Small Field Output Variation: Impact of Field Size
Abstract
Purpose
Large variation up to 8% in field output factor (FOF) among various investigators has been reported. This is critical for modern radiotherapy techniques. Field definition based on FWHM (Sclin) and S (jaw based) has been used but debated. This study investigates the repeatability of jaw-defined small fields and evaluates the FOF with plastic scintillator detector (PSD).
Methods
Measurements were performed on a Varian TrueBeam for jaw-defined fields from 0.5×0.5 cm² to 3×3 cm² for 6X, 6FFF, 10X, 10FFF beams. A Blue Physics, PSD (BP-PSD) system was used. which is integrated with dedicated water-tank control software enabling ultra-fast scanning and automated routines for detector centering and field size determination. After each jaw reset, the detector was automatically centered, and the field size was measured at full width at half maximum (FWHM) within a few seconds. Field center stability and field size repeatability were quantified. FOFs were analyzed as a function of the measured field size rather than nominal jaw settings.
Results
Jaw positioning showed excellent repeatability, with minimal variation in field center and profile shape after repeated jaw resets, even for the smallest field sizes. However, for fields smaller than approximately 2.5×2.5 cm², the measured field sizes were consistently smaller than their nominal values and more pronounced with decreasing field size. The measured FOF at depths and field size agree strongly with Monte Carlo–based reference data.
Conclusion
Modern linacs demonstrate excellent mechanical repeatability, producing true radiation field size. This study demonstrates that measuring and centering the detector for each individual small field and using the actual measured field size for output factor determination produces FOF that are comparable to MC data supporting accurate dosimetry for small field below 3×3 cm² to ensure reliable commissioning and quality assurance in advanced radiotherapy treatments.