Effect of magnesium oxide on interfraction prostate motion and rectal filling in prostate cancer radiotherapy: analysis of a randomized clinical trial.
OBJECTIVE
To investigate whether magnesium oxide reduces the interfraction motion of the prostate and the amount of rectal filling and rectal gas, which influences prostate position during radiotherapy for prostate cancer.


METHODS
From December 2008 to February 2010, 92 prostate cancer patients scheduled for intensity-modulated radiotherapy (77 Gy in 35 fractions) using fiducial marker-based position verification were randomly assigned to receive magnesium oxide (500 mg twice a day) or placebo during radiotherapy. In a previous study, we investigated the effect on intrafraction motion and did not find a difference between the treatment arms. Here, we compared the interfraction prostate motion between the two treatment arms as well as the amount of rectal filling and rectal air pockets using pretreatment planning computed tomography and magnetic resonance imagingscans.


RESULTS
There was no statistically significant difference between the treatment arms in translation and rotation of the prostate between treatment fractions, except for the rotation around the cranial caudal axis. However, the difference was less than 1° and therefore considered not clinically relevant. There was no significant difference in the amount of rectal filling and rectal air pockets between the treatment arms.


CONCLUSIONS
Magnesium oxide is not effective in reducing the interfraction prostate motion or the amount of rectal filling and rectal gas during external-beam radiotherapy. Therefore, magnesium oxide is not recommended in clinical practice for these purposes.
Department of Radiation Oncology, University Medical Center Utrecht, Heidelberglaan 100, 3584, Utrecht, The Netherlands.
Annemarie M
AM
den Harder
Carla H
CH
van Gils
Alexis N T J
AN
Kotte
Marco
M
van Vulpen
Irene M
IM
Lips
D000368:Aged; D019583:Dose Fractionation, Radiation; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D004334:Drug Administration Schedule; D057918:Fiducial Markers; D006801:Humans; D054368:Laxatives; D008277:Magnesium Oxide / Q000008:administration & dosage*; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009038:Motion; D056888:Patient Positioning; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D014057:Tomography, X-Ray Computed
D054368:Laxatives; D008277:Magnesium Oxide