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Background and purpose The strong magnetic field of integrated magnetic resonance imaging (MRI) and radiation treatment systems influences secondary electrons resulting in changes in dose deposition in three dimensions. To fill the need for volumetric dose quality assurance, we investigated the effects of strong magnetic fields on 3D dosimeters for MR-image-guided radiation therapy (MR-IGRT) applications. Material and methods There are currently three main categories of 3D dosimeters, and the following were used in this study: radiochromic plastic (PRESAGE®), radiochromic gel (FOX), and polymer gel (BANG™). For the purposes of batch consistency, an electromagnet was used for same-day irradiations with and without a strong magnetic field ( B 0 , 1.5 T for PRESAGE® and FOX and 1.0 T for BANG™). Results For PRESAGE®, the percent difference in optical signal with and without B 0 was 1.5% at the spectral peak of 632 nm. For FOX, the optical signal percent difference was 1.6% at 440 nm and 0.5% at 585 nm. For BANG™, the percent difference in R 2 MR signal was 0.7%. Conclusions The percent differences in responses with and without strong magnetic fields were minimal for all three 3D dosimeter systems. These 3D dosimeters therefore can be applied to MR-IGRT without requiring a correction factor. [ABSTRACT FROM AUTHOR]