Phantom Modification to Study Multiple Matrices Planning Using Matlab Software in Gammaknife

GAMMAKNIFE TREATMENT PLAN DOSE ANALYSIS BY GAFCHROMIC FILM USING MATLAB SOFTWARE

Gopishankar.N, R.K.Bisht, E.B.Rajmohan, V.Subramani, T.Ganesh, Pratikumar, SS.Kale,

G.K.Rath, A.K.Mahapatra,All India Institute of Medical Sciences, NewDelhi ,India.

Background:GammaKnife (GK) is the gold standard for Stereotactic Radiosurgery in treating

intracranial lesions.

Objective:To perform GK treatment planning verification using Gafchromic film in spherical polystyrene phantom with MATLAB Software.

Methods:Polystyrene phantom (PS),spherical ,16cm in diameter was used for this study which is usually fixed with some special plastic(acrylic) holders ,that is set to the collimator helmet. Routine fixing of phantom is such that it can be changed either in X or Z axis. We found a method of fixing the PS phantom in the Leksell frame to perform measurements in the phantom in any orientation. The frame with the phantom was fixed such that film cassette could be placed and removed with ease. We used carbon posts, two short and two long for this positioning. Instead of titanium screws we designed our own acrylic screws for fixing on patient to avoid artifacts in the CT images.CT indicator box with small rectangular copper strips embedded on all sides served as fiducial defining tool. For CT scanning, Somatom Volume zoom with Head Sequential RT protocol was used. Phantom was positioned in the frame with CT laser passing through the film cassette, placed in the middle. Film to be used for dosimetry was not placed in the film cassette to avoid amount of radiation getting absorbed in the film.

Small pinhead sized copper bit was placed in the middle of film cassette as reference marker for shot placement during planning. Image slices of 1mm thickness were generated and were transferred to LGP system for planning. Multiple matrix planning using 8mm collimators was done. Reference dose of 25Gy was prescribed at the periphery. In addition to the 50% isodose line, lower isodose levels were selected for analyzing the penumbral region.Gafchromic XRtypeR film was placed inside the film cassette and inserted inside the modified phantom. Subsequently the film was positioned for each shot and exposed for the time calculated by LGP system. Exposed film was scanned with a Flatbed scanner model Scanjet 3300 C. Before utilizing the film for multiple matrices planning analysis, a calibration procedure was carried out where film pieces were cut and exposed to known doses. Each film was scanned five times to minimize the variation (scanner noise) using the flat bed document scanner. The scanned images were analyzed as Pixel value (or) net O.D using MATLAB 6.5 software.

Results: The film for multiple matrix planning was placed in the PS phantom and doses in every matrix were verified using this graph. Hence the dose registered in the film was well correlating with the LGP TPS values. Contour plotting was done for the film in which multiple matrix planning was performed using MATLAB software. The isodose lines pattern obtained in LGP with scanned CT images and radiation exposed film analyzed in MATLAB software were similar in shape.

Conclusions:The existing solid polystyrene phantom can be fixed in the Leksell frame which was not possible earlier. It can also be used as isocentric tool to perform film dosimetry.

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Figure.1

A calibration graph for the exposed films was plotted. The results were compared

with the LGP TPS and found to be well correlating

Figure.2

Contour plotting was done for theGafchromic film in which multiple matrix GK

planning was analyzed using MATLAB version 6.5.The shape obtained with the

radiation exposed film was similar in pattern to the isodose pattern in LGP.