Image registration of BANG gel dose maps for quantitative dosimetry verification

The BANG (product symbol SGEL, MGS Research Inc., Guilford, CT) polymer gel has been shown to be a valuable dosimeter for determining three-dimensional (3D) dose distributions. Because the proton relaxation rate (R2) of the gel changes as a function of absorbed dose, MR scans of the irradiated gel can be used to generate 3D dose maps. Previous work with the gel, however, has not relied on precise localization of the measured dose distribution. This has limited its quantitative use, as no precise correlation exists with the planned distribution. This paper reports on a technique for providing this correlation, thus providing a quality assurance tool that includes all of the steps of imaging, treatment planning, dose calculation, and treatment localization.

The BANG gel formulation was prepared and poured into spherical flasks (15.3-cm inner diameter). A stereotactic head ring was attached to each flask. Three magnetic resonance imaging (MRI) and computed tomography (CT) compatible fiducial markers were placed on the flask, thus defining the central axial plane. A high-resolution CT scan was obtained of each flask. These images were transferred to a radiosurgery treatment-planning program, where treatment plans were developed. The gels were irradiated using our systems for stereotactic radiosurgery or fractionated stereotactic radiotherapy. The gels were MR imaged, and a relative 3D dose map was created from an R2 map of these images. The dose maps were transferred to an image-correlation program, and then fused to the treatment-planning CT scan through a rigid body match of the MRI/CT-compatible fiducial markers. The fused dose maps were imported into the treatment-planning system for quantitative comparison with the calculated treatment plans.

Calculated and measured isodose surfaces agreed to within 2 mm at the worst points within the in-plane dose distributions. This agreement is excellent, considering that the pixel resolution of the MRI dose maps is 1.56 x 1.56 mm, and the treatment-planning dose distributions were calculated on a 1-mm dose grid. All points within the dose distribution were well within the tolerances set forth for commissioning and quality assurance of stereotactic treatment-planning systems. Moreover, the quantitative evaluation presented here tests the accuracy of the entire treatment-planning and delivery process, including stereotactic frame rigidity, CT localization, CT/MR correlation, dose calculation, and radiation delivery.

BANG polymer gel dosimetry coupled with image correlation provides quantitative verification of the accuracy of 3D dose distributions. Such quantitative evaluation is imperative to ensure the high quality of the 3D dose distributions generated and delivered by stereotactic and other conformal irradiation systems.