Two other important effects on dose distribution are the patient contour and patient inhomogeneity, which are considered separately.
Field Size and Shape
The field size is directly related to the amount of primary radiation entering the patient and the resulting dose distribution. A dose at any point results from both the primary and scattered radiation. Larger field sizes lead to increased generation of scattered radiation, which tends to increase dose at a specified point. Conversely, small field sizes may lead to loss of electronic equilibrium and the central flat part of the beam.
As field size increases:
- Zmax increases (less rapidly for very large field sizes over 20 cm)
- PDD increases at other points along the central beam axis
- The ratio of the penumbra to the central portion of the field decreases
- For cobalt-60 beams, the curvature of the isodose curves at depth increases due to reduction in scatter from parts of the beam distant to the central axis. This may be corrected for with a flattening filter.
As field size decreases:
- Zmax decreases (rapidly for field sizes under 6 cm)
- PDD decreases at points along the central beam axis
- The ratio of the penumbra to the central portion increases, and may lead to complete loss of the central flat section
Irregular field shapes are more difficult to calculate, but in most cases corrections to a square field can be made. The blocked parts of the beam do not contribute to the primary radiation but scattered radiation still deposits dose in those regions. High energy megavoltage beams may lead to blurring of blocked areas due to increased range of scatter.
Source Surface Distance
The intensity of a radiation beam is related to the distance from the source by the inverse square law – if the distance is doubled then the intensity falls by four times. Although the dose rate at an increased distance is lower, the attenuation of the beam is reduced due to the slower fall off in intensity at greater distance.
For an increased SSD:
- Zmax and percent depth dose increase
- The geometric penumbra increases with increasing distance
Reduced SSD shows the opposite effect.
Beam Quality
The most commonly used beams are kilovoltage (under 200 keV), 60Co (between 1.1 and 1.37 MeV), 6 MV (under 6 MeV photons, median of 1.5 MeV) and 18 MV (under 18 MeV photons, median of 4 – 6 MeV).
In general, as beam energy increases:
- Zmax increases due to increased range of scattered electrons
- Percent depth dose increases
- Penumbra initially reduces in size (from kilovoltage to low megavoltage) before slightly increasing with high megavoltage energies (18 MV). This increase in penumbra at higher energies is due to increased lateral range of electrons generated by the higher energy photons.
Kilovoltage beams generally show bulging isodose curves due to increased lateral scatter of photons, unlike megavoltage beams which have much less lateral scatter of photons. Zmax is usually located on the surface.
Beam Output
Beam output has no effect on dose distribution, but has implications for treatment time.
