ESTRO 2025 - Abstract Book

S2684

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

4611

Digital Poster Influence of ripples on the uncertainty of three reference dosimetry methods for scanned particle beams Hugo Palmans Radiotherapy and Radiation Dosimetry, National Physical Laboratory, Teddington, United Kingdom. Medical Physics, MedAustron Ion Therapy Center, Wiener Neustadt, Austria Purpose/Objective For scanned particle beams, the current consensus is to perform reference dosimetry by point-dose measurements at shallow depth in a broad beam generated as an equidistant, equal-weighted spot pattern. It is, however, generally not the focus of planning systems to get the dose at shallow depth right and this choice of reference conditions is thus suboptimal. Furthermore, small spot misalignments lead to considerable lateral ripples on the lateral dose profiles affecting dosimetry but these are not well correlated with what is happening in the target region. Two other reference dosimetry methods have been proposed that, from these two perspectives, better correlate with the dose in the target region: dose-area-product measurements at shallow depth in a single pencil beam and point-dose measurements in a box field similarly as recommended for scattered beams. In this work, the influence of uncertainties due to dosimetric ripples on each calibration method is quantified and compared. Material/Methods Spot size models for realistic proton and carbon ion beams with 8mm FWHM at the phantom surface were used. The effect of random positioning errors within ±0.3mm on the dosimetric uncertainty was quantified for Roos-type and Markus-type plane-parallel ionization chambers (with 16mm and 5mm collecting electrode diameter, respectively) by integrating the lateral spot profiles over the chamber area considering also the effect of spot widening with depth. Results For the 16mm diameter chamber, the random spot position errors resulted in a type B relative standard uncertainty of 0.4% at shallow depth in both beams and at large depth in a carbon ion beam and 0.2% at large depth in a proton beam. For the 5mm diameter chamber, the corresponding uncertainty contributions are 1.0% and 0.3% showing that using a smaller chamber, the consensus method results in substantial extra uncertainty for proton beams. For point-dose measurements in a box field, energy-layering ripples with amplitude 1% add 0.7% uncertainty to reference dosimetry. Conclusion While it has been argued that the consensus method of reference dosimetry better reflects the clinical delivery by accounting for inadequacies of spot positioning, it should be realized that these inadequacies are washed out at target depth and the dose-area-product method, while currently still suffering from a few other larger uncertainty contributions than the consensus method, can result in a better correlation with target dose and thus lower overall uncertainty, while not suffering from ripple effects in depth as in the box field method.

Keywords: reference dosimetry, particle beams, uncertainty