Influence of intrafraction motion on margins for prostate radiotherapy.
OBJECTIVE
To assess the impact of intrafraction intervention on margins for prostate radiotherapy.


METHODS
Eleven supine prostate patients with three implanted transponders were studied. The relative transponder positions were monitored for 8 min and combined with previously measured data on prostate position relative to skin marks. Margins were determined for situations of (1) skin-based positioning, and (2) pretreatment transponder positioning. Intratreatment intervention was simulated assuming conditions of (1) continuous tracking, and (2) a 3-mm threshold for position correction.


RESULTS
For skin-based setup without and with inclusion of intrafraction motion, prostate treatments would have required average margins of 8.0, 7.3, and 10.0 mm and 8.2, 10.2, and 12.5 mm, about the left-right, anterior-posterior, and cranial-caudal directions, respectively. Positioning by prostate markers at the start of the treatment fraction reduced these values to 1.8, 5.8, and 7.1 mm, respectively. Interbeam adjustment further reduced margins to an average of 1.4, 2.3, and 1.8 mm. Intrabeam adjustment yielded margins of 1.3, 1.5, and 1.5 mm, respectively.


CONCLUSIONS
Significant reductions in margins might be achieved by repositioning the patient before each beam, either radiographically or electromagnetically. However, 2 of the 11 patients would have benefited from continuous target tracking and threshold-based intervention.
Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48109-0010, USA. litzen@umich.edu
Dale W
DW
Litzenberg
James M
JM
Balter
Scott W
SW
Hadley
Howard M
HM
Sandler
Twyla R
TR
Willoughby
Patrick A
PA
Kupelian
Lisa
L
Levine
D000465:Algorithms; D019583:Dose Fractionation, Radiation; D055590:Electromagnetic Phenomena / Q000295:instrumentation* / Q000379:methods; D006046:Gold / Q000008:administration & dosage; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000175:diagnosis* / Q000532:radiotherapy*; D019736:Prostheses and Implants; D011859:Radiography; D020266:Radiotherapy, Conformal / Q000379:methods; D012189:Retrospective Studies; D016683:Supine Position; D013997:Time Factors
D006046:Gold