Planning a Patient, Deciding on the Volumes and Fields and Plan Verification

Abstract

After having completed the simulation process, the therapist must then decide on the treatment volumes, beam positions and beam parameters; create treatment plans; and verify if the plans produced cover the targeted volume. In cases where only the dose along the central axis of the beam is sought, only the source-to-skin distance (SSD) is required. Simple algorithms, such as Clarkson’s method, may be used to determine the dosimetric effects of using standard or even customized blocks in the fields and to calculate the dose to off-axis points if their coordinates and SSD are measured. In simple computerized 2D treatment planning, a single skin contour taken at the point of the central axis of the beam is sufficient and can be prepared by using lead wire or a plaster cast at the time of simulation. The patient data requirements for conformal treatment planning are more elaborate than in 2D treatment planning. External shape of the patient must be outlined in all areas where the beams enter and exit and in the adjacent areas. The targets and internal structures have to be contoured for dose calculation, and electron densities for each volume element in the dose calculation matrix must be determined for correction for heterogeneities. Transverse CT scans contain all the information required for complex treatment planning and form the basis of CT simulation in modern radiotherapy treatment. This chapter will deal mainly with beam definition in 2D radiotherapy and set-up verification.