Palliative radiation for vertebral metastases: the effect of variation in prescription parameters on the dose received at depth.
OBJECTIVE
To assess the effect of prescription parameters on the dose received by the spine during palliative radiotherapy.


METHODS
In a survey, members of the Canadian Association of Radiation Oncologists were asked to define their prescription parameters for vertebral metastases. The depth of the spinal canal and vertebral body at 8 spinal levels was measured in 20 magnetic resonance imaging studies (MRIs). Survey results were applied to the measurements to assess the dose received at depth. The depth of spinal structures assessed at simulation and by diagnostic imaging was compared.


RESULTS
Prescriptions were most commonly to D(max) 3 cm or 5 cm using 60Co-6MV photons delivering 8-30 Gy in 1-10 fractions. Mean depths from MRI were: posterior spinal canal, 5.5 cm; anterior spinal canal, 6.9 cm; and anterior vertebral body, 9.6 cm. Application of the prescription parameters from the survey to these measurements showed a wide range in the dose at depth with variation in technique. Depths measured at simulation correlated well with diagnostic imaging.


CONCLUSIONS
The spinal canal and vertebral body lie >5 cm beneath the skin, and the dose received varies by up to 50% with changes in prescription depth. We suggest a suitable prescription point for vertebral metastases and a method for determining this at simulation.
Barton
Rachael
R
Palliative Radiation Oncology Program, Princess Margaret Hospital, University Health Network, University of Toronto, Toronto, Canada. rachaelbarton@doctors.org.uk
Robinson
Graham
G
Gutierrez
Eric
E
Kirkbride
Peter
P
McLean
Michael
M
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002170:Canada; D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D006801:Humans; D008279:Magnetic Resonance Imaging; D008875:Middle Aged; D010146:Pain / Q000532:radiotherapy; D010166:Palliative Care; D018787:Radiation Oncology / Q000592:standards*; D011879:Radiotherapy Dosage; D013125:Spinal Neoplasms / Q000532:radiotherapy* / Q000556:secondary*; D013131:Spine / Q000033:anatomy & histology*; D011795:Surveys and Questionnaires
D003037:Cobalt Radioisotopes