Author: Marcos Feijoo, Mamoru Ichiki, Tatsuya Maeda, Tatsuhiro Makino, Tomonori Megumi, Tadashi Nakabayashi, Yukino Nakata, Shuichi Ozawa, Daishi Takayama π¨βπ¬
Affiliation: Hiroshima High-Precision Radiotherapy Cancer Center, Blue Physics LLC, Imamura General Hospital, Accuray Inc. π
Purpose: This study aims to accurately measure the leaf open time (LOT) of the multi-leaf collimator (MLC) in tomotherapy using an advanced plastic scintillator detector (PSD) with high time resolution and to verify the feasibility of PSD-based LOT quality assurance (QA).
Methods: A water-equivalent slab phantom was positioned with its center aligned to the gantry rotation axis. Measurements were performed using the Blue Physics Model 10 PSD (1 mm in diameter, 1 mm in length), placed at the phantomβs center, with a sampling rate of approximately 750 ΞΌs. In the preliminary experiment, the PSD was positioned between leaves #32 and #33 of the 64 MLCs, and a manual irradiation procedure was conducted to detect leaf signals. Subsequently, LOT was evaluated for a QA treatment plan using TomoDirect technology. The plan, created in IMRT mode with a modulation factor (MF) of 2.5 using PlanningStation version 5.1.8, provided planned LOT values. The PSD was placed between leaves #32 and #33, and the measured LOT was compared with the planned LOT.
Results: Preliminary experiments confirmed PSD signal detection for leaves #32 and #33, while signals for leaves #31 and #34 were insufficient. In QA treatment plan measurements, setting thresholds at 20% and 70% of the maximum signal facilitated the analysis of the double waveform caused by differing LOTs between leaves #32 and #33. The difference between planned and measured LOT values was less than 2 ms.
Conclusion: This study demonstrated that a fast PSD provides a reliable and independent verification method for LOT, supporting MLC quality assurance testing for Tomotherapy users.