Quantification of the Plan Aperture Modulation of Radiotherapy Treatment Plans

TL;DR

There is no standardization or consensus on how to quantify plan modulation, partly due to the limitations of existing metrics, but many international guidelines emphasize the importance of quantifying plan modulation.

Abstract

This study introduces a novel metric, Plan Aperture Modulation (PAM), developed to quantify the modulation of radiotherapy treatment plans. PAM aims to provide a clear geometric interpretation, addressing the limitations of previous complexity metrics and facilitating its integration into treatment planning systems (TPSs) and clinical workflows. The PAM metric was defined as the average fraction of the target area outside the beam aperture, weighted over all control points in a treatment plan. The metric was evaluated in VMAT plans for two sites: prostate with lymph nodes and lung SBRT. Plans with varying complexities were generated using the Eclipse TPS, and PAM was compared to established metrics, including Plan Modulation (PM), Modulation Complexity Score (MCS), and monitor units per Gray (MU/Gy). The relationship between PAM and the Modulation Factor (MF), which quantifies the increase in MUs due to plan modulation, was also investigated. PAM provided a more intuitive assessment of plan modulation compared to the other metrics, and was validated across different delivery systems, such as C-arm linacs and Halcyon systems. The metric outperformed the previous metrics, indicated a zero modulation for Dynamic Conformal Arc plans, and was independent of confounding variables, such as treatment technique, beam energy, delivery system, and patient anatomy. Derived equations enabled the calculation of MF based on PAM, allowing for a robust quantification of plan modulation. PAM is a robust and intuitive metric for quantifying modulation in radiotherapy plans. It overcomes the limitations of previous metrics and can be readily implemented in TPSs to control plan modulation during optimization and for reporting. PAM is a promising tool for improving treatment planning workflows and for comparing and benchmarking radiotherapy plans in multi-institutional studies, clinical trials, and audits.

Figures (6)

• Figure 1: Example illustrating the beam aperture and target projection in the beam’s eye view, with the total target area ($A_{\text{total}}$), the blocked area ($A_{\text{blocked}}$), and the corresponding AM value.
• Figure 2: PAM values for prostate (high-dose prostate target) and lung SBRT plans. Symbols represent plans with varying complexity levels, and lines connect plans for the same clinical cases. X-axis labels represent complexity control settings: OFF (no control), ASC (Aperture Shape Controller set to moderate), ASC&MU (ASC set to moderate with MU restriction), and DCA (Dynamic Conformal Arc technique).
• Figure 3: Plan complexity values for prostate and lung SBRT plans for the ratio of Monitor Units to prescribed dose (MU/Gy, top row), Plan Modulation (PM, middle row), and Modulation Complexity Score (MCS, bottom row). Symbols represent plans with varying complexity levels, and lines connect plans from the same clinical cases. X-axis labels represent complexity control settings: OFF (no control), ASC (Aperture Shape Controller set to moderate), ASC&MU (ASC set to moderate with MU restriction), and DCA (Dynamic Conformal Arc technique).
• Figure 4: Relationship between PAM values and number of Monitor Units per Gray (MU/Gy, top row), Plan Modulation (PM, middle row), and Modulation Complexity Score (MCS, bottom row). Symbols represent plans with varying complexity for lung SBRT and prostate plans, with lines connecting plans from the same clinical cases.
• Figure 5: Top row: Relationship between Monitor Units (MU) and PAM for a lung SBRT case with plans using different treatment techniques --Volumetric Arc Therapy (VMAT) and Sliding Windows (SW)-- and a prostate with lymph nodes case with plans using different beam energies --6 MV, 6FFF, and 10 MV--. Symbols represent the values for the evaluated plans and lines show the MU-PAM relationship from Equation \ref{['eq:12']}. Bottom row: Modulation Factor for the same plans, with lines showing the theoretical predictions from Equation \ref{['eq:14']} and symbols indicating the ratio of MU to the predicted MU for PAM = 0 for each case.