Is a single isocenter sufficient for volumetric modulated arc therapy radiosurgery when multiple itracranial metastases are spatially dispersed?
Previous work demonstrated improved dosimetry of single isocenter volumetric modulated arc therapy (VMAT) of multiple intracranial targets when they are located ≤ 4cm from isocenter because of narrower multileaf collimators (MLCs). In follow-up, we sought to determine if decreasing isocenter-target distance (diso) by using 2 to 3 isocenters would improve dosimetry for spatially dispersed targets. We also investigated the effect of a maximum dose constraint during VMAT optimization, and the dosimetric effect of the number of VMAT arcs used for a larger number of targets (i.e., 7 to 9). We identified radiosurgery cases that had multiple intracranial targets with disoof at least 1 target > 5cm. A single isocenter VMAT plan was created using a standardized 4-arc technique with 18Gy per target. Each case was then replanned (1) using 2 to 3 isocenters, (2) including a maximum dose constraint per target, and in the case of 7 to 9 targets, (3) using 3 to 6 arcs. Dose evaluation included brain V6Gyand V12Gy, and conformity index (CI), gradient index (GI), and heterogeneity index (HI) per target. Two isocenters were sufficient to limit disoto ≤ 4cm and ≤ 5cm for 11/15 and 13/15 cases, respectively; after replanning with 2 to 3 isocenters, disodecreased from 5.8 ± 2.8cm (2.3 14.9) to 2.5 ± 1.4cm (0 5.2). All dose statistics improved on average, albeit modestly: V6Gy= 6.9 ± 7.1%, V12Gy= 0.9% ± 4.4%, CI = 2.6% ± 4.6%, GI = 0.9% ± 12.7%, and HI = 2.6% ± 5.2%; however, the number of arcs doubled and monitor units increase by nearly 2-fold. A maximum dose constraint had a negative effect on all dose indices, increasing V12Gyby 9.7 ± 6.9%. For ≥ 7 targets, increasing number of arcs to > 3 improved CI, V12Gy, and V6Gy. A single isocenter is likely sufficient for VMAT radiosurgery of multiple intracranial metastases. Optimal treatment plan quality is achieved when no constraint is placed on the maximum target dose; for cases with many targets at least 4 arcs are needed for optimal plan quality.
Department of Radiation Oncology, Duke University Medical Center, Durham, NC.
Jay
J
Morrison
Department of Radiation Oncology, Duke University Medical Center, Durham, NC.
Rodney
R
Hood
Department of Radiation Oncology, Duke University Medical Center, Durham, NC.
Fang-Fang
FF
Yin
Department of Radiation Oncology, Duke University Medical Center, Durham, NC.
Joseph K
JK
Salama
Department of Radiation Oncology, Duke University Medical Center, Durham, NC.
John
J
Kirkpatrick
Department of Radiation Oncology, Duke University Medical Center, Durham, NC. Electronic address: justus.adamson@duke.edu.
Justus
J
Adamson
D001932:Brain Neoplasms / Q000532:radiotherapy* / Q000556:secondary*; D006801:Humans; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012189:Retrospective Studies
Intracranial; Radiosurgery; Single isocenter; VMAT