Toward dynamic Monte Carlo source modeling: measurement variability and primary electron source-width sensitivity in very small photon fields
- 1Department of Radiation Physics, Maadi Armed Forces Medical Compound
Abstract
Background: Very small photon fields are sensitive to both measurement conditions and radiation-source modelling, but the connection between effective primary-electron source width, downstream photon angular phase space, and field-size-dependent dosimetric response remains incompletely characterized.
Methods: The study was restricted to 6 MV. Empirical measurement variability was evaluated using five ionization-chamber datasets acquired on a clinical Elekta Versa HD for fields from 20×20 to 3 × 3 cm ² . A generalized Versa HD/Agility EGSnrc/BEAMnrc model was evaluated with two prescribed stationary Gaussian source endpoints, M1 (FWHM x = FWHM y = 0.15 cm) and M2 (FWHM x = 0.35 cm, FWHM y = 0.60 cm). DOSXYZnrc dose responses and final pre-phantom photon phase-space data were analysed for 3×3, 2×2, 1×1, and 0.5×0.5 cm ² . The 3×3 cm ² field provided the experimental–Monte Carlo overlap; the smaller fields were controlled Monte Carlo endpoint-sensitivity analyses. M1 was additionally benchmarked against a 10×10 cm ² CC13 commissioning PDD.
Results: Mean experimental coefficient of variation increased from 2.46% at 20×20 cm ² to 15.45% at 3×3 cm ² . The M2 central-axis reductions were 8.48%, 8.27%, 29.59%, and 56.34% for 3×3, 2×2, 1×1, and 0.5×0.5 cm ² , respectively. The corresponding M2/M1 ratios of the 95% angular-containment solid angle were 1.275, 1.498, 2.415, and 3.835. The four-field comparison demonstrates clear field-size progression in angular containment, while showing that amplification is observable-dependent rather than universally monotonic. The M1 PDD benchmark supported reference depth-dose behaviour, whereas lateral-profile findings were restricted to controlled numerical sensitivity because absolute lateral agreement was not independently established in this study alone.
Conclusion: The demonstrated result is a stationary endpoint-sensitivity chain linking effective primary-electron source representation, linac-head transport, photon angular phase space, and small-field dose/profile response. A reduced covariance formulation is retained only as a prospective framework for testing whether a stationary effective source can represent unresolved accelerator-source states; no time-resolved source dynamics were measured or simulated.
Related
- Optical retardance measurement with constant sensitivity using vector vortex beams
- Inferring cell property distributions from high-throughput measurement data with cell-to-cell variability
- Comment on arXiv:2608.29450: "First Measurement of Solar Neutrinos through Elastic Neutrino-Electron Scattering at the keV Scale"
- QCD corrections to light-quark dipole sensitivity via transverse spin asymmetries at the Electron-Ion Collider
- Reducing hyperparameter sensitivity in measurement-feedback based Ising machines
BibTeX
@article{habib2026toward,
title = {{Toward dynamic Monte Carlo source modeling: measurement variability and primary electron source-width sensitivity in very small photon fields}},
author = {Mina M. Habib},
journal = {Frontiers in Oncology},
year = {2026},
month = oct,
doi = {10.3389/fonc.2026.1968378},
url = {https://www.frontiersin.org/articles/10.3389/fonc.2026.1968378/full},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Toward dynamic Monte Carlo source modeling: measurement variability and primary electron source-width sensitivity in very small photon fields
AU - Mina M. Habib
PY - 2026
DA - 2026/10/28
JO - Frontiers in Oncology
DO - 10.3389/fonc.2026.1968378
UR - https://www.frontiersin.org/articles/10.3389/fonc.2026.1968378/full
AB - Background: Very small photon fields are sensitive to both measurement conditions and radiation-source modelling, but the connection between effective primary-electron source width, downstream photon angular phase space, and field-size-dependent dosimetric response remains incompletely characterized. Methods: The study was restricted to 6 MV. Empirical measurement variability was evaluated using five ionization-chamber datasets acquired on a clinical Elekta Versa HD for fields from 20×20 to 3 × 3 cm ² . A generalized Versa HD/Agility EGSnrc/BEAMnrc model was evaluated with two prescribed stationary Gaussian source endpoints, M1 (FWHM x = FWHM y = 0.15 cm) and M2 (FWHM x = 0.35 cm, FWHM y = 0.60 cm). DOSXYZnrc dose responses and final pre-phantom photon phase-space data were analysed for 3×3, 2×2, 1×1, and 0.5×0.5 cm ² . The 3×3 cm ² field provided the experimental–Monte Carlo overlap; the smaller fields were controlled Monte Carlo endpoint-sensitivity analyses. M1 was additionally benchmarked against a 10×10 cm ² CC13 commissioning PDD. Results: Mean experimental coefficient of variation increased from 2.46% at 20×20 cm ² to 15.45% at 3×3 cm ² . The M2 central-axis reductions were 8.48%, 8.27%, 29.59%, and 56.34% for 3×3, 2×2, 1×1, and 0.5×0.5 cm ² , respectively. The corresponding M2/M1 ratios of the 95% angular-containment solid angle were 1.275, 1.498, 2.415, and 3.835. The four-field comparison demonstrates clear field-size progression in angular containment, while showing that amplification is observable-dependent rather than universally monotonic. The M1 PDD benchmark supported reference depth-dose behaviour, whereas lateral-profile findings were restricted to controlled numerical sensitivity because absolute lateral agreement was not independently established in this study alone. Conclusion: The demonstrated result is a stationary endpoint-sensitivity chain linking effective primary-electron source representation, linac-head transport, photon angular phase space, and small-field dose/profile response. A reduced covariance formulation is retained only as a prospective framework for testing whether a stationary effective source can represent unresolved accelerator-source states; no time-resolved source dynamics were measured or simulated.
SN - 2234-943X
ER -