A phantom for dosimetric characterization of small radiation fields: design and use

An acrylic phantom was designed and constructed for the acquisition and verification of basic dosimetric data of narrow fields in stereotactic radiotherapy/radiosurgery (SRT/SRS) using thermoluminescent (TL) dosimetry. An array of holes to accommodate up to 426 dosimeters was used to allow the assessment of dose distribution in circular fields with a 1-mm spatial dose resolution with minimal field perturbation. It was found experimentally that there must be a minimum gap of 1 mm between neighboring dosimeters in 6-MV photon fields. Most of the dosimetric characteristics of a 6-MV x-ray SRS/SRT unit assessed using TL dosimetry and ion chamber dosimetry were in good agreement when the longitudinal axis of the chamber was parallel to the central beam axis. TL dosimetry showed that the penumbra width increased with increasing collimator aperture. The low cost of the phantom and the widespread use and familiarity of TL dosimetry in radiotherapy departments offer a significant advantage in the use of the proposed methodology.