Quantification of the aperture modulation in radiotherapy treatment plans

BACKGROUND: Many international guidelines, including those from the American Association of Physicists in Medicine (AAPM), emphasize the importance of quantifying plan modulation and recommend clearly documenting the modulation amount and range of radiotherapy treatment plans. However, there is no standardization or consensus on how to quantify plan modulation, partly due to the limitations of existing metrics. PURPOSE: This study introduces two novel metrics to quantify the modulation of treatment plans: the plan aperture modulation (PAM) and the modulation factor (MF). The aim of these metrics is to provide a clear and intuitive assessment of the aperture modulation in radiotherapy plans, addressing limitations of previous metrics and facilitating integration into treatment planning workflows. METHODS: PAM was defined as the average fraction of the target area located outside the beam aperture, weighted across all control points in a treatment plan. It was evaluated on volumetric modulated arc therapy (VMAT) plans for prostate with lymph nodes and lung stereotactic body radiation therapy. Plans with varying complexities were generated using the Eclipse treatment planning system (TPS), and PAM was compared to established metrics including plan modulation (PM), modulation complexity score (MCS), and monitor units per gray (MU/Gy). MF was defined in this paper as the relative increase in MUs due to plan modulation, and the relationship between PAM and MF was investigated using analytical expressions. RESULTS: PAM provided an intuitive and geometrically clear assessment of plan modulation and was validated across different delivery platforms, such as C-arm linacs and Halcyon systems. Derived equations allowed calculation of MF based on PAM, which is useful to assess and control the MU increase due to aperture modulation. PAM and MF outperformed the previous metrics, indicating zero modulation in Dynamic Conformal Arc plans, and demonstrated independence from confounding variables such as treatment technique, beam energy, delivery system, and patient anatomy, enabling an absolute quantification of plan modulation. CONCLUSIONS: PAM and MF offer a clear and absolute quantification of the aperture modulation in radiotherapy plans. They address limitations of previous metrics and can be readily implemented in TPSs to control plan modulation during optimization and for reporting, thus facilitating improvements in treatment planning workflows and plan benchmarking in multi-institutional studies, clinical trials, and audits.