SU‐E‐T‐282: Magnitude and Impact of Localization Error in Stereotactic Radiosurgery(SRS) Treatments

Purpose: Highly accurate target positioning in SRS is achieved by use of localizer. Several critical factors contributing to localization uncertainty were investigated. Methods: BrainLAB CT/X‐ray localizer and Head&Neck localizer were scanned in sequential mode with Siemens Somatom CT scanner with 2.4mm slice width (SW) and localized in BrainLAB iPlan 4.1 based on high density PVC rods. Localization quality was quantitatively evaluated. Manual correction was performed based on knowledge of the geometry and structure of the localizer. A head phantom was scanned with SW of 2.4mm and 1.2mm before and after an intentionally introduced 0.75 degree horizontal rotation to study localization uncertainty caused by localizer misalignment and choice of SW. Variation of the localized image slice thickness (ST) and image/contours distortion were checked to evaluate localization quality. Results: Image localization was affected significantly by copper wires in X‐ray inserts for CT/X‐ray localizer. Due to this localization error, ST varied by as much as 2mm and in‐plane shift by 0.15mm (2.4mm SW). Manual correction reduced ST variation from about ±84% to ±23% while mean ST remained the same; it also reduces in‐plane shift by 15% and 25% in the two orthogonal directions, respectively. Head&Neck localizer doesn't have X‐ray inserts, its localization uncertainty was much lower and ST varied by only ±20% without correction. The head phantom study showed that localization accuracy is affected by not only the SW but also misplacement of PVC rods due to copper wire interference. Thinner SW demonstrated higher localization quality while the 0.75 degree horizontal rotation doesn't have much effect. Conclusions: Copper wires in X‐ray inserts are used for angiographic localization; its higher density affects the localization of slices where PVC and copper are close. The resulting ST variation causes contouring inaccuracy and non‐negligible dosimetric impact on small target SRS planning. Manual correction significantly improves localization quality.