Investigation of optimal planning strategy in gamma knife perfexion for vestibular schwannoma tumor using hybrid plan technique.

Purpose.Stereotactic radiosurgery (SRS) for vestibular schwannoma (VS) is clinically challenging because of surrounding critical structures. We generated and compared the forward plan (FP), inverse plan (IP), and hybrid plan (HP) for the optimal planning strategy in Gamma Knife stereotactic radiosurgery (GKSRS) for vestibular schwannoma tumors (VS).Methods and materials. In this study, 51 planning scenarios of 17 patients with VS were planned for GKSRS using FP, IP, and HP in Leksell Gamma plan (LGP10.1) using the TMR10 algorithm. The planning images were obtained using the following MRI (GE, USA) scan parameters: T1W images-MPRAGE sequence, FOV-256 mm × 256 mm, matrix size-512 mm × 512 mm, and the slice thickness 1 mm. The total dose was prescribed12Gy and normalized at 50% isodose level.Results and discussion. The plan parameters were compared dosimetrically by maintaining FP as a base plan. The statistical analysis, including one-factor, repeated measures ANOVA and Bonferroni correction tests, were performed. The p-value for planning parameters such as brainstem dose, beam ON time, and gradient index significantly favored HP.Conclusion. Overall results show that HP is an efficient method for GKSRS of VS The p-value was less than 0.001 and statistically significant for various plan indices.

Verification of Gamma Knife extend system based fractionated treatment planning using EBT2 film.

PURPOSE: This paper presents EBT2 film verification of fractionated treatment planning with the Gamma Knife (GK) extend system, a relocatable frame system for multiple-fraction or serial multiple-session radiosurgery. METHODS: A human head shaped phantom simulated the verification process for fractionated Gamma Knife treatment. Phantom preparation for Extend Frame based treatment planning involved creating a dental impression, fitting the phantom to the frame system, and acquiring a stereotactic computed tomography (CT) scan. A CT scan (Siemens, Emotion 6) of the phantom was obtained with following parameters: Tube voltage--110 kV, tube current--280 mA, pixel size--0.5 × 0.5 and 1 mm slice thickness. A treatment plan with two 8 mm collimator shots and three sectors blocking in each shot was made. Dose prescription of 4 Gy at 100% was delivered for the first fraction out of the two fractions planned. Gafchromic EBT2 film (ISP Wayne, NJ) was used as 2D verification dosimeter in this process. Films were cut and placed inside the film insert of the phantom for treatment dose delivery. Meanwhile a set of films from the same batch were exposed from 0 to 12 Gy doses for calibration purposes. An EPSON (Expression 10000 XL) scanner was used for scanning the exposed films in transparency mode. Scanned films were analyzed with inhouse written MATLAB codes. RESULTS: Gamma index analysis of film measurement in comparison with TPS calculated dose resulted in high pass rates >90% for tolerance criteria of 1%∕1 mm. The isodose overlay and linear dose profiles of film measured and computed dose distribution on sagittal and coronal plane were in close agreement. CONCLUSIONS: Through this study, the authors propose treatment verification QA method for Extend frame based fractionated Gamma Knife radiosurgery using EBT2 film.

MAGAT gel and EBT2 film-based dosimetry for evaluating source plugging-based treatment plan in Gamma Knife stereotactic radiosurgery.

This work illustrates a procedure to assess the overall accuracy associated with Gamma Knife treatment planning using plugging. The main role of source plugging or blocking is to create dose falloff in the junction between a target and a critical structure. We report the use of MAGAT gel dosimeter for verification of an experimental treatment plan based on plugging. The polymer gel contained in a head-sized glass container simulated all major aspects of the treatment process of Gamma Knife radiosurgery. The 3D dose distribution recorded in the gel dosimeter was read using a 1.5T MRI scanner. Scanning protocol was: CPMG pulse sequence with 8 equidistant echoes, TR = 7 s, echo step = 14 ms, pixel size = 0.5mm × 0.5mm, and slice thickness of 2 mm. Using a calibration relationship between absorbed dose and spin-spin relaxation rate (R2), we converted R2 images to dose images. Volumetric dose comparison between treatment planning system (TPS) and gel measurement was accomplished using an in-house MATLAB-based program. The isodose overlay of the measured and computed dose distribution on axial planes was in close agreement. Gamma index analysis of 3D data showed more than 94% voxel pass rate for different tolerance criteria of 3%/2 mm, 3%/1 mm and 2%/2 mm. Film dosimetry with GAFCHROMIC EBT 2 film was also performed to compare the results with the calculated TPS dose. Gamma index analysis of film measurement for the same tolerance criteria used for gel measurement evaluation showed more than 95% voxel pass rate. Verification of gamma plan calculated dose on account of shield is not part of acceptance testing of Leksell Gamma Knife (LGK). Through this study we accomplished a volumetric comparison of dose distributions measured with a polymer gel dosimeter and Leksell GammaPlan (LGP) calculations for plans using plugging. We propose gel dosimeter as a quality assurance (QA) tool for verification of plug-based planning.