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External beam radiotherapy has a role in each histological type of thyroid cancer. For treatment confined to the thyroid bed, an antero--oblique wedged pair of beams is simple to apply. More frequently, however, the volume needs to include both sides of the neck and adequate dose must be given down to the level of the suprasternal notch, precluding use of lateral fields. Thus, anterior and posterior fields are usually necessary, extending from the tips of the mastoid processes or hyoid down to the carina and laterally to include both sides of the neck and supraclavicular fossae. The mandible and infraclavicular portions of both lungs are shielded, but there is no midline lead in the phase one volume. A mid-plane dose of 46 Gy given in 23 daily fractions results in an acceptable early radiation reaction and will avoid late damage to the spinal cord. Because of considerable variation of interplanar distance along the length of the volume, a lateral simulator film should be taken to determine the maximum cord dose. For most patients, a three-dimensional CT planned phase two volume will be required and should be determined prior to completing phase one. The optimum beam arrangement, usually incorporating conformal beam shaping assisted by use of a multileaf collimator, should avoid further dose to the cord. If there is additional dose to the spinal cord, this phase can be introduced before 46 Gy is reached.

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