Comparison of dose decrement from intrafraction motion for prone and supine prostate radiotherapy.
OBJECTIVE
Dose effects of intrafraction motion during prone prostate radiotherapy are unknown. We compared prone and supine treatment using real-time tracking data to model dose coverage.


METHODS
Electromagnetic tracking data were analyzed for 10 patients treated prone, and 15 treated supine, with IMRT for localized prostate cancer. Plans were generated using 0 mm, 3 mm, and 5mm PTV expansions. Manual beam-hold interventions were applied to reposition the patient when translations exceeded a predetermined threshold. A custom software application (SWIFTER) used intrafraction tracking data acquired during beam-on model delivered prostate dose, by applying rigid body transformations to the prostate structure contoured at simulation within the planned dose cloud. The delivered minimum prostate dose as a percentage of planned dose (Dmin%), and prostate volume covered by the prescription dose as a percentage of the planned volume (VRx%) were compared for prone and supine treatment.


RESULTS
Dmin% was reduced for prone treatment for 0 (p=0.02) and 3 mm (p=0.03) PTV margins. VRx% was reduced for prone treatment only for 0mm margins (p=0.002). No significant differences were found using 5 mm margins.


CONCLUSIONS
Intrafraction motion has a greater impact on target coverage for prone compared to supine prostate radiotherapy. PTV margins of 3 mm or less correlate with a significant decrease in delivered dose for prone treatment.
Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO 63110, USA.
Jeffrey R
JR
Olsen
Parag J
PJ
Parikh
Michael
M
Watts
Camille E
CE
Noel
Kenneth W
KW
Baker
Lakshmi
L
Santanam
Jeff M
JM
Michalski
D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D019583:Dose Fractionation, Radiation; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D009038:Motion; D009361:Neoplasm Invasiveness / Q000473:pathology; D009367:Neoplasm Staging; D056888:Patient Positioning / Q000379:methods; D016684:Prone Position; D011471:Prostatic Neoplasms / Q000401:mortality / Q000473:pathology / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012189:Retrospective Studies; D018570:Risk Assessment; D016683:Supine Position; D016019:Survival Analysis; D016896:Treatment Outcome