Electromagnetic transponders indicate prostate size increase followed by decrease during the course of external beam radiation therapy.
OBJECTIVE
Real-time image guidance enables more accurate radiation therapy by tracking target movement. This study used transponder positions to monitor changes in prostate volume that may be a source of dosimetric and target inaccuracy.


METHODS
Twenty-four men with biopsy-proven T1c-T3a prostate cancer each had three electromagnetic transponders implanted transperineally. Their coordinates were recorded by the Calypso system, and the perimeter of the triangle formed by the transponders was used to calculate prostate volumes at sequential time points throughout the course of radiation therapy to a dose of 81 Gy in 1.8-Gy fractions.


RESULTS
There was a significant decrease in mean prostate volume of 10.9% from the first to the final day of radiation therapy. The volume loss did not occur monotonically but increased in most patients (75%) during the first several weeks to a median maximum on Day 7. The volume increased by a mean of 6.1% before decreasing by a mean maximum difference of 18.4% to nadir (p < 0.001 for both increase and decrease). Glandular shrinkage was asymmetric, with the apex to right base dimension varying more than twice that of the lateral dimension. For all dimensions, the mean change was <0.5 cm.


CONCLUSIONS
Real-time transponder positions indicated a volume increase during the initial days of radiation therapy and then significant and asymmetric shrinkage by the final day. Understanding and tracking volume fluctuations of the prostate during radiation therapy can help real-time imaging technology perform to its fullest potential.
King
Benjamin L
BL
Radiation Oncology Department, University of Washington, Seattle, WA, USA.
Butler
Wayne M
WM
Merrick
Gregory S
GS
Kurko
Brian S
BS
Reed
Joshua L
JL
Murray
Brian C
BC
Wallner
Kent E
KE
D000368:Aged; D000726:Androgen Antagonists / Q000627:therapeutic use; D019583:Dose Fractionation, Radiation; D004574:Electromagnetic Fields; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D009929:Organ Size / Q000528:radiation effects; D011467:Prostate / Q000473:pathology* / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000188:drug therapy / Q000473:pathology* / Q000532:radiotherapy*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012371:Robotics; D047368:Tumor Burden / Q000528:radiation effects*
D000726:Androgen Antagonists