reduction in scatter reaching the point of dose compensation.

The required thickness of a tissue-equivalent compensator that gives the same dose at the point of interest as would a bolus of thickness equal to the tissue deficit is called the thickness ratio (tau).

0.70. The overall dosimetric accuracy of a compensator is approximately plus or minus 5% considering all variables.

Depending on the tissue deficits encountered in a particular TBI technique, compensator material should be selected so that the compensator is not too bulky or of too high a density that small errors would amount to large errors in dose.

Compensators can be designed to take into account not only tissue deficit, but also tissue inhomogeneities such as lungs. In the latter case, a bulk density correction is used to calculate radiologic path length through the inhomogeneity.

After a particular TBI technique has been established and commissioned for clinical use, it is recommended that an invivo dosimetry check be performed on the first 20 or so patients. Thermoluminescent dosimeter (TLD) capsules or chips, surrounded by suitable buildup bolus, may be placed on the patient's skin at strategic locations and doses measured for the actual treatments given. TLD results should be compared with expected doses, calculated by summing entrance and exit doses at the location of the TLDs and taking into account thickness variation, compensation, and off-axis ratios at the depth of TLDs. An agreement of plus or minus 5% between the calculated and measured doses is considered reasonably good. An overall dose uniformity of plus or minus 10% is considered acceptable for most protocols.