Radiation dose to the lens during craniospinal irradiation-an improvement in proton radiotherapy technique.
OBJECTIVE
To evaluate the effect of angle modification of cranial field proton beam therapy on the radiation dose delivered to the lens during craniospinal irradiation (CSI).


METHODS
Thirty-nine patients with central nervous system tumors who received CSI with a posterior fossa boost were analyzed for the radiation dose to the lens. Thirteen patients received cranial field treatment using standard opposed-lateral proton beams, and 26 patients received treatment with angled posterior-oblique proton beams. The lens dose in a test case also was evaluated by comparing conventional X-rays with the two proton beam planning methods by using a CMS/Xio three-dimensional planning system.


RESULTS
Substantial lens dose sparing was realized with the angling of the cranial proton beams 15 degrees -20 degrees to the posterior. In the 39 treated patients who were analyzed (median age, 7 years), average dose delivered to the lens was decreased by approximately 50% by angling of the proton beams, with the average maximum dose decreasing from 74% to 40% of the prescribed dose (p < 0.0001). Significant lens sparing was seen in patients 10 years and younger (median age, 6 years; p < 0.0001), whereas an insignificant decrease was seen in older patients (median age, 16 years; p = 0.14). With the opposed-lateral technique (median age, 6 years), the lens dose increased significantly with decreasing age (p = 0.002), whereas there was no effect of age on lens dose in the angled beam-treated group (median age, 8.5 years; p = 0.73).


CONCLUSIONS
The present study clearly shows an advantage in sparing of the lens dose by angling the beams used during proton beam CSI. This effect is most pronounced in patients 10 years and younger because of anatomic effects of sinus development.
Cochran
David M
DM
Division of Health Sciences and Technology, Harvard Medical School and Massachusetts Institute of Technology, Cambridge, MA, USA.
Yock
Torunn I
TI
Adams
Judith A
JA
Tarbell
Nancy J
NJ
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D016543:Central Nervous System Neoplasms / Q000532:radiotherapy*; D002528:Cerebellar Neoplasms / Q000532:radiotherapy; D002648:Child; D002675:Child, Preschool; D016371:Cranial Irradiation / Q000379:methods*; D005260:Female; D006801:Humans; D007908:Lens, Crystalline / Q000528:radiation effects*; D008297:Male; D008527:Medulloblastoma / Q000532:radiotherapy; D061766:Proton Therapy; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control; D011522:Protons
D011522:Protons