Beam incidence.

In order to achieve good homogeneity and a small uncertainty in the dose absorbed at different points, opposing beams can be used. This can be obtained either by using a single radiation source and then changing patient position or by using multiple opposing sources. There is no ideal technique for irradiating the whole body in a homogenous and precise way and shielding the lungs optimally. In 1986, in European centres, AP-PA incidence (50%), bilateral (35%) and combined techniques (15%) were used^[3]. Today the application of anterior/posterior irradiation has increased.

Beam quality.

In order to compare dosimetrical data, a beam quality index^[30] has been defined:

I = D₂₀ / D₁₀ ( f+20 / f+10 )²

where D₂₀/D₁₀ represents the quotient of dose. Under TBI conditions, these doses have to be measured at 20 and 10 cm depth respectively in a phantom of at least 30 x 30 x 30 cm³. f is the source to phantom surface distance. Any beam quality can be used in AP-PA TBI. Bilateral TBI requires high energies.

Technical and geometrical data.

The treatment geometry to be selected will depend on the local conditions. In most institutions, there is no dedicated treatment room for TBI. In this case, it is necessary to develop a technique by means of which the whole patient can be irradiated homogeneously, reproducible and reliable.

The more usual option is to provide a large source-patient distance (ranging from 3 to 6 m), and the use of the diagonal of the field for the positioning of the patient. It must be remembered, that the flattened part of the beam is always much smaller than that indicated by the optical alignment. There are techniques which utilize smaller treatment distances by using multiple parallel and adjacent beams, or moving beams or patient translation; see figure 1^[3,6]

Patient position.

The position of the patient could be prone, supine, lateral, foetal, standing or seated. The main criteria for choosing one of these are patient comfort during treatment and uniformity of the radiation field but taking into account any local and individual requirements.

Treatment verification.

Individual verification procedures should be used specially if there are any problems with patient positioning, beam modifier alignment, TBI reproducibility or any other parameters related with treatment stability.

The most usual procedures for verification are the use of portal imaging methods to control the position of the shielding or alignment of detectors for in vivo dosimetry.



Figure 1.- TBI, single source: techniques using horizontal photon beams for bilateral (a,b) and for AP-PA irradiation in decubitus and standing position (c,d) and techniques using vertical photon beams for AP-PA irradiation with a large aperture ⁶⁰Co-irradiator (e), large distance -two floors- (f), moving beam by head rotation (g), by patient translation (h), and horizontal source scan (i). Multiple beam TBI: techniques using adjacent fields (j) or direct and oblique fields (k). Multiple source TBI techniques (l,m).