(Non)-migration of radiopaque markers used for on-line localization of the prostate with an electronic portal imaging device.
OBJECTIVE
Radiopaque gold markers can be implanted in the prostate to visualize its position on portal images during radiation therapy. This procedure assumes that the markers do not move within the prostate. In this work we test this assumptiom.


METHODS
Three markers were implanted transrectally in the prostate of patients undergoing external radiation therapy. An orthogonal pair of portal images was acquired periodically throughout the course of radiation therapy with an a-Si electronic portal imaging device (EPID). The marker coordinates were determined, and the distances between the implanted markers were recorded. The distance time trend is used to evaluate the magnitude of marker migration.


RESULTS
The average standard deviation (SD) of the distances between markers was 1.3 mm (range 0.44 to 3.04 mm). Three of the 11 patients show a SD larger than 2 mm. For these patients, all three distances show a simultaneous reduction with time, compatible with a shrinking of the prostate. All had been treated with neoadjuvant hormone therapy. For 1 of the 3 patients, this reduction in volume was confirmed with a repeat computed tomographic scan.


CONCLUSIONS
None of the 33 markers studied migrated significantly. The implantation of three radiopaque gold markers enables accurate and precise on-line verification of the prostate position during external beam radiation therapy. The use of three markers provides a tool to monitor prostate position and volume changes that can occur over time due to hormone or radiation therapy.
Pouliot
Jean
J
Department of Radiation Oncology, University of California San Francisco, Comprehensive Cancer Center, San Francisco, CA, USA. pouliot@radonc17.ucsf.edu
Aubin
Michele
M
Langen
Katja M
KM
Liu
Yu-Ming
YM
Pickett
Barby
B
Shinohara
Katsuto
K
Roach
Mack
M
D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D013997:Time Factors; D014057:Tomography, X-Ray Computed
D006046:Gold