Three-dimensional dosimetry using polymer gel and magnetic resonance imaging applied to the verification of conformal radiation therapy in head-and-neck cancer

Background and purpose: It was our aim to investigate NMR-based BANG gel do simetry as a three-dimensional dosimetry technique in conformal radiotherap y. Materials and methods: The BANG gel consisting of gelatin, water and co-mon omers was first validated in a cylindrical glass flask for a single standar d beam. Next, the gel contained in a human neck-shaped cast was used to ver ify a treatment plan for the conformal irradiation of a concave tumour in t he lower neck. Magnetic resonance relaxation rate images were acquired and, based on an appropriate calibration of the gel, converted to absorbed dose distributions. The resulting maps were compared with dose distributions me asured using radiographic film. Results: The gel-measured dose profiles of standard beams agreed within 3% (root mean square difference) with the profiles measured with high spatial resolution by a diamond detector. For the multi-beam conformal treatment, t he difference map between gel-measured and him-measured dose distributions revealed a noise component and a more systematic deviation including struct ural or space-coherent patterns. The mean absolute value of the difference amounted to 8%, A number of possible causes for this deviation are designat ed. Conclusions: Polymer gel dosimetry in combination with magnetic resonance i maging is a promising method for dosimetric verification of conformal radio therapy. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.