8.4.2 - Field Size Modification (kV photons)
Kilovoltage photons have different mechanics to megavoltage photons. They rely on the photoelectric effect more than incoherent scattering, meaning that small thickness of high Z materials will attenuate a large percentage of the beam. They also have a reduced SSD, typically 15 or 25 cm. Field sizes are usually no more than a 10 cm circle, but extended SSD treatments are possible in some situations.
Modification of field size
There are two commonly used methods of field shaping.
- Kilovoltage treatments are usually aimed at small skin lesions, often circular. Most kilovoltage machines will have a set of cylindrical applicators that can be used at an SSD of 15 cm and project a circular field.
- The kilovoltage treatment cone is able to treate a circle of 10 cm diameter. This can be modified by using specially created cutouts made of lead consisting of 2 TVL, attenuating most of the beam.
Increasing field size will have a corresponding increase in dose rate, and therefore less time is required for larger field sizes. Custom blocking of the larger applicator requires correction of dose rate to account for the attenuated dose and corresponding reduced dose rate.
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