Comparison of output factors of different photon energy which using various detectors in small fields

Small radiation fields which commonly used in advanced radiotherapy applications, such as intensity modulated radiotherapy and stereotactic treatment techniques, are generally considered to be fields smaller than 4 cm × 4 cm. Small field dosimetry is more sensitive and complex than large fields due to lack of lateral electronic equilibrium, steep dose gradients and volume averaging effect. Lack of lateral electronic equilibrium and steep dose gradients are radiation related problems; the volume averaging effect is related to the detector. The aim of this study is to determine which of these detectors are more suitable for output measurements in small fields using some clinically available detectors. In the study TLD and five ionization chambers in different volumes were used and output measurements were taken at reference depth (d = 10 cm) and 6 MV and 18 MV photon energies for different field sizes. The data obtained from these measurements were normalized to the output value of the 10 cm × 10 cm reference field size and the output factors (OF) were found. Statistical analysis of the output factors was performed by using the Friedman test in the SPSS program. When we compare detectors in groups of 6 and 4 for both energies, it was determined that there is a significant difference between OF of very small and conventional small fields. Standard deviations of OF for detectors in large fields (≥4 × 4 cm2) are smaller than 3% for 6 MV and less than 2% for 18 MV. As the field size decreases (<4 × 4 cm2), the difference between the OF of detectors is increasing. Especially in very small fields (≤1,5 × 1,5 cm2) standard deviations increased to 10% for 6 MV and to 9% for 18 MV. Although the mini detectors (CC01 and CC04) have a larger volume than the microdetectors (TLD), the OF in very small fields are larger on both energies since they have wall material with air equivalent density. Thus, it is determined that the ideal detector to be used for small fields OF measurements can not be said to be the "smallest volume" detector, and that it is necessary to consider the properties of the building material as well as to the effective measurement volume of the detectors in the detector selection. Keywords: Radiation Detectors; Small Field Dosimetry; Output Factor