Breast Radiotherapy. Part 3

Target volume definition

A more risk-adapted approach to the volume treated may be defined with the premise that the total dose should match the pattern of local relapse more closely, in doing so delivering a lower dose to low-risk volumes and a higher dose to high-risk areas.

Within external beam radiotherapy accurate tumour bed localisation is crucial not only for PBI but also in the context of tumour bed dose escalation. Both of these techniques are currently being tested within randomised controlled trials against WBRT.

Clearly, the benefits of radiotherapy should not be undermined by the side effects. The EBCTCG systematic overview of radiotherapy does confirm an excess of cardiac mortality seen mostly in the under-50 age group. One potential gain from treating a smaller volume of breast tissue to a high dose is to reduce the dose to adjacent organs at risk, for instance the myocardium and/or the main coronary vessels.

Tumour bed localisation has been evaluated by several methods:

CT This gives much more spatial information regarding organs at risk, including the relationship to the chest wall, lung and heart. However, the tumour bed volume can be difficult to define unless there is an obvious seroma or focal scarring. It is affected by CT windowing and so should ideally be used with another method to corroborate the location, for instance fiducial markers such as surgical clips or gold seeds. These have the disadvantage of variations with insertion techniques with regard to positioning, and the possibility of marker migration. Due to their high density, they are visible with megavoltage portal imaging and may therefore also be used in the verification of the treatment accuracy and reproducibility.

MRI This has the advantage of better visualisation of the tumour bed. However, there are technical limitations, such as the problem of co-registration with CT.

Ultrasound 2D ultrasound is already used in some centres for the planning of direct simple electron fields to determine the distance between the skin surface and the anterior pectoral fascia, to optimise selection of beam energy and in turn treatment depth. Ultrasound may be used in conjunction with the CT data for more complex planning but it may be spatially limited and may underestimate the tumour volume.

Optimisation of treatment planning

Intensity-modulated radiotherapy (IMRT)

With standard breast radiotherapy planning using a simulator, the planning cannot take account of the 3D anatomy and changes in tissue density unique to a given patient. This in turn leads to significant dose inhomogeneities which can in some cases account for a worse cosmetic outcome, particularly in women with larger breasts. CT (3D) planning allows an appreciation of these important variables and in turn facilitates sophisticated planning and treatment delivery.

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