Equivalence in dose fall-off for isocentric and nonisocentric intracranial treatment modalities and its impact on dose fractionation schemes.
OBJECTIVE
To investigate whether dose fall-off characteristics would be significantly different among intracranial radiosurgery modalities and the influence of these characteristics on fractionation schemes in terms of normal tissue sparing.


METHODS
An analytic model was developed to measure dose fall-off characteristics near the target independent of treatment modalities. Variations in the peripheral dose fall-off characteristics were then examined and compared for intracranial tumors treated with Gamma Knife, Cyberknife, or Novalis LINAC-based system. Equivalent uniform biologic effective dose (EUBED) for the normal brain tissue was calculated. Functional dependence of the normal brain EUBED on varying numbers of fractions (1 to 30) was studied for the three modalities.


RESULTS
The derived model fitted remarkably well for all the cases (R(2) > 0.99). No statistically significant differences in the dose fall-off relationships were found between the three modalities. Based on the extent of variations in the dose fall-off curves, normal brain EUBED was found to decrease with increasing number of fractions for the targets, with alpha/beta ranging from 10 to 20. This decrease was most pronounced for hypofractionated treatments with fewer than 10 fractions. Additionally, EUBED was found to increase slightly with increasing number of fractions for targets with alpha/beta ranging from 2 to 5.


CONCLUSIONS
Nearly identical dose fall-off characteristics were found for the Gamma Knife, Cyberknife, and Novalis systems. Based on EUBED calculations, normal brain sparing was found to favor hypofractionated treatments for fast-growing tumors with alpha/beta ranging from 10 to 20 and single fraction treatment for abnormal tissues with low alpha/beta values such as alpha/beta = 2.
Department of Radiation Oncology, University of California, San Francisco, CA 94143, USA. lijunma@radonc.ucsf.edu
Lijun
L
Ma
Arjun
A
Sahgal
Martina
M
Descovich
Young-Bin
YB
Cho
Cynthia
C
Chuang
Kim
K
Huang
Normand J
NJ
Laperriere
Dennis C
DC
Shrieve
David A
DA
Larson
D001921:Brain / Q000528:radiation effects*; D001932:Brain Neoplasms / Q000473:pathology / Q000532:radiotherapy / Q000601:surgery*; D016371:Cranial Irradiation / Q000379:methods; D019583:Dose Fractionation, Radiation; D006801:Humans; D008962:Models, Theoretical; D011832:Radiation Injuries / Q000517:prevention & control; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods; D012062:Relative Biological Effectiveness