Brain Stereotactic Radiosurgery Planning Using the Single-Isocenter Dynamic-Arc Technique

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Brain stereotactic radiosurgery (SRS) treatment using a single isocenter for treating multiple metastases can save treatment time but poses a challenge to SRS planning using the dynamic-arc technique. The objective of this study is to develop a strategy for SRS planning using the single-isocenter dynamic-arc technique. We hypothesize that the single-isocenter dynamic-arc technique can produce satisfactory SRS plans for treating multiple targets and save treatment time. We have developed a planning strategy for single-isocenter dynamic-arc technique and used it clinically since February 2015. In this technique, the targets are first split into small batches so that all targets in a batch are within 5 cm of each other. The center of gravity of all targets in a batch is the isocenter, with which multiple treatment groups are associated. Each treatment group typically consists of 3 dynamic-arc beams to irradiate 1-3 targets. For each arc, the collimator angle is adjusted to minimize the “island” between targets that cannot be blocked with the multileaf collimators (MLCs). Each treatment group is normalized so that the maximal dose is 125% of the prescription dose. If the maximal dose to a target is less than 123%, an additional arc is used to boost the maximal dose of that target to 125%. To evaluate the plan quality we randomly selected 5 cases planned with this technique. There were totally 21 planning target volumes (PTVs), and 13 isocenters were used to treat all these targets. We replanned them using multiple isocenters, 1 isocenter for each target. The prescription for each target was 20 Gy with a maximal dose of 25 Gy, which was equivalent to 20 Gy to 80% isodose line. Plan quality indexes including PTV coverage, mean dose, V12Gy, conformity index (CI), and V50%/VPTV were compared. Satisfactory PTV coverage (V100%=95% and V95%=100%) were achieved for all plans using either technique. Most PTVs have a maximal dose between 24.9 and 25.1 Gy, with 2 PTV between 24.5 and 24.9 Gy. The ratios of mean dose and CI between the single- and multiple-isocenter techniques were respectively 1.05±0.035 and 1.06±0.0, indicating similar plan quality globally and in the high dose area. The difference was more pronounced for the mid-to-low dose spillage with the ratios of V12Gy and V50%/VPTV being 1.12±0.055 and 1.14±0.045, respectively. Overall, the plan quality was slightly better for the multi-isocenter technique because the single-isocenter technique uses MLCs instead of jaws to block the radiation between targets and thus has a higher leakage dose. The treatment time was reduced by 30%-50% with the single-isocenter technique. The single-isocenter dynamic-arc technique can produce satisfactory SRS plans for treating multiple targets and can significantly reduce the treatment time. This time saving can be further improved using a larger allowable distance between targets. However, this tactic might worsen the rotational error to an unacceptable level.