432 Matching half beams in head and neck radiotherapy. planning and dosimetry

Abstract

<h3>Introduction</h3> In the treatment planning of the head and neck tumours, usually matching fields are necessary. There are inherent hazards with this technique in that overlapping of these portals may result in hot and cold spots at the field edges. The use of linacs with independent collimators enables to eliminate the beam divergency, matching then the central axes of adjacent fields where there is no divergence. On the other hand, the 3D radiation treatment planning systems allow us to calculate the dose variation through the field's junction. <h3>Technical description</h3> We describe a technique consistent in three isocentric half-fields (two upper laterals and one lower anterior). Patients are immobilized with a thermoplastic mask. The upper limit of the lateral fields and the midline are outlined in the mask. It's not necessary to move either the patient or the treatment couch for the different fields of treatment. The SSD for each field is calculated at simulation and/or planihcation time. The lateral fields are shielded with individual focalized Cerrobend's blocks, leaving the half field limit free. <h3>Conclusion</h3> In this technique there isn't beam divergency at the junction's fields, so the homogeneity is maximal. A dosimetric study with a 3D planning system shows a good distribution of dose. The dose contribution of each half field in the other half field is <2%. The new linacs working with asymmetric collimators enable us to use the split matching technique in a simple, reproducible and accurate form.