Small photon field dosimetry using EBT2 Gafchromic film and Monte Carlo simulation

Small photon fields are increasingly used in modern radiotherapy especially in intensity modulated radiation therapy (IMRT) and stereotactic radiosurgery (SRS) treatments. Accurate beam profile and central axis depth doses measurements of such beams are complicated due to the electron disequilibrium. Hence the EBT2 (external beam therapy) Gafchromic film was used for dosimetry of small circular photon beams because of its high spatial resolution. The results of EBT2 film were compared with PinPoint measurements and Monte Carlo calculations. Four small field sizes (5, 10, 20 and 30 mm diameters) were produced by circular cones coupled to a Varian Clinac-2100 linear accelerator at 6MV photon beam energy. Experimental measurements were carried out using two dosimeters: The Gafchromic EBT2 film and the 0.015 cm3 PinPoint ionization chamber (PTW, type 31006). The EGSnrc/BEAMnrc code was used to calculate dosimetric parameters for the above small fields. The results showed that the PDD values measured by EBT2 film had maximum local differences 4% and 3% with PinPoint measurements and MC calculations respectively. The measurements of penumbra (80-20%) at 5 cm depth in a water phantom and SSD=100 cm by EBT2 film were up to 0.8 and 0.4 mm smaller than PinPoint measurements and MC calculation respectively. Our results show good agreement between EBT2 film measurements and MC calculation for small field output factors but PinPoint measurements need correction factors. This study showed that the Gafchromic EBT2 film is suitable detector for small field dosimetry especially for penumbra and output factor measurements.