Image registration of BANG (R) gel dose maps for quantitative dosimetry verification

Background: The BANG(R) (product symbol SGEL, MGS Research Inc., Guilford, CT) polymer gel has been shown to be a valuable dosimeter for determining t hree-dimensional (3D) dose distributions. Because the proton relaxation rat e (R2) of the gel changes as a function of absorbed dose, MR scans of the i rradiated 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 d istribution. This has limited its quantitative use, as no precise correlati on exists with the planned distribution. This paper reports on a technique for providing this correlation, thus providing a quality assurance tool tha t includes all of the steps of imaging, treatment planning, dose calculatio n, and treatment localization. Methods and Materials: The BANG(R) gel formulation was prepared and poured into spherical flasks (15.3-cm inner diameter). A stereotactic head ring wa s attached to each flask. Three magnetic resonance imaging (MRI) and comput ed tomography (CT) compatible fiducial markers were placed on the flask, th us defining the central axial plane. A high-resolution CT scan was obtained of each Bask. These images were transferred to a radiosurgery treatment-pl anning program, where treatment plans were developed. The gels were irradia ted using our systems for stereotactic radiosurgery or fractionated stereot actic 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 s can through a rigid body match of the MRI/CT-compatible fiducial markers. T he fused dose maps were imported into the treatment-planning system for qua ntitative comparison with the calculated treatment plans. Results: 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 l-mm dose grid. All points within the dose distribution were well wit hin the tolerances set forth for commissioning and quality assurance of ste reotactic treatment-planning systems. Moreover, the quantitative evaluation presented here tests the accuracy of the entire treatment-planning and del ivery process, including stereotactic frame rigidity, CT localization, CT/M R correlation, dose calculation, and radiation delivery. Conclusion: BANG(R) polymer gel dosimetry coupled with image correlation pr ovides quantitative verification of the accuracy of 3D dose distributions. Such quantitative evaluation is imperative to ensure the high quality of th e 3D dose distributions generated and delivered by stereotactic and other c onformal irradiation systems. (C) 1999 Elsevier Science Inc.