Position verification for the prostate: effect on rectal wall dose.
OBJECTIVE
To evaluate the effect of gold marker (GM)-based position correction on the cumulative dose in the anorectal wall compared with traditional bony anatomy (BA)-based correction, taking into account changes in anorectal shape and position.


METHODS
A total of 20 consecutive prostate cancer patients, treated with curative external beam radiotherapy, were included. Four fiducial GMs were implanted in the prostate. Positioning was verified according to the shift in BA and GMs on daily electronic portal images. Position corrections were determined using on- and off-line position verification protocols according to the position of the GMs (GM-on and GM-off) and BA (BA-off). For all patients, intensity-modulated radiotherapy plans were made for the GM (8-mm planning target volume margin) and BA (10-mm planning target volume margin) protocols. The dose distribution was recomputed on 11 repeat computed tomography scans to estimate the accumulated dose to the prostate and anorectal wall while considering internal organ motion.


RESULTS
The dose that is at least received by 99% of the prostate was, on average, acceptable for all protocols. The individual patient data showed the best coverage for both GM protocols, with >95% of the prescribed dose for all patients. The anorectal wall dose was significantly lower for the GM protocols. The dose that is at least received by 30% of the rectal wall was, on average, 54.6 Gy for GM-on, 54.1 Gy for GM-off, and 58.9 Gy for BA-off (p <.001).


CONCLUSIONS
Position verification with GM and reduced planning target volume margins yielded adequate treatment of the prostate and a lower rectal wall dose, even when accounting for independent movement of the prostate and anorectal wall.
Haverkort
Marie A D
MA
Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands. m.a.haverkort@amc.uva.nl
van de Kamer
Jeroen B
JB
Pieters
Bradley R
BR
van Tienhoven
Geertjan
G
Assendelft
Esther
E
Lensing
Andrea L
AL
van Herk
Marcel
M
de Reijke
Theo M
TM
Stoker
Jaap
J
Koning
Caro C E
CC
D001003:Anal Canal / Q000000981:diagnostic imaging / Q000528:radiation effects; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D009068:Movement; D056888:Patient Positioning; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000175:diagnosis / Q000000981:diagnostic imaging / Q000532:radiotherapy; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging / Q000528:radiation effects*; D014057:Tomography, X-Ray Computed / Q000379:methods; D047368:Tumor Burden; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000528:radiation effects
D006046:Gold