Purpose: To develop a cassette capable of holding custom-sized radiochromic film in a reproducible manner for in-water dose measurements and quality assurance (QA) of Cs-137 blood irradiators.
Methods: The currently developed film cassette was made of two thin (0.8mm) Lexan sheets cut to 19.4cm x 12.3cm. It contained a rectangular hole at the center (9.7cm above the bottom of the canister) and circular holes at six off-center locations (2.5cm and 5cm lateral to the center, and 8 cm above and below the center). The holes facilitated uniform exposure within the designated regions of measurement without any interference from the cassette material. To measure and verify the dose output and dose distribution of Cs-137 blood irradiators, a GAFChromic EBT-3 film was sandwiched in the cassette and placed diametrically vertical inside the water-filled canister. The film was then irradiated for the specified set time to deliver the desired dose (~8 Gy) based on the manufacturer-quoted dose rate. The film was scanned using Epson Expression 1000XL scanner one day after the irradiation. The measured optical density of the film was converted to the dose using the film calibration curve derived from prior measurements with an ionization chamber-calibrated (reference) Cs-137 irradiator. The film calibration curve was verified on the date of measurement by exposing two 2cm x 2cm pieces of the same film to a known dose (~8 Gy) using the reference Cs-137 irradiator. Another 2cm x 2cm piece of unexposed film provided the background reading.
Results: The measured doses at the center and off-center locations using the film cassette agreed with the manufacturer-stated doses within ±5% over three periodic QA sessions.
Conclusion: The currently developed film cassette was found efficient and reliable for in-water dose measurements and periodic QA of Cs-137 blood irradiators.