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Introduction The analysis of Winston‐Lutz (WL) test for SRS QA is time consuming and subjects to human observation uncertainties. In this work, a novel template technique is described to analyze the low contrast portal images of WL test. Methods/Materials Linac based Steotactic Radiosurgery requires overall end‐to‐end accuracy of sub‐millimeter (<1mm). In our department, a BrainLab® microMLC is utilized to perform SRS procedure. A WL test is performed by imaging a lead ball of size of 5mm in diameter via a Siemens Optivue EPID imager, 1 MU is delivered and a field size of 6mm×6mm is employed for 7 different gantry/table combinations. The acquired EPID images are enhanced with a Mexican Hat Function which is a band limited filter. By using different dilation parameters, either the radiation field or the ball in the field can be enhanced. A template binary image with known radiation isocenter, representing the ideal radiation field of size of 6mm×6mm, is generated. In addition the two dimensional projection of a lead ball is created for feature localization. The defined field and ball template images are then matched with the enhanced EPID images by using normalized cross‐correlation method. The location of radiation center and mechanical isocenter can then be determined. The location difference between the beam and mechanical isocenter represents alignment accuracy for the particular field. Results This technique has been test on 151 images. The analysis of 145 images indicated alignment accuracy of better than 1mm. The review of 6 images failed the tolerance (>1mm) and needed to be realigned according to our protocol. The result is agreed with our visual observation. Conclusions By using modern image processing and analysis technique, we developed a template based algorithm to analyze WL test images for SRS quality assurance.

Purpose: Proper evaluation of a stereotactic radiosurgery (SRS) plan is challenging due to lack of a comprehensive evaluation index and variation of dose prescription. In this study, a new treatment plan index is proposed to evaluate the quality of an SRS treatment plan by incorporating planning parameters that describe tumor coverage, dose to surrounding normal tissues, and tumor dose uniformity. Method and Materials: The proposed index contains four factors. The first factor is MDPD (ratio of the maximum dose to the prescription dose), which specifies the tumor uniformity. The second factor is V100, percent tumor volume covered by 100% prescription dose. The third factor is characteristic of dose spillage to normal tissue. The fourth factor is a piecewise function used to differentiate the deviation of the treatment plans from the requirement of RTOG protocol. Thirty‐four SRS cases fulfilling the requirement of RTOG protocol were analyzed by our proposed index and Paddick's index. Two dose prescription approaches were considered for this comparison: one is to maximize the tumor coverage and maintain plan uniformity; the other is to maximize the tumor coverage and spare the surrounding normal tissue. Results: Our result indicates that the proposed index becomes negative value when the tumor coverage or PITV significantly deviates from RTOG protocol. The treatment plan index for different lesion size, irregularity and location of critical structure relative to tumor target were calculated with various techniques at two different dose prescription approaches. Conclusion: Comparison of the plan index recommended by Paddick and that proposed in this study for clinical 34 SRS cases in Baystate Medical Center indicates that our plan index is reliable and sensitive to the lesion size, irregularity, and location of critical structures and is easier for the selection of the treatment techniques and dose prescription during the process of SRS.

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