Faculty of Physics - University of Bucharest

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FACULTY OF PHYSICS

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2026-06-11 23:58

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Conference: Bucharest University Faculty of Physics 2026 Meeting

Section: Biophysics; Medical Physics

Title:

Assessment of the Relationship between the MONACO Modulation Factor and Gamma Analysis Outcomes in VMAT Quality Assurance

Authors:

Ion Marius TOMA (1), Claudia CHILOM (1), Cristian CIOABĂ (2)

Affiliation:

1) Faculty of Physics, University of Bucharest

2) Spitalul „Sfântul Nicolae” Pitești

E-mail

tomamarius1717@gmail.com

Keywords:

Volumetric Modulated Arc Therapy (VMAT), Modulation Factor (MF), Monaco Treatment Planning System (TPS), Gamma Index Analysis, Patient-Specific Quality Assurance (PSQA), Gamma Passing Rate (GPR)

Abstract:

TThe widespread use of volumetric modulated arc therapy (VMAT) has significantly improved treatment compliance and delivery efficiency. However, the inherent mechanical complexity of VMAT requires patient-specific quality assurance (PSQA), an essential yet highly resource-intensive clinical protocol. This study aims to evaluate whether the Monaco Modulation Factor (MF), which quantifies the complexity of the treatment plan, can predict the results of Gamma index analysis, thereby reducing the workload and optimizing the clinical workflow. A cohort of 36 VMAT treatment plans were delivered using the Elekta Synergy LINAC. The MF was extracted from the Monaco treatment planning system. Physical dosimetric verification was performed using the Octavius 4D phantom, with a pass rate of 95% of the Gamma Pass Rates (GPRs) obtained via VeriSoft software, using standard criteria of 3%/3 mm. The relationship between MF and GPR was evaluated using rigorous nonparametric statistical methods and receiver operating characteristic (ROC) analysis. The results demonstrated a strong negative correlation between MF and GPR. Categorical analysis (classifying planes into low, medium, and high complexity) confirmed that higher modulation significantly increases the probability of QA failure. The ROC analysis yielded an impressive area under the curve (AUC) value of 0.971, identifying an optimal mathematical cutoff value of MF = 4.07. However, prioritizing absolute patient safety (100% sensitivity), a stricter and safer clinical threshold of MF ≤ 3.00 was established, below which plans consistently pass quality control. Based on the analyzed data, it can be concluded that the modulation factor serves as an extremely reliable a priori metric for predicting VMAT deliverability and assessing quality control failure. Although the proposed threshold (MF = 3.00) represents more of an institution-specific benchmark than a universal constant, these findings provide a solid, data-driven justification for transitioning from a universal physical measurement protocol to a modern and efficient risk-based quality assurance framework.

References:

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