In small-field radiotherapy, accurate characterization of the penumbra is critical for precise dose delivery, particularly in treatments like stereotactic radiosurgery (SRS), where steep dose gradients are required to spare surrounding healthy tissues. The objective of this study was to quantify the penumbra region in small radiation fields to improve dose delivery accuracy and minimize unnecessary exposure. Three detectors—the Semiflex 3D ion chamber, PinPoint 3D ion chamber, and microDiamond detector—were used to measure beam profiles for field sizes ranging from 1 × 1 cm2 to 4 × 4 cm2 using 6 MV photon energy. These measured profiles were compared against data from the Monaco treatment planning system (TPS) to evaluate consistency and detector performance. For larger field sizes (4 × 4 cm2 and 3 × 3 cm2), the Semiflex 3D detector showed the smallest deviations from TPS data, with errors of 14.5% and 14.3%, respectively. The PinPoint 3D and microDiamond detectors exhibited larger deviations, with microDiamond showing the greatest discrepancies (30.7% and 34.5%). In the 2 × 2 cm2 field, Semiflex maintained a deviation of 14.9%, while PinPoint 3D and microDiamond showed errors of 28.1% and 38.4%. For the smallest field (1 × 1 cm2), all detectors underestimated penumbra width: 11.3% (Semiflex 3D), 27.1% (PinPoint 3D), and 32.7% (microDiamond). Despite strong overall correlations between measured and planned profiles, noticeable deviations were observed, particularly in smaller fields. The Semiflex 3D, with the largest collecting volume, showed the best agreement with TPS data across all field sizes. This study highlights the importance of detector selection in small-field dosimetry and underscores the need for accurate penumbra quantification to ensure optimal treatment outcomes.
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