ESTRO 2025 - Abstract Book

S2571

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

760

Digital Poster Sensitivity analysis of output factor with field size uncertainties and focal spot in small-field dosimetry George X Ding 1 , Indra J Das 2 1 Radiation Oncology, Vanderbilt University, Nashville, USA. 2 Radiation Oncology, Northwestern University Feinberg School of Medicine, , Chicago, USA Purpose/Objective: Accurate dosimetry for small field is governed by lack of lateral electron equilibrium; source occlusion by the collimating system; and the size of detectors to accurately measure dose[1]. It is known that the output is sensitive to focal spot size and field size uncertainties, but the sensitivity analysis is not known. Since experimental methods cannot separate steep output variation caused by focal spot size and field size uncertainties, the Monte Carlo technique is used to quantify the impact of focal spot size and field size uncertainties. Material/Methods: The Monte Carlo Simulations are performed using the EGSnrc system with user code BEAMnrc/DOSXYZnrc[2] for generating 6 MV beams from a Varian Clinac 23iX and modern TrueBeam linear accelerator. The beam parameters used in the simulation including the incident electrons distribution on the beam target were previously benchmarked against measurements[3, 4]. For Clinac accelerator the incident electrons’ intensity distribution[3] is taken to be Gaussian with the same FWHM in X and Y of 1.2 mm while for TrueBeam the incident electrons’ distribution[4] is taken to be Gaussian with FWHM in X of 1.618 mm and in Y of 1.793 mm respectively. The beam output sensitivity is investigated as a function of field sizes defined by the X and Y jaws between different focal spot sizes. Field output factors (FOF) are calculated relative to a 10 x 10 cm 2 field at SSD=90 cm and depth=10 cm. Results: Figure 1 presents field output factors as a function of square field sizes ranging from 4.5 x 4.5 mm 2 to 30 x 30 mm 2 . Several observations are noticed: 1) uncertainties of 0.1 mm in field size defined by the jaws can lead to 2-3% variation in the FOF at field size of 5 x 5 mm 2 ; 2 ) FOF are extremely sensitive to the beam focal spot size for field sizes < 12 x 12 mm 2 ; 3) 20% decrease in FOF at field of 5 x 5 mm 2 is observed when FWHM of Gaussian increases by 0.5 mm; 4) the effect of beam focal spot size is negligible for FOF at field size > 15 x 15 mm 2 .