Accurate dosimetry in small photon fields is essential for advanced radiotherapy techniques such as stereotactic radiosurgery (SRS) and volumetric modulated arc therapy (VMAT). However, small-field dosimetry possesses challenges due to lateral charged particle disequilibrium and detector volume averaging effects. This study experimentally determines the field output correction factor (FOCF) for the Semiflex 3D ion chamber, offering valuable data for emerging detectors in clinical applications. Additionally, it compares the performance of various detectors in measuring small photon field output factors. The evaluation includes five different detectors: a microDiamond detector, three PTW ionization chambers (Semiflex 3D, Pinpoint, and Pinpoint 3D), and Gafchromic EBT3 film. Measurements were conducted on an Elekta Versa HD linear accelerator for photon energies (6 MV FF, 6 MV FFF, 10 MV FF, and 10 MV FFF). A solid water phantom was used for film dosimetry, while a PTW MP3 scanning water phantom was used for ionization chambers and micro-Diamond measurements. Field output factors were measured for different field sizes, and results were compared with literature values. The microDiamond detector and Gafchromic EBT3 film demonstrated superior accuracy in small fields (<1 × 1 cm2), with deviations from literature data within 2% - 3%. Ionization chambers exhibited significant underestimation due to volume averaging effects, particularly in fields below 2 × 2 cm2. Field output correction factors (FOCF) were determined for the PTW 31021 Semiflex 3D detector, filling a gap in the IAEA TRS 483 guidelines. The microDiamond and Gafchromic EBT3 film provide reliable results, making them the preferred choices for accurate dose measurement in small photon fields. The findings provide insights into detector-specific correction factors that improve clinical dosimetry protocols, particularly in advanced radiotherapy techniques such as SRS and VMAT. This study contributes to refining dosimetric protocols and improving clinical accuracy in radiotherapy.
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