Intensity modulated radiation therapy for thyroid malignancies

External beam radiation therapy can be used to treat thyroid malignancies, such as follicular, papillary, and anaplastic carcinomas. For these cases, given a typical prescription of 60 Gy to the planning target volume (PTV), conventional forward treatment planning is a challenging task due to: (a) the proximity of the spinal cord with a tolerance dose of 45-50 Gy; (b) the complex shape of the PTV; and (c) the varying PTV depth in cranial-caudal direction. Given these factors, delivering target doses with inhomogeneity of about 20% are not uncommon. We have improved the treatment technique for thyroid malignancies by employing inverse treatment planning and intensity modulated beams delivered by a step-and-shoot technique. In our approach, patients are CT scanned in supine position and the PTV and the spinal cord are outlined. Four beams with lateral opposed fields covering the superior aspect of the PTV and an anterior oblique wedge pair treating the inferior portion of the PTV are employed for inverse treatment planning. The resulting beam intensities are segmented and delivered with few (4-5) multi-leaf collimated subfields. The leaf sequences are verified by film measurements and the cumulative dose from each beam is measured at the prescription point in phantom. To date, 5 thyroid patients have been treated with this technique at our institution. While respecting the tolerance dose of the spinal cord, the resulting distributions show excellent coverage of the PTV (±5%). Our experience with thyroid treatments points to the possible replacement of conventional techniques by intensity modulated radiation treatment.