Radiochromic film for verification of superficial x-ray backscatter factors.

Accurate calculation and knowledge of backscatter factors (BSF) in superficial x-ray radiotherapy is required to perform accurate absorbed dose determination. These measurements have been performed historically with small thin parallel plate ionisation chambers and Thermoluminescent Dosimeters (TLD's). This note investigates the use of a low energy dependence radiochromic thin film (GAFCHROMIC EBT) for measurement and verification of backscatter factors. A single layer film and an extrapolation method with multiple films have been investigated. 50kVp to 150kVp beams were analysed and results for BSF were measured and compared to IPEMB (Institution of Physics and Engineering in Medicine and Biology UK) derived results. Agreement within 2% (1 SD) was found using both the single layer and extrapolation techniques with IPEMB derived results at 30cm SSD and equivalent photon energies. A 150kVp beam was found to have BSF of 1.12 +/- 0.02 (2cm circle), 1.24 +/- 0.01 (5cm circle), 1.36 +/- 0.02 (10cm circle) respectively compared to 1.11, 1.23 and 1.36 for IPEMB derived results. In summary a single layer film provided an accurate measurement and verification of BSF and was found to be within 2% of derived IPEMB results in all cases. The extrapolation method in general provided a slightly closer match to IPEMB results (<1%) but with no extra discernable accuracy than the single layer film most likely due to the already small thickness (0.3mm) of one film piece. GAFCHROMIC EBT, Radiochromic film provides a very simple and easy method for measurement and verification of BSF for x-ray energies commonly used for superficial x-ray therapy.