Impact of two-dimensional dosimetric leaf gap on SRS-VMAT for single-isocenter multi-target brain metastases

Abstract

To explore the effects of two-dimensional dosimetric leaf gap (DLG) on the gamma pass rate (GPR) for single-isocenter multi-target (SIMT) stereotactic radiosurgery for patients with brain metastases. Two-dimensional DLG data were measured by an electronic portal imaging device (EPID) with the sweeping gap methods provided by Varian. A total of 56 arc fields from 28 SIMT patients were analyzed, including 19 with multiple targets and 9 with single targets. The leaf gap in patients' plan were modified using an in-house program, which adjusted the multileaf collimator positions based on the DLG values from the two-dimensional DLG distribution. Dose measurements were performed with EPID for both the original and modified plans. Comparisons were made between the measurements and the predicted dose planes from the treatment planning system using 3%/2 mm gamma criteria. Additionally, factors influencing the improvements in measurement results were analyzed.Compared to the original plan, the GPR for sweep test plans with leaf gaps of 2 mm, 4 mm, 6 mm, 10 mm, 14 mm, 16 mm and 20 mm increased by 12.2%, 17.6%, 25.4%, 58.2%, 83.6%, 84.3%, and 82%, respectively, after modification. For the clinical VMAT fields, GPR improvements fluctuated significantly due to the variations of modulation complexity score, the monitor units, as well as the number and volume of targets. The maximum GPR improvements with 16% and 36.3% were observed for the single-isocenter single-target and SIMT patients respectively. A linear relationship was found between GPR improvement and both modulation complexity score and the number of targets, whereas no significant correlation was found with monitor units or target volume. The two-dimensional DLG significantly impacts treatment plans for SIMT and single-isocenter single-target patients, particularly for SIMT cases. The GPR improvement correlates linearly with increases in modulation complexity score and the number of targets, while the monitor units and target volume show minimal correlation with GPR improvement. This study underscores the importance of accurate beam modeling as a key factor influencing GPR improvement.

Keywords: Dosimetric leaf gap; Electronic portal image device; Gamma pass rate; Multileaf collimators; Multiple target brain metastases; Sweep gap.

Fig. 1

Illustration of irradiation path through the MLC with rounded end and the relationship between the optical field, radiation filed and DLG.

Fig. 2

Acquisition process of 2D DLG distribution map. (a) The EPID detection results of the sliding window gap MLC gap plans with gap of 2 mm to 20 mm. (b) Extrapolation graph to calculate the DLG value of each pixel. (c) 2D DLG distribution map.

Fig. 3

The flowchart of in-house developed software-DLGTuner to adjust the MLC position according to the 2D DLG map.

Fig. 4

The comparison between the measured and calculated dose for Plan_org. (a) dose distribution calculated by TPS. (b) dose distribution measured by EPID. (c) dose profile comparison between measured and calculated in X direction. (d) dose profile comparison between measured and calculated in Y direction.

Fig. 5

The comparison between the measured and calculated dose for Plan_new. (a) dose distribution calculated by TPS. (b) dose distribution measured by EPID. (c) dose profile comparison between measured and calculated in X direction. (d) dose profile comparison between measured and calculated in Y direction.

Fig. 6

The relationship between the GPR improvements and (a) MCS, (b) target number, (c) number of MUs and (d) target volumes for the multiple targets brain metastasis.

Fig. 7

Dose profiles of sweeping gap field (2 mm width) acquired by the EPID.