It is important to understand the basic techniques of radiotherapy planning because these will help when developing complex plans. These techniques, which are taught at the First FRCR level (Royal College of Radiologists), are used by the treatment centre's Physics Department for checking the validity of calculations before they are applied to patients, and anyone interpreting plans will need to be familiar with them to know whether it is worth adjusting treatment plans during review. Advanced radiotherapy techniques will be discussed in Chapter 6.
There are several useful reviews of radiotherapy physics in the literature. One by Shiu and Mellenberg (2001) includes sections on isodose planning. Another, by Purdy (2000), provides a perspective on future directions in 3D treatment planning. Radiotherapy is a rapidly developing field and it is important to ensure that new methods satisfy safety and effectiveness requirements before being adopted as routine treatments.
This chapter will deal with the general principles of developing isodose plans that are suitable for treatment and will use specific examples to highlight particular points. It will focus mainly on external beam radiotherapy with megavoltage photons; there will be shorter sections on the use of electrons and brachytherapy. Some aspects of radiotherapy planning using lower energy (kilovoltage) photons will be discussed in Chapter 36. The use of unsealed radiation sources for prostate, thyroid and childhood cancers are discussed in Chapters 22, 38 and 40, respectively.
Treatment planning overview
Radiotherapy planning can be divided into stages as follows.
• Patient preparation, position and immobilisation.
• Localisation method (e.g. orthogonal films, CT scanning, and image co-registration).
• Definition of target volumes and organs at risk.
• Radiotherapy technique, including beam arrangements, beam energy, size and shape, weighting, wedges and production of isodose plan.
• Prescription, including number of phases, dose, energy and fractionation.
• Verification (i.e. checking the geometrical set-up of the treatment). This can take place before treatment in the simulator or CT simulator, and/or during treatment using portal imaging.
Quality assurance (QA) is essential and it is of paramount importance for each member of the planning team to be familiar with the overall process.
Patient position and immobilisation
For radiotherapy to be effective it must be delivered accurately, and it is important to understand the methods of immobilisation and the inevitable uncertainties in the delivery of treatment.