Beam energy, source-surface distance, field size / shape and collimation all effect the dose distribution seen with megavoltage electron beams.
Linear accelerators are usually capable of producing electrons from 4 MeV up to 20 MeV. The difference in dose distribution is quite marked between the low and high end of the spectrum.
Low dose beams will usually have:
- A lower surface dose relative to zmax, due to rapid attenuation of electrons over a short distance, leading to a peak of dose at depth
Note that the surface dose is not physically less at lower energies, it is just the ratio of dose at surface to dose at zmax.
- Shallower R50 and Rp
- Retain bulging of low dose isodose lines
- No lateral restriction at depth of high dose isodose lines
By contrast, high dose electron beams will demonstrate:
- A higher surface dose (over 90% of zmax), due to increased electron energy and travelling distance and minimal lateral scatter at high electron energies
- Deeper R50 and Rp
- More prominent bulging of the low dose isodose lines at depth
- Lateral restriction at depth of high isodose lines
zmax is not dependent on beam energy but rather on individual machine construction and use of accessories. The more important values are the pre- and post-max 90% distances, which describe the useful range of the beam. Importantly, low energy beams usually require bolus to acheive dose at the skin surface, whereas high energy beams do not.
Source Surface Distance
As SSD increases, the machine output required to generate significant dose also increases (as per the inverse square law). Importantly, the ‘source’ used for inverse square law calculations is a virtual source from which the electrons ‘appear’ to be emanating from. This can be determined experimentally and is used for corrections.
With extended SSD treatments, there is a tendency for the surface dose to increase due to increased lateral scatter of electrons within air. Electrons travelling at a tangent away from the central beam axis will tend to deposit their dose more superficially.
Another important feature is that the beam edge will become blurred at a longer distance from the treatment head. This will lead to ineffective treatment and beam collimation should be placed close to the skin surface to sharpen the field edges. See the section on collimation below.
Field Size and Shape
If the lateral scatter range of electrons is less than the distance from the central axis to the field edge, central electronic equilibrium will occur and percent depth dose will not alter significantly with larger field sizes. The only exception is the surface dose, which will increase with larger fields as there will be increased numbers of electrons scattering in the volume of air relative to the surface area.
As field size decreases below this range, electronic equilibrium is lost and the percent depth doses will decrease.
Placement of Collimation
If the air gap between the end of the electron applicator and the surface increases, the high isodose lines tend to converge along the central axis and the low dose isodose lines spread laterally. This is due to increased scattering occurring in the air. This enlarged penumbra may cause difficulties in generating a suitable dose distribution and it is recommended that the distance from the applicator to the surface be as low as possible. Alternatively, for extended SSD treatments, collimation or shielding can be placed adjacent to the patient surface to correct for this problem.
This effect is acceptable when the collimation is under 5 cm from the surface. When collimation is greater than 10 cm from the surface, the penumbra begins to enlarge to the point where a therapeutic beam is difficult to achieve.
Bolus is used for two purposes with electron beam treatment:
- For an irregular contour (which could generate hot and/or cold spots), compensating bolus may be applied to the surface to give a flat incidence
- For low energy electron beams, build up contour will 'pull up' the isodose lines towards the surface, allowing treatment of superficial lesions.
Tissue equivalent material is used for electron bolus in nearly all cases. High energy electron beams do not require build up bolus as they have significant dose at the surface.