(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: Our institution recently installed a multiple brain-metastases TPS. The system requires dosimetric evaluation for commissioning and patient specific QA. The purpose of this study is to develop a three-dimensional radiochromic film dosimetry phantom and method to validate the calculated absolute dose and dose distribution.
Methods: A special phantom simulating human head dimensions was constructed using acrylic slabs. The phantom consists of 22 slabs, each measuring 20x20 cm² and 8.5 mm thickness. The phantom is housed in a frame made of the same material for accuracy and reproducibility. The phantom construction allows for easy film placement. Nine radiopaque markers were attached to the phantom for film location and identification. An EBT3 radiochromic film sheet is placed above the slabs with a radiopaque marker. Five reflective markers were placed on the phantom for setup. A CT simulation scan with axial 0.75 mm slice thickness was generated. Single tumor and multiple tumor SRS cases were planned on the phantom for testing. Multiple tumor patient SRS cases were projected onto the phantom. The phantom was accurately placed at isocenter using reflective markers and the ExacTrac system. Exposed films were scanned and analyzed using FilmQA Pro. Gamma analysis was performed for planned dose and measured dose distributions using a 3%/1 mm criteria.
Results: A single tumor phantom plan had an agreement of 100 %. Both phantom and patients’ multiple tumor plans had passing rate ranging from 94%-98% depending on distance of tumor from isocenter. The lowest passing rate is 91.1% which is for a patient lesion which is 7.9 cm from isocenter.
Conclusion: Treating multiple brain tumors with a single isocenter requires high linac geometric accuracy (e.g. 0.5 mm and 0.5°). This method for dosimetric evaluation proved to be simple and cost-effective. However, for patients with distant tumors, additional isocenter plans might be required.
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