Scholars@Duke publication: Is a single isocenter sufficient for volumetric modulated arc therapy radiosurgery when multiple itracranial metastases are spatially dispersed?

Is a single isocenter sufficient for volumetric modulated arc therapy radiosurgery when multiple itracranial metastases are spatially dispersed?

Publication , Journal Article

Morrison, J; Hood, R; Yin, F-F; Salama, JK; Kirkpatrick, J; Adamson, J

Published in: Med Dosim

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 diso of 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 V6Gy and V12Gy, and conformity index (CI), gradient index (GI), and heterogeneity index (HI) per target. Two isocenters were sufficient to limit diso to ≤ 4cm and ≤ 5cm for 11/15 and 13/15 cases, respectively; after replanning with 2 to 3 isocenters, diso decreased 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 V12Gy by 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.