The Effect of Heterogeneity in Small Electron Fields: A Dosimetric Study
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Abstract
Background: Small electron fields are used in radiotherapy for superficial tumors and areas close to the skin. However, the impact of tissue heterogeneity on dose distribution in these fields poses considerable challenges. To explore how the variability of cold foam affects dose distribution in small electron fields using Semiflex 3D and Advanced Markus dosimeters.
Methods: Dosimetric measurements were performed using an Elekta Vera-HD linear accelerator with 10 MeV and 12 MeV electron beams. Square field sizes of 2×2, 3×3, 4×4, 5×5, and 6×6 cm² were investigated. Dose distributions were assessed using Semiflex 3D and Advanced Markus ionization chambers. Percentage Depth Dose (PDD) curves were analyzed, revealing that at 10 MeV, the depth of maximum dose (d_max) was 2.2 cm, while at 12 MeV, it increased to 2.7 cm.
Results: The results confirm that the OF increases with both field size and beam energy. Larger field sizes enhance lateral electron scattering, and higher beam energy enables deeper penetration and broader dose distribution, further increasing the OF. A minimum field size of 3 cm × 3 cm is recommended, as differences between dosimeters were observed in 2 cm × 2 cm and 3 cm × 3 cm fields, but remained below 2% for larger fields. The study also found that increasing heterogeneity reduces the OF, with air-equivalent heterogeneities consistently decreasing the OF across all field sizes and energy levels.
Conclusion: Monte Carlo algorithms in treatment planning systems (TPS) model heterogeneities using CT images and Hounsfield Unit (HU) values. However, their accuracy depends on CT image quality and device calibration, and HU-to-parameter conversion may not fully account for tissue variability. Combining computational simulations with experimental validation is recommended to improve TPS accuracy, especially in electron mode and when dealing with heterogeneities.
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| Keywords | ||
| Tissue heterogeneity small electron fields radiotherapy Semiflex 3D Advanced Markus | ||
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