Analysis of dose conformity and normal-tissue sparing using two different IMRT prescription methodologies for irregularly shaped CNS lesions irradiated with the Beak and 1-cm MIMiC collimators.
OBJECTIVE
To determine whether intensity modulated sequential tomotherapy using the NOMOS Beak provides superior dose conformity and organ sparing to the MIMiC "1-cm" mode, and if so, to identify a subset of patients most likely to benefit from Beak intensity modulated sequential tomotherapy.


METHODS
Twelve patients with irregularly shaped central nervous system tumors were selected for intensity modulated radiation therapy planning. Two treatment plans, one using the Beak collimator and the other using the 1-cm MIMiC collimator, were generated for each patient with identical anatomic contouring, prescriptions, and optimization algorithms. The Beak attaches to the MIMiC collimator and truncates the 1-cm MIMiC mode beamlet size from 1.00 x 0.85 cm(2) to 1.00 x 0.39 cm(2) at isocenter. Conformity indexes were calculated for each lesion using two different prescription methodologies, and mean doses to critical structures were recorded.


RESULTS
For the first prescription methodology using uniform prescribed isodose, mean conformity index was 2.19 (range, 1.33-3.90) for the Beak compared to 2.67 (range, 1.64-4.75) for the 1-cm mode (p = 0.0003). Mean doses to the brainstem, right orbit, and left optic nerve were significantly lower with the Beak than with the 1-cm mode (p = 0.0150, 0.0068, and 0.0284, respectively). For the second prescription methodology using uniform target volume coverage prescription, mean conformity index was 2.04 (range, 1.56-2.70) for the Beak compared to 2.73 (range, 1.70-8.58) for the 1-cm mode (p = 0.07). Mean doses to the brain, brainstem, optic chiasm, right optic nerve, left optic nerve, and left orbit were significantly lower with the Beak than with the 1-cm mode (p = <0.0001, <0.0026, <0.0016, <0.0076, <0.0007, and <0.046, respectively).


CONCLUSIONS
Beak intensity modulated sequential tomotherapy is superior to the 1-cm MIMiC mode for irregularly shaped central nervous system tumors, because it provides better conformity and critical organ sparing. These differences may allow for safer dose escalation and retreatment, so the method presents an alternative to gamma knife stereotactic radiosurgery.
Zinkin
Heather D
HD
Department of Radiation Oncology, Tufts-New England Medical Center, Tufts University School of Medicine, Boston, MA 02111, USA. hzinkin@tufts-nemc.org
Rivard
Mark J
MJ
Mignano
John E
JE
Wazer
David E
DE
D000465:Algorithms; D016543:Central Nervous System Neoplasms / Q000532:radiotherapy*; D002813:Chondrosarcoma / Q000532:radiotherapy; D005909:Glioblastoma / Q000532:radiotherapy; D006801:Humans; D008545:Melanoma / Q000532:radiotherapy / Q000556:secondary; D008577:Meningeal Neoplasms / Q000532:radiotherapy; D008579:Meningioma / Q000532:radiotherapy; D009464:Neuroma, Acoustic / Q000532:radiotherapy; D010911:Pituitary Neoplasms / Q000532:radiotherapy; D011832:Radiation Injuries / Q000517:prevention & control; D011835:Radiation Protection; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation / Q000379:methods*