An inexpensive method of small photon field dosimetry with EBT3 radiochromic film

Abstract

The relative output factor for small field photon beams with EBT3 (External beam therapy, Gafchromic ™) Radiochromic film were obtained using indigenously developed a program in MATLAB. MLC and JAWS created square small field sizes for dosimetry. To calibrate Radiochromic film, we have cut the film into several small segments of 6 × 6 cm2. These small pieces of film were irradiated to known doses ranging from 25 to 300 cGy by SAD technique using 6 MV photon beam from Linear accelerator. A Program in MATLAB was written to analyze the scanned film data and all the images of the films were imported into this Program. To validate the accuracy of calibration curve, radiochromic films were irradiated with different field sizes of 10 × 10 cm2, 15 × 1 5 cm2, 20 × 20 cm2 to known dose of 200 cGy each by SAD technique. Field sizes were defined using two methods, in the first method fields were defined using X-Y collimator Jaws with the MLC retracted to their maximum position. In the Second method, MLCs were used to define square field sizes of 1 × 1 cm2 to 5 × 5 cm2, 10 × 10 cm2 and 15 × 15 cm2. Calibration curve created using gafchromic film show percentage variation between measured and delivered dose within 3% of standard field size. Output factors measured by Gafchromic EBT3 film showed close agreement with those measured using the other detector for field sizes of 2.0 × 2.0 cm2 and above. EBT3 film has properties such as linear response, energy, and dose-rate independence. It makes it suitable detector for small field dosimetry. Our Indigenous developed MATLAB Program for film dosimetry gave the desired results. From this study, we can conclude that EBT3 film can be used for relative output factors ranging from small field to large fields.