A quality assurance test for the validation of the spatial and dosimetric accuracy of a new technique for the treatment of multiple brain mestastases. (September 2016) - Kalaitzakis, G.
Abstract : Introduction: Radiation treatment planning, delivery and patient monitoring have been enriched with the increased use of MRI in radiotherapy clinical practice. MRI Polymer gel dosimetry has been proposed as a sensitive 3D dosimetry and QA tool. Purpose: To develop and implement a QA test for the validation of spatial and dosimetric accuracy of a recently introduced dynamic conformal arcs technique for the treatment of multiple brain metastases. Materials and methods: A 3D head avatar phantom with radiologically bone-equivalent material, filled with a sensitive polymer gel dosimeter was fabricated using anonymized CT scans of a specific patient. Irradiation was performed utilizing a single setup isocenter dynamic conformal arcs technique. A 1.5 T MRI clinical scanner was used as the reading device for dose quantification on gel material. Calculated and measured 3D dose distributions were compared in terms of spatial agreement as well as dose profiles, 3D gamma indices (5%/2 mm, 15% dose threshold), DVHs and dose-volume indices. Results: A spatial agreement within 2 mm was observed between high dose regions and gel polymerized areas on MRI. Detected offset is a combination of MR-CT registration inaccuracies, set-up errors and MRI-related geometric distortions. Comparison of relative dose profiles showed good dosimetric agreement between the two datasets, while corresponding 3D gamma index passing rate reached 90%. Dosimetric agreement was further verified in terms of DVH comparison, especially for structures lying in high dose areas. Conclusion: The 3D head avatar phantom and methods used, demonstrated efficacy in validating spatial and dosimetric accuracy of the specific radiotherapy technique.