The IMRT is proper implement to get high dose deliver to tumor as its shape and selective approach in radiation therapy. Since the IMRT is performed as modulated the radiation fluence by the MLC created the open shapes and its irradiation time, the dose of segment of radiation field effects on the cumulated portal dose. The accurate output factor of small and step shape of segment is important to improve the determination of deliver tumor dose as it is directly proportional to dose. This experiment performed with the 6 MV photon beam of Clinac Ex(Varian) from 3×3 cm² to 0.5×0.5 cm² small field size for collimator jaw in MLC free and/or for MLC open field in fixed collimator jaw 10×10 cm² using the CC01 ion chamber, SFD diode, diamond detector and X-Omat film dosimetry. As results of normalized to the reference field of 10×10 cm² of MLC, the output factor of 3×3 cm² showed 0.899±0.0106, 0.855±0.0106 for 2×2 cm2, 0.764±0.0082 for 1×1 cm² and 0.602±0.0399 for 0.5×0.5 cm². The output factor of MLC open field has shown a maximum 3.8% higher than that of the collimator jaw open field.

KeywordsSmall field, Output factor, Film dosimetry