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Dosimetric Effects Due to Indexing Clips On An Immobilization Device

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K Verdecchia

K Verdecchia*, S Balik , P Xia , The Cleveland Clinic Foundation, Cleveland, OH


SU-I-GPD-T-76 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The safety of radiotherapy patients highly depends on accurate delivery of a treatment plan to relatively small targets. Various immobilization devices and indexing base boards are used for patient setup, which are secured to the couch-top by indexing clips. Recently, the immobilization device used in our clinic had either aluminum or high density plastic mounting clips. This study investigated dosimetric effects due to these clips being in the radiation field during treatment.

Methods: The board was tested by a Varian Edge machine using 6MV, 6MV FFF, 10MV and 10MV FFF energy beams. First, an open field (20X20 cm) at gantry angle 115° was irradiated at the central axis through the middle of the clamp to represent a worst-case scenario. Dosimetric measurements were acquired using an ion chamber array (24X24 cm active area) without the board, and with the board with metal clips and with high density plastic clips. The attenuation of the clips was investigated by dosimetric comparison. Also, patient specific IMRT-QA for two SBRT patients (two half-arc VMAT plans), which contained similar target sizes as the clips, were performed with the board containing metal clips and without the board.

Results: The attenuation of the plastic and metal clips were 17% and 22% (6MV), 16% and 24% (10MV), 12% and 19% (10MV) and 12% and 13% (10MV FFF), respectively. No clinically significant differences were observed between the measured IMRT-QA using the base board with metal clip and no board (3%/3mm was used for gamma analysis).

Conclusion: Using the base plate with the plastic or metal clip caused significant attenuation of the beam. For static gantry angles, the patient setup should avoid direct radiation beams going through the clips. As expected, the overall effect was much lower for VMAT beams and did not change the gamma passing rate significantly.

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