The dosimetry of a single “hockey stick” portal for treatment of tumors of the cranio-spinal axis

Authors

  • Glasgow Glenn P.,

    1. Division of Radiation Oncology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110
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  • Marks James E.

    1. Division of Radiation Oncology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110
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Abstract

Conventional treatment of tumors of the cranio-spinal axis portal usually involves multiple-field, moving junction treatments to avoid overlapping fields over the spinal cord. To avoid these problems, we irradiate the cranio-spinal axis using a single “hockey stick” portal and the 25-MV x-ray beam from a Varian Clinac-35® linear accelerator. Patients are positioned prone on the floor 229 cm from the radiation source and the collimators are rotated 45° so the maximum diagonal dimension of the field 116 cm at 229 cm is coincident with the cranio-spinal axis. The head is alternately rotated to treat the right-hand side one day and the left-hand side the next day. Thermoluminescent dosimetry in an anatomical phantom reveals that, relative to the 100% dose delivered at 4-cm depth on the central axis of the blocked field, the midline posterior fossa dose is about 100%, with a maximum dose of about 105% to the extreme posterior portion of the skull. The midline neck dose is about 95% and the dose to the inferior portion of the spinal cord is about 105%. The doses to other critical organs are also presented.

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