In vivo proton beam range verification using spine MRI changes.
OBJECTIVE
In proton therapy, uncertainty in the location of the distal dose edge can lead to cautious treatment plans that reduce the dosimetric advantage of protons. After radiation exposure, vertebral bone marrow undergoes fatty replacement that is visible on magnetic resonance imaging (MRI). This presents an exciting opportunity to observe radiation dose distribution in vivo. We used quantitative spine MRI changes to precisely detect the distal dose edge in proton radiation patients.


METHODS
We registered follow-up T1-weighted MRI images to planning computed tomography scans from 10 patients who received proton spine irradiation. A radiation dose-MRI signal intensity curve was created using the lateral beam penumbra in the sacrum. This curve was then used to measure range errors in the lumbar spine.


RESULTS
In the lateral penumbra, there was an increase in signal intensity with higher dose throughout the full range of 0-37.5 Gy (RBE). In the distal fall-off region, the beam sometimes appeared to penetrate farther than planned. The mean overshoot in 10 patients was 1.9 mm (95% confidence interval, 0.8-3.1 mm), on the order of the uncertainties inherent to our range verification method.


CONCLUSIONS
We have demonstrated in vivo proton range verification using posttreatment spine MRI changes. Our analysis suggests the presence of a systematic overshoot of a few millimeters in some proton spine treatments, but the range error does not exceed the uncertainty incorporated into the treatment planning margin. It may be possible to extend our technique to MRI sequences that show early bone marrow changes, enabling adaptive treatment modification.
Gensheimer
Michael F
MF
Vanderbilt University School of Medicine, Nashville, TN 02114, USA.
Yock
Torunn I
TI
Liebsch
Norbert J
NJ
Sharp
Gregory C
GC
Paganetti
Harald
H
Madan
Neel
N
Grant
P Ellen
PE
Bortfeld
Thomas
T
D000293:Adolescent; D000328:Adult; D001853:Bone Marrow / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects*; D002528:Cerebellar Neoplasms / Q000532:radiotherapy*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008527:Medulloblastoma / Q000532:radiotherapy*; D061766:Proton Therapy; D011879:Radiotherapy Dosage; D012189:Retrospective Studies; D013125:Spinal Neoplasms / Q000532:radiotherapy*; D013131:Spine / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects*; D014057:Tomography, X-Ray Computed; D055815:Young Adult; D011522:Protons
D011522:Protons