3-DIMENSIONAL VISUALIZATION AND MEASUREMENT OF CONFORMAL DOSE DISTRIBUTIONS USING MAGNETIC-RESONANCE-IMAGING OF BANG POLYMER GEL DOSIMETERS

Purpose/Objective: The measurement of complex dose distributions (thos e created by irradiation through multiple beams, multiple sources, or multiple source dwell positions) requires a dosimeter that can integra te the dose during a complete treatment. Integrating dosimeter devices generally are capable of measuring only dose at a point (ion chamber, diode, TLD) or in a plane (film). With increasing use of conformal do se distributions requiring shaped, noncoplanar beams, there will be an increased requirement for a dosimeter that can record and display a 3 D dose distribution. The use of a 3D dosimeter will be required to con firm the accuracy of treatment plans produced by the current generatio n of 3D treatment-planning computers. Methods and Materials: The use o f a Fricke-infused gel and magnetic resonance imaging (MRI) to demonst rate the localization of stereotactic beams has been demonstrated (11) . The recently developed BANG polymer gel dosimetry system (MGS Resear ch, Inc., Guilford, CT), based on radiation-induced chain polymerizati on of acrylic monomers dispersed in a tissue-equivalent gel, surpasses the Fricke-gel method by providing accurate, quantitative dose distri bution data that do not deteriorate with time (6, 9). The improved BAN G2 formulation contains 3% N,N'-methylene-bisacrylamide, 3% acrylic ac id, 1% sodium hydroxide, 5% gelatin, and 88% water, where all percenta ges are by weight. The gel was poured into volumetric flasks, of dimen sions comparable to a human head. The gels were irradiated with comple x beam arrangements, similar to those used for conformal radiation the rapy. Images of the gels were acquired using a Siemens 1.5T imager and a Hahn spin-echo pulse sequence (90 degrees-tau-180 degrees-tau-acqui re, for different values of tau). The images were transferred via netw ork to a Macintosh computer for which a data analysis and display prog ram was written. The program calculates R2 maps on the basis of multip le TE images, using a monoexponential nonlinear least-squares fit base d on the Levenberg-Marquardt algorithm. The program also creates a dos e-to-R2 calibration function by fitting a polynomial to a set of dose and R2 data points, obtained from gels irradiated in test tubes to kno wn doses. This function can then be applied to any other R2 map, so th at a dose map can be computed and displayed. Results: Through exposure to known doses of radiation, the gel has been shown to respond linear ly,vith dose in the range of 0 to 10 Gy, and its response is independe nt of the beam energy or modality. Dose distributions have been imaged in orthogonal planes, and can be displayed in a convenient form for c omparison with isodose plans. The response of the gel is stable; the g el can be irradiated at any time after its manufacture, and imaging ca n be conducted any time following a brief interval after irradiation. Conclusion: The polymer gel dosimeter has been shown to be a valuable device for displaying three-dimensional dose distributions. The imaged dose distribution can be compared easily with calculated dose distrib utions, to validate a treatment planning system. In the future, gels m ay be prepared in anthropomorphic phantoms, to confirm unique patient dose distributions. (C) 1997 Elsevier Science Inc.