When a photon beam is incident on an irregular surface the dose distribution is altered and isodose charts need to be corrected. Modern computers are able to calculate changes behind the scenes but some other methods may be used.
Impact of changing contour
Photons begin attenuating as soon as they encounter the patient contour. This means that if parts of the patient are closer to the beam source, the beam will be attenuated more than in parts of the patient distant to the beam source. This can lead to undesired changes in isodose curves at depth.
Correction Techniques
Effective Source-Surface Distance
This method adjusts dose at a point by a factor based on the inverse square law and the alteration in SSD for the irregular contour.
Tissue-Air Ratio or Tissue-Maximum Ratio Method
Instead of adjusting for SSD, the TAR or TMR methods apply a correction based on the dose ratio at the point compared with the dose ratio if the surface was flat.
Isodose Shift Method
This allows correction of an entire isodose chart at once rather than individual points. The isodose curves are shifted based on the surface contour multiplied by a k factor. The k factor is a constant based on the beam energy being used.
Compensating for tissue contour
Instead of accepting the dose distribution produced by an irregular contour, the isodose lines can be flattened by using bolus or a compensator.
Note on bolus: Khan says that bolus used to compensate for contour irregularity should be called bolus, and bolus used to increase the dose to skin be called build up bolus.
Bolus
Bolus is tissue equivalent material applied to the surface of the patient to even out their contour. This leads to flatter isodose lines at depth, but has the problem of increasing skin dose.
Compensators
Compensators were used to correct for an irregular patient contour while keeping the skin sparing effect. They are loaded into the treatment head, and designed to attenuate parts of the beam in a similar way to bolus. This evens out of the isodose lines at depth but also retains the skin sparing effect. Compensators are difficult to design and increase treatment time.
