Voxel-Based Conversion of Hypofractionated Radiotherapy Dose Distributions to 2 Gy-Equivalent OAR Constraints: Proof-of-Concept Demonstrating the Radiobiological Benefits of Hypofractionation in a Prostate Radiotherapy Case

Mazen Moussallem; Dima Mahmoud; Antoine Nassif

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arXiv · 2604.10227

Voxel-Based Conversion of Hypofractionated Radiotherapy Dose Distributions to 2 Gy-Equivalent OAR Constraints: Proof-of-Concept Demonstrating the Radiobiological Benefits of Hypofractionation in a Prostate Radiotherapy Case

Mazen Moussallem·Dima Mahmoud·Antoine Nassif

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Abstract

Objectives: Existing voxel-based dose converters transform hypofractionated dose distributions into biologically effective dose (BED) or equivalent dose in 2 Gy fractions (EQD2), but they are not reliably applicable to organ-at-risk (OAR) dose constraints, particularly in low-dose regions, which may lead to dose misinterpretation. This study develops and demonstrates a voxel-based method to convert hypofractionated dose distributions into 2 Gy-equivalent OAR constraints. Methods: To2GyConstraints converter (www.healthy-innovations.com) was applied to a prostate cancer case. The method uses the Linear Quadratic (LQ) model for doses per fraction less than or equal to 7.5 Gy and the Linear Quadratic Linear (LQ-L) model for higher doses. For voxel fraction doses below a threshold defined as the mean between the prescribed hypofractionated fraction dose and 2 Gy, an equivalent number of fractions is calculated. The method then applies an EQDx-type conversion, rather than EQD2, using this calculated fraction number to better reproduce normofractionated dose behavior. Results: For doses above the defined threshold, unlike BED, the To2GyConstraints model produced results consistent with EQD2 and provided clinically realistic dose values comparable to standard dosimetric constraints, thereby offering a clearer demonstration of the radiobiological benefits of hypofractionation in prostate cancer. For doses below the threshold, unlike EQD2, the To2GyConstraints model showed behavior consistent with BED, yielding higher dose estimates when converted to a normofractionation scheme. Conclusions: To2GyConstraints converter shows promising results for radiobiological interpretation of hypofractionation. Further multicenter validation is required. Advances in knowledge: A voxel-based method enabling application of normofractionation OAR constraints to hypofractionated dosimetry after conversion.

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Mazen Moussallem, Dima Mahmoud, Antoine Nassif. 2026-04-15. Voxel-Based Conversion of Hypofractionated Radiotherapy Dose Distributions to 2 Gy-Equivalent OAR Constraints: Proof-of-Concept Demonstrating the Radiobiological Benefits of Hypofractionation in a Prostate Radiotherapy Case. https://arxiv.org/abs/2604.10227

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