Application of Optical CT Scanning in Three-Dimensional Radiation Dosimetry

There has been much research effort in the development of an accurate and reliable threedimensional dose verification system, prompted by the advances of technologies in radiation treatment of cancer patients. New radiotherapy techniques such as intensity modulated radiation therapy (IMRT), stereotactic radiosurgery (SRS) and high dose rate (HDR) brachytherapy are aimed at dose deliveries that are highly localized within the tumor volumes. The dose distributions from these treatment methods are typically characterized with high dose gradients around the boundaries of the targets, which present certain challenges for one or two-dimensional dosimeters such as film, TLD, and ion chamber (Webb, 2001). In recent years, gel dosimeter has emerged as a promising candidate for 3D dosimetry (Baldock et al, 2010). Extensive studies have been done on the development of different types of gel formulas (Maryanski et al, 1996; Baldock et al, 1998; Pappas et al, 1999; Fong et al, 2001; Adamovics and Maryanski, 2006). Between the two classes of gel dosimeters that have been studied so far, the radiochromic Fricke gel is easier to make and handle than the polymer gel, but it suffers from a major error of diffusion. The blurring of the dose distribution image within time as short as an hour makes it inconvenient to be implemented into clinics. The polymer gel has the advantage of preserving the spatial accuracy of the dose distribution, but is sensitive to oxygen contamination and thus needs to be made freshly before each measurement. Furthermore, both types of gels suffer from a long-term drift of the baseline optical density, either due to self-oxidation of the ferrous ions in the Fricke gel or polymerization initiated by the redox reactions in the polymer gel. The readout of gel dosimeters was initially conducted by magnetic-resonance imaging (MRI) (Gore et al, 1984; Maryanski et al, 1993; Audet and Schreiner 1997; De Deene et al, 1998; Low et al, 1999; Lepage et al, 2002) and subsequently extended to other imaging modalities such as optical computed tomography (CT) (Gore et al, 1996; Kelly et al, 1998; Doran et al, 2001; Xu et al, 2003), X-ray CT (Hilts et al, 2000) and ultrasound (Mather and Baldock, 2003). The idea of optical CT was first introduced to gel dosimetry in 1996 (Gore et al, 1996) together with a new tissue equivalent polymer gel dosimeter (Maryanski et al, 1996). Since then, optical CT scanners in a variety of forms have been built for imaging gel phantoms irradiated with photon, electron, proton beams as well as brachytherapy radiation sources (Islam et al, 2003; Wuu et al, 2003; Oldham and Kim, 2004; Xu et al, 2004; Doran et al, 2006; DeJean et al, 2006;