Uniform irradiation of the craniospinal axis with a penumbra modifier and an asymmetric collimator.
OBJECTIVE
A technique is described that uses an independent, asymmetric collimator and a penumbra modifier to uniformly administer radiation over the craniospinal axis.


METHODS
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.


CONCLUSIONS
This single-field isocentric technique allows more uniform irradiation of the craniospinal axis than do previously described techniques.
Sohn
J W
JW
Cleveland Clinic Foundation, Department of Radiation Oncology, OH 44195.
Schell
M C
MC
Dass
K K
KK
Suh
J H
JH
Tefft
M
M
D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D011878:Radiotherapy / Q000295:instrumentation*; D013116:Spinal Cord / Q000528:radiation effects*