Dose verification of single shot gamma knife applications using VIPAR polymer gel and MRI

Abstract

This work describes an experimental procedure with potential to assess the overall accuracy associated with gamma knife clinical applications, from patient imaging and dosimetry planning to patient positioning and dose delivery using the automated positioning system of a Leksell Gamma Knife model C. The VIPAR polymer gel-MRI dosimetry method is employed due to its inherent three-dimensional feature and linear dose response over the range of gamma knife applications. Different polymer gel vials were irradiated with single shot gamma knife treatment plans using each of the four available collimator helmets to deliver a maximum dose of 30 Gy. Percentage relative dose results are presented not only in the form of one-dimensional profiles but also planar isocontours and isosurfaces in three dimensions. Experimental results are compared with corresponding Gammaplan treatment planning system calculations as well as acceptance test radiochromic film measurements. A good agreement, within the experimental uncertainty, is observed between measured and expected dose distributions. This experimental uncertainty is of the order of one imaging pixel in the MRI gel readout session (<1 mm) and allows for the verification of single shot gamma knife applications in terms of acceptance specifications for precision in beam alignment and accuracy. Averaging net R(2) results in the dose plateau of the 4 mm and 18 mm collimator irradiated gel vials, which were MR scanned in the same session, provides a crude estimate of the 4 mm output factor which agrees within errors with the default value of 0.870.

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

“…A 9% methacrylic‐based gel was found to give linear doses response up to 40 Gy and is ideal for performing radiosurgery verification (31) . Several studies exist for Gamma Knife verification with polymer gel dosimetry 3 , 13 , 31 , 32 , 33 , 34 , 35 , 36 . Interestingly, 3D measurement based on gel dosimetry has not been used so far for plug planning verification which is the goal of this study.…”

Mechanical and dose delivery accuracy evaluation in radiosurgery using polymer gels

“…The maximum dose delivered to the center of each shot, which also corresponds to the maximum dose measured at each target, was determined by averaging the values of a number of voxels $(>\hspace{0.17em}20)$ in the dose plateau region of each shot, (10) and it is also presented in Table 1. Comparison with corresponding TPS calculations (20 Gy for PABIG and 30 Gy for VIPAR) shows that measured dose values are lower by 9%, on average, for both gel formulations.…”

“…( 10 , 11 ) Major points in the manufacturing procedure include the purification of monomers before preparation and the implementation of a thorough deoxygenating procedure necessary to ensure the minimum possible oxygen presence in the gel. Both gel formulations exhibit comparable dose sensitivity and a wide linear dose‐response region extending up to about 40 Gy.…”

“…( 10 , 11 , 14 ) Three markers, which are circular stickers of 1.5 cm diameter and 0.3 cm thickness with a circular hollow center of 0.5 cm diameter (MM 3005, IZI Medical Products Corp., Baltimore, MA), were adhered on predetermined positions of the gel vial using a custom‐built aluminum tool in order to establish a 3D coordinate system of reference. This coordinate system can be reconstructed in both the irradiation planning and readout phases of the experiment and thus facilitates the comparison of planned and measured dose distributions.…”

Characterization of a new polymer gel for radiosurgery dosimetry using Magnetic Resonance Imaging

“…Corresponding HDR brachytherapy data for the original VIPAR formulation [5] are also presented for comparison. Both data sets for the VIP formulation present Figure 1.…”