Small-Field Dosimetry in Single-Isocenter Stereotactic Irradiation for Brain Metastases Using a Detector-Specific Field Output Correction Factor
T Ono1*, M Nakamura2, M Uto1, T Mizowaki1, (1) Department of Radiation Oncology and Image-applied Therapy, Graduate School of Medicine, Kyoto University, Kyoto, Japan, (2) Division of Medical Physics, Department of Information Technology and Medical Engineering, Human Health Sciences, Kyoto University Graduate School of Medicine, Kyoto, Japan
Presentations
WE-IePD-TRACK 6-7 (Wednesday, 7/28/2021) 12:30 PM - 1:00 PM [Eastern Time (GMT-4)]
Purpose: In small-field dosimetry such as stereotactic irradiation for brain metastases, the measurement dose may include large uncertainties. Recently, IAEA-TRS483 has reported detector-dependent correction factors for small irradiation fields that are expected to reduce the uncertainty of the measured dose in small irradiation fields. This study aimed to evaluate the measured doses considering the correction factors in single-isocenter stereotactic irradiation for brain metastases.
Methods: A total of 14 sites: 9 cases of single brain metastasis and 2 cases of multiple brain metastases (including 2 sites and 3 sites) were included in this study. Single-isocenter stereotactic irradiation plans were created with a dynamic conformal arc using XVMC (calculation grid size: 1 mm) in Elements Multiple Brain Mets SRS (Brainlab). The dose was measured with a water-equivalent phantom using a cc04 chamber (IBA). The multi-leaf collimator (MLC) field size of each target was calculated for each control point (CP) using Python. Each correction factor was calculated from the MLC field size referred to in the IAEA-TRS483 report. Finally, the plan correction factor was calculated from the average correction factor weighted by the monitor unit for each CP. The measured doses with and without the correction factor were compared with the values calculated by the XVMC, and the differences were evaluated as the mean value ± standard deviation.
Results: The difference between the measured and calculated doses was −3.2% ± 1.5% (range: −5.3–0.6%) without correction and −1.8% ± 1.2% (range: −3.6–1.0%) with correction, indicating that the difference between the measured and calculated doses decreased with correction.
Conclusion: The measured doses considering the correction factors were evaluated in single-isocenter stereotactic irradiation for brain metastases. A more reliable evaluation of the dose in patients is expected by considering the output correction that takes into account the detector characteristics in small-field dosimetry.
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