8.4.1 - Field Size Modification (MV Photons)

Field Shape and Size Modification

Design of Field Blocks

Blocks are designed to reduce dose within parts of the field to insignificant levels. Transmission of less than 5% is usually acceptable. This equates to a thickness of lead at least 4.5 – 5 times the half value layer of the beam. While easily done for kilovoltage beams, megavoltage photon beams may require thickness of up to 7 cm.

Divergent blocks

Blocks may be designed to follow the natural geometric divergence of the beam. This requires a significant time investment for the casting of blocks and does not always have a large clinical effect. If it is vital that a sharp penumbra be maintained then custom blocks may be cast. Some departments maintain a collection of divergent blocks for commonly used fields; this has become less important since MLCs become available.

Methods of Field Shaping

Blocking was initially done through the use of custom blocks. Independent jaws allowed determination of field size. Multileaf collimators are newer devices that allow for rapid creation of irregular field shapes.

Custom blocks

Custom blocks are typically made of a low melting point alloy such as Cerrobend™. The low melting point allows the metal to be poured into the desired shape with a small furnace. A Styrofoam cutout is used as a template for the block construction.
Custom blocks define fields very well with a smooth outline, but have the drawback of being heavy and time intensive to produce. Treatments that use multiple fields (such as conformal therapy) necessitate entering the bunker and exchanging the block with every beam.

Independent jaws

Linear accelerators typically have two sets of jaws, which are capable of forming a rectangular field of differing dimensions. They are unable to create irregular fields aside from rectangles. The movable jaws align to the beam divergence and always form a divergent edge.
Independent jaws are able to generate fields with a non-divergent edge and do not suffer from interleaf transmission associated with MLCs. They are motorised and therapists do not need to enter the room to change their position.

Multileaf collimators

A multileaf collimator contains a set of about 80 leaves that are able to attenuate a variable part of the beam. Each leaf is motorised and can be placed in position to block out part of a field. Custom field shapes are created by using a combination of the leaves. The leaves are structured to reduce interleaf transmission to acceptable levels. Some linear accelerators replace a set of independent jaws with a multileaf collimator, whereas others install the MLC as a tertiary collimator. The latter require curved leaf ends to generate a uniform penumbra. The former generally use a divergent end technique to minimise transmission penumbra.
Although MLCs can create a variety of fields, they are unable to produce an island field. A small amount of transmission (<5%) occurs between each leaf. The field defined by the MLC is not as smooth as for custom blocking. The advantages of MLCs are the reduced time required for treatment and their potential use in intensity modulated radiation therapy.

Effect of Field Size

A larger field means that there is an increase in the amount of scatter from the treatment head as well as an increase in the amount of scatter for interactions in air. This leads to electron contamination and is an important factor in field size changes. Very small field sizes may cause disequilibrium of electron scatter in the central part of the beam, leading to loss of the flat portion. This limits the utility of very small field sizes.

In general:

  • As field size increases, surface dose will increase due to increased electron contamination of the beam
  • As field size increases, zmax will move closer to the surface due to electron contamination and scatter from the outer parts of the beam
  • As field size increases, the depth dose of points beyond zmax will increase due to increase scatter from lateral parts of the beam increasing the dose at depth.