A Full Width at Half Maximum (FWHM) Based Approach to Treatment Planning System Validation for Precise Stereotactic Radiotherapy

Accurate characterization of small radiation fields is essential for high-precision radiotherapy techniques such as Stereotactic Radiosurgery (SRS). This study evaluates the effectiveness of Full Width at Half Maximum (FWHM) analysis as a method for validating a Treatment Planning System (TPS), particularly for detector-dependent variations in small-field dosimetry. Beam profiles and percent depth dose (PDD) data were collected for field sizes ranging from 1 × 1 cm² to 4 × 4 cm² using three detectors, PTW Semiflex 3D, PinPoint 3D, and microdiamond, and compared to Monte Carlo-generated data from the Monaco TPS. An analysis was done of the FWHM results obtained from each detector and from the TPS. The decrement line method was implemented to analyse isodose widths derived from these measurements, as an extra means of comparison. Results show that discrepancies between measured and TPS-predicted FWHM values increase with decreasing field size, with the microDiamond detector showing the largest deviations. However, when full beam dosimetry (including PDD data) was considered, the isodose distributions exhibited closer agreement across the detectors. These findings emphasize the importance of detector selection during commissioning, as well as the role of FWHM and isodose analysis in enhancing TPS validation, QA processes, and ultimately clinical safety in small-field radiotherapy.