LEGO-type Multi-purpose Dosimetry Phantom for Intensity-Modulated Radiation Therapy (IMRT) Dose Verification

초록

In the present study, a LEGO-type multi-purpose dosimetry phantom was developed for intensity-modulated radiation therapy (IMRT) dosimetry. Polystyrene, polyethylene, Teflon, and polyurethane foam were selected for muscle, fat, bone, and lung tissue, respectively, after considering mass density, atomic composition, effective atomic number, and interaction coefficients. The phantom is composed of a number of small pieces, similar to LEGO pieces, and it is possible to construct different designs of phantoms as necessary. Inhomogeneities of different tissue materials or air cavities can be inserted in the phantom. The phantom also includes dosimeter holders for several different types of dosimeters including films frequently used in IMRT dosimetry. The developed phantom was used to determine the variation of measured dose as a function of the field size for two different dosimeters (diode detector and ion chamber). Our results show that these two dosimeters do not agree well if the field size is less than 1.5 × 1.5 cm2 for polystyrene and Teflon, or less than 3 × 3 cm2 for air cavity. The discrepancy was as large as ~50% for air cavity when the field size was 0.7 × 0.7 cm2, which clearly shows the challenges in IMRT small field dosimetry. The LEGO-type phantom was developed for IMRT small field dosimetry, but it can also be used for one- and two-dimensional dosimetry studies in other conventional techniques when the heterogeneity of tissue is a critical issue in patient dose evaluation.