(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: To determine the dosimetric characteristics and therapeutic potential of an Orthopantomograph x-ray unit for use as a novel small animal irradiation system.
Methods: A Siemens Orthopantomograph, formally repurposed as a low-kVp beamlet system for choroidal melanoma (Esquivel et. al., 2006), has the potential to provide radiobiologists a cost effective, in-laboratory, small animal irradiator, without the shielding or bureaucratic requirements of isotope based systems. A collaboration between diagnostic and therapeutic physicists provided expertise in determining the quality and quantity of the radiation produced for assessing feasibility. The system operates as fluoroscope, producing energies between 57 and 90 kVp, continuously outputting at between 5 and 12 mA. Profiles determining field size were collected with Gafchromic XR film. Positioning of the detector system was accomplished with repurposed screen-film system and semi-transparent layer of phosphorescent-polymer. A Fluke UNFORS system was utilized to characterize exposure rate of set modalities and an Exradin A16 chamber was cross calibrated for measuring smaller fields involved. Measurements were collected within air and PMMA phantoms at multiple distances from the focal spot. In-room operator scatter-exposure was also determined.
Results: Output for the unit was determined to be 600 cGy/min at 7.3 cm from the source. System output demonstrated a systematic instability during the initial 14 sec of operation if each exposure before stabilizing. A depth dose curve in PMMA was determined for the prediction of dose at depths of tissue. Testing additionally revealed system over-heating, preventing reliable output after prolonged periods of use.
Conclusion: While a potentially cost effective irradiator, the repurposed Orthopantomograph is not without challenges. Field alignment was accomplished with recycled phosphor screen and outputs were determined to be viable. Issues of output stability may be remedied with in-vivo TLD and it is recommended that a monitor chamber system be installed. Additional issues of LET remain unresolved.
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