Impact of Dosimetric Parameters on Interplay Effects in 6 MV Flattening Filter-Free Photon Beams to Treat Lung Cancer

Context: Interplay effects have become the significant problem in lung cancer radiotherapy. Since these effects yield dose variation within the target and surrounding tissues.

Aim: The aim of this study is to investigate the effect of the dosimetric parameters of interplay effects in 6 MV flattening filter-free (FFF) photon beams for lung cancer.

Settings and design: This study performed planning, measurement, and data analysis sections for examining different breathing amplitudes and phases, doses, dose rates, field sizes, and fractionations.

Subjects and methods: Standard and clinical plans were created on the eclipse treatment planning system. The static and dynamic measurements were performed using a robotic platform and two-dimensional (2D) diode array. The gamma passing rates were defined as the percent of dose variation caused by the interplay effects.

Statistical analysis used: Unpaired t-test.

Results: The outcomes showed three trends between gamma passing rates (γ) and dosimetric parameters. First, a decreasing trend was breathing amplitudes. The lowest γ of maximum amplitudes (2 cm) in both one dimensional and 2D were <25%. Second, an increasing trend was field sizes. The lowest γ of minimum field size (4 cm × 4 cm²) was <55%. Third, constant outcomes were breathing phases, doses, dose rates, and a number of fractions. The γ values of these factors were 53.1%, 55.1%, 34.7%, and 36.7%, respectively.

Conclusions: Lung tumor motion-induced interplay effects in 6 MV FFF photon beams are more pronounced for higher breathing amplitudes and smaller field sizes.