Two-dimensional dose distribution of a miniature x-ray device for stereotactic radiosurgery

Abstract

The Photon Radiosurgery System is a miniature x-ray device developed for the treatment of small intracranial neoplasms. The x-rays are generated at the tip of a 10-cm-long, 3-mm-diam probe with a nearly isotropic distribution. Results from measurements of the two-dimensional dose distribution around the x-ray source are presented using two methods: (1) dose measurement with an ionization chamber and a water phantom system and (2) dose measurement with radiochromic film and a solid water phantom. The shape of the two angular dose distributions in the axial plane agree with each other to with approximately 10% and the dose at 10 mm from the source, orthogonal to the probe axis, was about 20% lower than at the same distance along the axis. The relative dose difference of 20% corresponds to a change in distance from the source of +/- 0.3 mm at 10 mm. It is shown that the anisotropy of radiation distribution in the axial plane can be improved to approximately 10% by adjusting the electron beam with a 12% reduction in the overall radiation output.

Quality assurance and independent dosimetry for an intraoperative x‐ray device

“…This approach is used by the manufacturer for annual factory calibrations, 28 with a version available as a commercial product for the end user, and was used by a number of early investigators of the system. 7,16,29 The early investigators used a small parallel plate chamber (0.02 cm 3 , type 30334, PTW) in a computed controlled water tank with a solid water chamber holder containing a window of 1 mm thickness facing the (PRS) probe. The accuracy of movement in the tank was 0.1 mm.…”

“…Radiochromic film is selfdeveloping, reducing the variation due to wet processing, and more tissue equivalent, but requires careful handling to reduce other uncertainties, such as nonuniformity. Early investigators of the PRS system used radiochromic film to measure isotropy 6, 7, 42 and depth doses, 22,29,42 and more recently it has been used to intercompare different sources, 44 validate novel applicator designs, 12 or for in vivo dosimetry. 24 Hakim et al 42 used Gafchromic MD-810 film (International Specialty Products, ISP) in a solid water phantom (Gammex) to confirm the inverse cube dependence of dose on distance from the source (without applicator attached).…”

“…Agreement with TLDs in an acrylic phantom was worse at larger depths, which may have been because of the beam hardening at depth compared to calibration conditions, or the different phantom materials used. Yasuda et al 29 also used Gafchromic film (brand not given, ISP) to measure depth doses and isotropy by cutting a section from a sheet of film to match the dimensions of the probe, and placing the film and probe between two shaped pieces of solid water. Film pieces for calibration were sandwiched between sheets of solid water and polystyrene at a distance of 5 mm from the source and a 15% correction was applied to account for differences between sold water and polystyrene.…”

Dosimetry measurements with an intra-operative x-ray device

“…Detailed device description was performed for the original design, which had a maximum energy of 40 keV (Dinsmore et al 1996, Beatty et al 1996, Biggs and (This figure is in colour only in the electronic version) 1996). The spectrum and isotropy of the output were validated by Monte Carlo simulation (Yanch and Harte 1996) and radiochromic film (Yasuda et al 1998). Beatty et al (1996) measured a half value layer (HVL) of 0.10 mm Al for the unattenuated beam and 0.71 mm Al at a depth of 10 mm in Solid Water (Gammex RMI, WI).…”

Patient-Specific Quality Assurance Using a 3D-Printed Chest Phantom for Intraoperative Radiotherapy in Breast Cancer

“…In this case, the main cause of uncertainty in the measurement was the positioning of the detector. As has been determined in previous studies (24,25), both the ionization chamber and the dosimetry of the film can be performed with an accuracy of 5%-10%, when considering the largest error due to the positioning of the dosimeter. Based on this fact, the steep dose gradients produced by the INTRABEAM system yielded a 10% difference in dose for a 1 mm difference in distance.…”