Physiologic and radiographic evidence of the distal edge of the proton beam in craniospinal irradiation.
OBJECTIVE
Fatty replacement of bone marrow resulting from radiation therapy can be seen on T1-weighted magnetic resonance (MR) images. We evaluated the radiographic appearance of the vertebral bodies in children treated with proton craniospinal irradiation (CSI) to illustrate the distal edge effect of proton radiotherapy.


METHODS
The study cohort consisted of 13 adolescents aged 12-18 years who received CSI with proton radiotherapy at Massachusetts General Hospital. Ten of these patients had reached maximal or near-maximal growth. Proton beam radiation for these 10 patients was delivered to the thecal sac and exiting nerve roots only, whereas the remaining 3 patients had a target volume that included the thecal sac, exiting nerve roots, and entire vertebral bodies. Median CSI dose was 27 [range, 23.4-36] cobalt gray equivalent (CGE) given in 1.8-CGE fractions. Magnetic resonance images of the spine were obtained after completion of radiotherapy.


RESULTS
Magnetic resonance images of patients who received proton radiotherapy to the thecal sac only demonstrate a sharp demarcation of hyperintense T1-weighted signal in the posterior aspects of the vertebral bodies, consistent with radiation-associated fatty marrow replacement. Magnetic resonance images of the patients prescribed proton radiotherapy to the entire vertebral column had corresponding hyperintense T1-weighted signal involving the entire vertebral bodies.


CONCLUSIONS
The sharp delineation of radiation-associated fatty marrow replacement in the vertebral bodies demonstrates the rapid decrease in energy at the edge of the proton beam. This provides evidence for a sharp fall-off in radiation dose and supports the premise that proton radiotherapy spares normal tissues unnecessary irradiation.
Krejcarek
Stephanie C
SC
Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
Grant
P Ellen
PE
Henson
John W
JW
Tarbell
Nancy J
NJ
Yock
Torunn I
TI
D000293:Adolescent; D001921:Brain / Q000473:pathology* / Q000528:radiation effects; D001930:Brain Injuries / Q000175:diagnosis / Q000209:etiology / Q000517:prevention & control; D016543:Central Nervous System Neoplasms / Q000175:diagnosis / Q000532:radiotherapy*; D002648:Child; D016371:Cranial Irradiation / Q000009:adverse effects; D019317:Evidence-Based Medicine; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D061766:Proton Therapy; D011522:Protons / Q000009:adverse effects*; D011832:Radiation Injuries / Q000175:diagnosis* / Q000209:etiology / Q000517:prevention & control; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D012189:Retrospective Studies; D018570:Risk Assessment; D012542:Scattering, Radiation; D013116:Spinal Cord / Q000473:pathology* / Q000528:radiation effects; D013119:Spinal Cord Injuries / Q000175:diagnosis / Q000209:etiology / Q000517:prevention & control; D016896:Treatment Outcome
D011522:Protons