Small Field Output Factor Measurement: A Comparison of Microdiamond, Scintillation Detectors, and Radiochromic Film
Abstract
Purpose
Small field sizes used in stereotactic radiosurgery (SRS) pose significant dosimetric challenges due to heightened sensitivity to detector size, geometry, and response in small-beam conditions. Accurate determination of small field output factors (SFOFs) is therefore critical for the safe clinical implementation of SRS. This study evaluates the performance of three detectors—the PTW microDiamond detector (model 60019), Medscint Hyperscint plastic scintillation detector, and EBT4 radiochromic film—for measuring SFOFs in SRS-relevant small fields. Measured SFOFs were compared with treatment planning system (TPS) calculations to assess measurement accuracy and reliability.
Methods
All detectors and film were positioned at a source-to-surface distance of 95 cm and a depth of 5 cm in water or solid water. Field sizes were defined using high-definition multileaf collimator (HDMLC) apertures of 3×3, 2.5×2.5, 2×2, 1.5×1.5, 1×1, 0.7×1, 0.5×1, and 0.5×0.5 cm², with jaws fixed at 3×3 cm². SFOFs were normalized to a 10×10 cm² jaw-defined reference field. Measurements were performed using 6X-FFF and 10X-FFF beams on a Varian Edge TrueBeam linear accelerator. Field size–dependent output correction factors were applied to the microDiamond detector measurements.
Results
Measured SFOFs showed close agreement with TPS calculations for all three measurement techniques. Agreement improved with increasing field size for both photon energies. The largest discrepancies were observed for the smallest field (0.5×0.5 cm²), particularly for the microDiamond detector at 10X-FFF and for film measurements at both energies. For field sizes ≥1×1 cm², all measurements agreed with TPS within 2%.
Conclusion
All evaluated detectors are suitable for SRS small field dosimetry, though increased uncertainty exists for sub-centimeter fields. Careful detector selection, appropriate correction factors, and cross-validation using complementary techniques are recommended to ensure accurate SFOF commissioning for clinical SRS applications.