Dosimetric accuracy of Gafchromic EBT2 and EBT3 film for in vivo dosimetry.

Radiochromic film has the potential to provide accurate in vivo dosimetry measurements. However, it is not known whether small film pieces can still provide accurate dosimetric results. The use of small film pieces is of particular interest in regions of interest (ROIs) such as the eye, or where the patient's contour changes rapidly. This study examines the dosimetric accuracy of Gafchromic EBT2 and EBT3 models of radiochromic film and its dependence on film size, ROI size, and height above the scan bed for 6 MV photons and 9 MeV electrons. Films cut to sizes of 5.0 × 5.0, 10.0 × 10.0, 20.0 × 20.0, and 40.0 × 40.0 mm² were tested and it was found that there was no increase in uncertainty of dose when even the smallest film sizes were used. For a film 5.0 × 5.0 mm², ROIs of 1.4 × 1.4, 2.1 × 2.1 and 3.5 × 3.5 mm² were tested and it was found that the ROI size of 2.1 × 2.1 mm² was the most accurate. The standard deviation of the EBT3 placed on the glass (2.1%) was larger than the standard deviation of the EBT3 film raised above the glass (1.2%), therefore it is recommended that film is scanned raised above the scan bed. The general dosimetric performance of EBT3 was comparable to EBT2. We conclude that film pieces as small as 5.0 × 5.0 mm² could be used for the purpose of in vivo dosimetry of radiotherapy treatments.