PHSOR02 Presentation Time: 12:35 PM: Practical Considerations for Dosimetric Verification of Ru-106 Eye Plaque Vendor Certificate Using Radiochromic Film

Abstract

<h3>Purpose</h3> To develop a practical and efficient method for the dosimetric verification of Ru-106 plaques using radiochromic film measurements. <h3>Materials and Methods</h3> A hemispherical Solid Water^TM phantom with two quadrants was constructed with 12 mm radius to fit with eye plaque models - CCA, CCB, CCD and COB (Eckert & Ziegler BEBIG GmbH, Berlin, Germany). Each Ru-106 plaque was placed on the convex surface of the phantom for the film irradiations. Two types of radiochromic Gafchromic^TM dosimetry films (Ashland Advanced Materials, Bridgewater, NJ, USA) were studied - HD-V2 film with 12 μm active layer on a 97 μm clear polyester substrate and EBT-XD film with 25 μm active layer sandwiched between two 125 μm thick clear polyester substrates. A hemispherical film cut was sandwiched between the two quadrants of the phantom. Irradiation time for each plaque was calculated based on the dose rate from the vendor source certificate per AAPM TG 221 protocol. For each plaque, at least three irradiations were completed with each film type. AAPM TG 55 recommendations were used for film handling and scanning. Film dose measurements were analyzed at clinically relevant depth of 2 mm - 5 mm. For each film type, an optical density to dose calibration curve was obtained using a NIST calibrated Sr-90 source and a NIST-traceable 6 MV clinical linac beam. Measured film dose obtained using the calibration curve was corrected with MCNP derived Solid Water^TM to water dose conversion factors. Measured film absolute dose to water was compared with predicted dose to water value from the vendor source certificate for each eye plaque. Type A and B uncertainty components were reported. <h3>Results</h3> Using 6 MV calibration for HD-V2 film, the average percentage dose difference between film-measured vs. calculated dose per AAPM TG 221 from the vendor certificate at depth of 2 mm (6MV-Cal_HD-V2_DD%) for CCA, CCB, COB and CCD plaques were -9.2%, -6.3%, 0.9% and -7.7% respectfully. Similarly, Sr-90-Cal_HD-V2_DD% were -11.1%, -2.4%, 0.1% and -6.7% respectively. For EBT-XD, 2.5 mm measurements were taken instead of 2 mm, because of artifacts from the film cutting technique at shallower depth. Hence at 2.5 mm, 6MV-Cal_EBT-XD_DD% were -10.2%, -7.2%, -7.4% and -10.1% respectively; and Sr-90-Cal_EBT-XD_DD% were -5.5%, -3.0%, -3.1% and -6.1% respectively. All the DD% were within the uncertainty of the film measurements performed (13% at 2 mm - 5 mm depth, k = 2 with Sr-90 film calibration) and reported by the vendor (11% at 2 mm depth, k = 2). Results from the 6 MV vs. Sr-90 calibration curve were within 2% for HD-V2 film and within 5% for EBT-XD film. The average DD% between 2 mm-5 mm depths were within 15% for all plaques for both film types, using either 6 MV or Sr-90 calibration methods. DD% was smaller at shallow depths and increased slightly and gradually at the deeper depths. EBT-XD film was more prone to film damage due to polyester layer separation and provided more heterogenous DD% results for 2.5 mm-5 mm depths. EBT-XD film also had more variance between the two film calibration methods. <h3>Conclusions</h3> We have provided a simple and efficient method for dosimetric verification of Ru-106 eye plaque vendor certificate with radiochromic film measurements. Based on our experience, the HD-V2 film calibrated with 6 MV clinical linac beam provides the most practical solution for dosimetric verification of the data provided in the Ru-106 eye plaque vendor certificate.