UNIFORM IRRADIATION OF THE CRANIOSPINAL AXIS WITH A PENUMBRA MODIFIER AND AN ASYMMETRIC COLLIMATOR

Purpose: A technique is described that uses an independent, asymmetric collimator and a penumbra modifier to uniformly administer radiation over the craniospinal axis. Method and Materials: An isocenter is used at the junction of the cranial and the upper spinal fields. These fields are defined by a single isocenter at spinal cord depth and an independent, asymmetric collimator. From the isocenter, the cranial and upper spinal fields are extended 1 cm inferiorly and superiorly, respectively. A modifier provides a 2-cm wide penumbra at the central axis of the beam (overlapping region) and is attached to a wedge tray in an accessory slot. This modifier allows the fields to be matched so a uniform dose is delivered over the isocenter and junction. Dose distribution was measured with an anthropomorphic head-and-neck wax phantom that included the seventh cervical vertebrae. A film was placed in a coronal cut at the spinal cord level. Results: The administered dose varied less than 10% through the craniospinal axis. Reproducibility with portal films has been very good. Advantages include dose homogeneity, easy reproducibility, and decreased setup time. Conclusion: This single-field isocentric technique allows more uniform irradiation of the craniospinal axis than do previously described techniques.