Room: AAPM ePoster Library
Purpose: To develop an ionization chamber mounting jig for in-air output calibration and quality assurance of the Mark I-68 Cs-137 irradiator used for preclinical studies.
Methods: The current jig design employs a minimal amount of scattering material (Lexan) to prevent scattered photons from reaching the ionization chamber’s thimble. The jig consists of top and bottom horizontal plates (5 cm × 10 cm × 0.5 cm) connected to two vertical side plates (5 cm × 0.5 cm× 28 cm). A 30 cm long screw (0.93 mm/turn) was inserted from top to the bottom plate. A cage, which holds the chamber, has two supporting plates in the back-end connected to a larger front-end plate via two thin plate angular elbows. The first back-end plate of the cage is inserted into the rotatable screw while the second plate is glanced through another plate (3.5 cm × 0.5 cm × 28 cm). This design enables precise adjustment of the cage (i.e., source-to-chamber distance). The jig’s bottom plate is screwed onto the irradiator table and the ionization chamber is gently inserted horizontally through the arms of the elbows. The center of the ionization chamber thimble is adjusted to line up with the center mark of the table. The thimble axis is set to a desired height above the bottom plate. The measurements are then performed to provide in-air output calibration. The AAPM TG-61 protocol was used to determine in-air dose rates based on the measured ionization data.
Results: The measured output values using the currently developed jig agreed well (within 2%) with the reference dose rates decayed to the measurement dates.
Conclusion: An ionization chamber mounting jig has been designed and fabricated to perform in-air output calibration of the Mark I-68 Cs-137 irradiator in an efficient and reliable manner.