Introduction

The treatment planning process consists of a series of patient-related work tasks that eventually result in a custom plan of the external beam treatment and will enable the radiation dose prescription to be applied. The Radiation Treatment Planning (RTP) system provides a 3D dose distribution of the beams arranged around the body using a mathematical model of the megavoltage x-ray field. The composite dose map is displayed in relation to the target volume and the critical anatomical structures within the body.

Integral to the planning process are devices that ensure the treatment is reproducible on a daily basis; the most important of these is a method of reducing movement of the patient during treatment, and this is called ‘immobilization’. The specification of such a device is dependent on the area of the body for which it is required. For treatments of the head and neck, some form of immobilization device is essential to ensure reproducible set-up and to avoid displacement of the plan isocentre from its intended position. The relatively small field sizes used in the head and neck, compared with pelvic or thorax plans, requires a high degree of beam positional accuracy, because of the proximity to critical organs (e.g. eye, spinal cord), which depends on effective immobilization of the head. Other devices that facilitate reducing movement of the patient are site specific. For example, external beam radiation treatment of the breast utilizes a board that supports the patient at an inclined angle and provides hand grips that raises their arms above their head to meet the requirements for glancing beams arranged to the breast. Other devices stop the patient from moving their legs to reduce lower body movement or can make the treatment more sustainable over the few minutes of the beam exposure time.

Complete eradication of patient movement is impossible to achieve, although reducing this to within acceptable tolerance (e.g. 3 mm for head and neck, 5 mm for thorax and pelvis) is commonly achieved. Another technique to improve beam positional accuracy is to track the movement of the patient or movement of anatomical landmarks using an external device that is integral with the treatment accelerator – this method is called ‘Image Guided Radiotherapy (IGRT)’. IGRT requires monitoring of the patient using a real-time imaging device (e.g. video, ultrasound, x-ray) and the information is compared with the patient plan to correct for any positional inaccuracy. This technique enables a precise level of beam positioning control that can allow for any inadequacies in immobilization or effects of organ movement (e.g. lung displacement during respiration).

Anatomical information of the patient in the treatment position is required in order to undertake 3D treatment planning. The process of ‘localization’ includes the acquisition of either radiological images from a simulator and/or tomographic scanner (e.g. x-ray computer tomography, CT). The localization process provides external contour information and anatomical data that enable definition of planning target volume (PTV) contours and organs at risk (OAR); methods of providing this information range from x-ray CT, magnetic resonance imaging (MRI) through to positron emission tomography (PET). However, the basic requirement, to define the external contour and internal structures for treatment planning, are provided by a CT data set of the patient that can be transferred to the ‘Treatment Planning System’ (TPS) where attenuation data are converted to ‘electron density’ values for heterogeneity correction of the megavoltage beam data. Non-x-ray CT imaging modalities cannot provide this attenuation correction.

The production of an optimized treatment plan for external beam megavoltage treatment is highly dependent on the following:

• the size and shape of the PTVs

• the limiting (tolerance) dose to the critical structures (OAR)

• the positional reproducibility of the linear accelerator and couch support system

• limitation dictated by the patient’s position within the immobilization device

• selection of optimum beam parameters (e.g. field size collimator rotation).

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