S1694
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
uncertainties from large- vs small-field protocols [3,4]. Results:
Figure 1 (table) summarizes for the five phantom materials and six chambers, including
volumetric corrections
and
reference
values. TG-21
entries carry visibly
larger one-sigma uncertainties than TRS-483, consistent with protocol expectations for small fields.
Across chambers, the absolute spread of between parallel and perpendicular settings is modest (≈0.3–1.5%), but manufacturer-specific trends are evident: PTW chambers tend to have slightly in the perpendicular orientation, whereas Exradin A16 shows minimal orientation sensitivity, and IBA chambers show small but measurable orientation dependence. higher
Conclusion: We provide a compact, protocol-ready set of small- field correction factors for widely used GK chambers and phantoms (Figure 1), and confirm a robust linear
relationship between
and phantom electron
density (Figure 2). These results enable (i) quick
interpolation of for material substitutions, (ii) informed selection of chamber–orientation pairs with minimal perturbation, and (iii) reduced uncertainty when implementing TRS-483/TG-155 in GK calibration and QA. Together, they advance accurate, efficient GK dosimetry for stereotactic radiosurgery [3, 4]. References: [1] Kawrakow I, Rogers DWO, Mainegra-Hing E, Tessier F, Townson RW, Walters BRB.EGSnrc toolkit for Monte Carlo simulation of ionizing radiation transport,doi:10.4224/40001303 (2000).[2] Mora G, Maio A, Rogers D. Monte Carlo simulation of a typical 60Co therapy source. Med Phys. 1999;26:2494. [3] INTERNATIONAL ATOMIC ENERGY AGENCY, Dosimetry of Small Static Fields Used in External Beam Radiotherapy, Technical Reports Series No. 483, IAEA, Vienna (2017)[4] I. J. Das et al., “Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non ‑ equilibrium conditions,” Medical physics, vol. 48, no. 10, pp. e886– e921, 2021, doi: 10.1002/mp.15030. Keywords: Small Field Dosimetry, Gamma Knife, Ion Chambers
Figure 2 demonstrates a near-linear increase
of with phantom electron density from LW (1.00) through ABS/SW/PS (≈1.02–1.04) to PMMA (≈1.15–1.16). Linear fits (parallel data only) yield high goodness-of-fit for all chambers, supporting a practical material-scaling rule. PMMA consistently produces the
largest
, while LW anchors the lower bound,
with chamber-to-chamber offsets of
.
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