Quality Assurance of Multiple Brain Metastases with a Single-Isocenter Plan using Portal Dosimetry and Independent Dose Calculation

Abstract

Recently, a multiple metastases element (MME) for a linear accelerator (LINAC)-based radiation system was released commercially. It covers multiple brain metastases with a single-isocenter. The purpose of this study is to evaluate a quality assurance (QA) program for multiple targets with a single-isocenter plan. We verify this plan by portal dosimetry using an electronic portal imaging device (EPID) and independent dose calculation. We created 10 QA plans with a median of 3.5 targets (range, 2.0–9.0) using noncoplanar dynamic conformal arcs by MME. The median number of arcs was 6.5 (range, 4.0-9.0). The median planning target volume (PTV) was 0.76 cm3 (range, 0.45-2.76). These plans were delivered from a medical linear accelerator with a 6-MV X-ray beam. First, dosimetric comparison was conducted between MME with pencil beam (PB) and a treatment planning system with analytical anisotropic algorithm (AAA). The plan was transferred to the treatment planning system from the MME. The dose was then recalculated using the AAA with the same monitor unit (MU) and beam arrangement as the plans created using MME. Thereafter, the point dose of the PTV using the different dose-calculation algorithms were compared. Second, a portal dosimetry plan of the imported MME plan was created by an oncology information system. For the gamma analysis, a calculated portal image was acquired through a portal dose calculation algorithm in the treatment planning system, and a measured portal image was obtained using an EPID. Thereafter, the gamma analysis was performed using the portal dosimetry software. The resulting dose distributions were then compared with the corresponding planned dose distributions using the 2D gamma index with 3%/2 mm and 3%/1 mm passing criteria. In addition, LINAC delivery log files were acquired after the delivery of each arc plan. An in-house software was used to analyze the root mean square of the multi leaf collimator (MLC) error which was defined as the difference between the planned position and the actual position. The point dose for AAA was higher than the dose for PB by 0.5 ± 1.4 %. However, the differences were not statistically significant for AAA and PB (p-value=0.051). Only minor differences were observed in dose when comparing the two-treatment planning software related to the same plan. The average pass rates using the EPID images were 99.6±0.6% (3%/2 mm) and 97.3 ± 1.9% (3%1 mm). The methods showed a good agreement between calculation and measurement results in MUs and field shape. The MLC positioning between plan and delivered values were 0.019 ± 0.004 mm. There was no correlation between MLC errors and the pass rate. Our results indicate that the use of the recalculated dose and EPID are viable methods for the QA of multiple targets with single-isocenter plans. This QA method is useful as a rapid and convenient verification tool.