OPTICAL CT RECONSTRUCTION OF 3D DOSE DISTRIBUTIONS USING THE FERROUS-BENZOIC-XYLENOL (FBX) GEL DOSIMETER

In recent years, magnetic-resonance imaging of gelatin doped with the Fricke solution has been applied to the direct measurement of three-di mensional (3D) radiation dose distributions. However, the 3D dose dist ribution can also be imaged more economically and efficiently using th e method of optical absorption computed tomography. This is accomplish ed by first preparing a gelatin matrix containing a radiochromic dye a nd mapping the radiation-induced local change in the optical absorptio n coefficient. Ferrous-Benzoic-Xylenol (FBX) was the dye of choice for this investigation. The complex formed by Fe3+ and xylenol orange exh ibits a linear change in optical attenuation (cm(-1)) with radiation d ose in the range between 0 and 1000 cGy, and the local concentration o f this complex can be probed using a green laser light (lambda=543.5 n m). An optical computed tomography (CT) scanner was constructed analog ous to a first-generation x-ray CT scanner, using a He-Ne laser, photo diodes, and rotation-translation stages controlled by a personal compu ter. The optical CT scanner itself can reconstruct attenuation coeffic ients to a baseline accuracy of <2% while yielding dose images accurat e to within 5% when other uncertainties are taken into account. Optica l tomography is complicated by the reflection and refraction of light rays in the phantom materials, producing a blind spot in the transmiss ion profiles which, results in a significant dose artifact in the reco nstructed images. In this report we develop corrections used to reduce this artifact and yield accurate dosimetric maps. We also report the chemical reaction kinetics, the dose sensitivity and spatial resolutio n (<1 mm(3)) obtained by optical absorption computed tomography. The a rticle concludes with sample dose distributions produced by ''cross-fi eld'' 6 MV x-ray beams, including a radiosurgery example. (C) 1998 Ame rican Association of Physicists in Medicine.