Planning target margin calculations for prostate radiotherapy based on intrafraction and interfraction motion using four localization methods.
OBJECTIVE
To determine planning target volume (PTV) margins for prostate radiotherapy based on the internal margin (IM) (intrafractional motion) and the setup margin (SM) (interfractional motion) for four daily localization methods: skin marks (tattoo), pelvic bony anatomy (bone), intraprostatic gold seeds using a 5-mm action threshold, and using no threshold.


METHODS
Forty prostate cancer patients were treated with external radiotherapy according to an online localization protocol using four intraprostatic gold seeds and electronic portal images (EPIs). Daily localization and treatment EPIs were obtained. These data allowed inter- and intrafractional analysis of prostate motion. The SM for the four daily localization methods and the IM were determined.


RESULTS
A total of 1532 fractions were analyzed. Tattoo localization requires a SM of 6.8 mm left-right (LR), 7.2 mm inferior-superior (IS), and 9.8 mm anterior-posterior (AP). Bone localization requires 3.1, 8.9, and 10.7 mm, respectively. The 5-mm threshold localization requires 4.0, 3.9, and 3.7 mm. No threshold localization requires 3.4, 3.2, and 3.2 mm. The intrafractional prostate motion requires an IM of 2.4 mm LR, 3.4 mm IS and AP. The PTV margin using the 5-mm threshold, including interobserver uncertainty, IM, and SM, is 4.8 mm LR, 5.4 mm IS, and 5.2 mm AP.


CONCLUSIONS
Localization based on EPI with implanted gold seeds allows a large PTV margin reduction when compared with tattoo localization. Except for the LR direction, bony anatomy localization does not decrease the margins compared with tattoo localization. Intrafractional prostate motion is a limiting factor on margin reduction.
Beltran
Chris
C
Department of Radiation Oncology, Mayo Clinic College of Medicine, Rochester, MN, USA. chris.beltran@stjude.org
Herman
Michael G
MG
Davis
Brian J
BJ
D019583:Dose Fractionation, Radiation; D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D010384:Pelvic Bones / Q000033:anatomy & histology; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D019736:Prostheses and Implants
D006046:Gold