The use of small photon fields in radiotherapy techniques has increased substantially due to the wider clinical implementation of stereotactic radiosurgery (SRS), stereotactic body radiotherapy and intensity modulated radiotherapy. Accuracy of dose delivery from such special techniques is largely dependent on the accuracy of beam data commissioned in the treatment planning system (TPS). The commissioning of TPS with incorrectly measured or extrapolated beam data, specially nominal output and field output factor, can result in systematic error in dose delivery which subsequently may lead to erroneous patient treatments and radiation incidents or accidents. 1 The measurement of accurate beam data from small photon fields still remains challenging mainly due to source occlusion, lack of lateral electronic equilibrium,

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Purpose: To measure the output factor (OF) and output correction factors (OCF) of small circular cones using various active and passive detectors. Methods: The OF of BrainLab circular cones of 4, 6, 7.5, 10, 12.5 and 15mm diameter were measured isocentrically at 15mm depth for 6 MV X‐rays at 1000 MU/min from Novalis‐Tx Linac using high resolution diodes (SFD, Edge, PTW60008), plastic scintillator (ExtradinW1), small ionization chamber (CC01), EBT3 film and Al2O3:C OSLDs (nanoDot). OCF was calculated as a ratio of the OF of each detector relative to EBT3. Long term overall measurement uncertainty were estimated from the 15 repeat measurement performed for each detector over a period of 1year Results: The mean OF measured using various detectors are shown in the figure. In comparison to EDR3 measurement, nanoDot, CC01 and ExtradinW1 showed consistently lower OF upto −25.7% (nanoDot) which gradually improves with cone size (Table). The variation in OF measure with CC01 and ExtradinW1 for cone size >10mm is within −1.4% and −1.9%. Whereas, Edge and PTW60008 measured consistently larger OF except for 4mm cone. All diodes showed better agreement with EDR3 with maximum variation within ±1.7% (SFD), 2.5% (Edge) and 4.4% (PTW60008). The OCF was largest for nanoDot (range 1.04–1.35) and least for the diodes (range 0.97–1.04). The estimated overall long term standard uncertainty (k=2, 95% confidence) were highest for nanoDot at 3% followed by 2.43% for EDR3, 1% for SFD, 0.49% for ExtradinW1, 0.42% for CC01 and PTW60008, 0.16% for edge. Conclusion: Largest OF for the smallest cone was observed from EDR3. Although correction factor needed for all diodes were very similar, Edge showed most consistent result. nanoDot in its present form is not suitable for OF measurement from small fields. ExtradinW1 with its stem oriented perpendicular to beam axis underestimate OF for field size ≤10mm and need further investigation.

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