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An Assessment of Setup Accuracy and Deep Inspiration Breath Hold Reproducibility Using Intra-Fraction Portal Imaging for Conformal Breast Radiotherapy

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A Gray

A Gray1*, T Erven1, S Arumugam2 , (1) Liverpool and Macarthur Cancer Therapy Centres, Campbelltown, NSW, Australia (2) Liverpool and Macarthur Cancer Therapy Centres, Liverpool, NSW, Australia.


SU-I-GPD-J-4 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To assess the dosimetric impact of geometrical uncertainties in patient setup and Active Breathing Coordinator™ (ABC) assisted Deep Inspiration Breath Hold (DIBH) in conformal breast radiotherapy.

Methods: Eight left breast cancer patients received ABC assisted DIBH conformal radiotherapy on an Elekta linear accelerator (linac) with treatment gated by the ABC signal. The patient setup was supine on a breast board, set up to tattoos, anterior-posterior source-to-surface distance checked, then shifted to isocenter. Setup images for days 1-3 were assessed and an average shift subsequently applied if greater than 5mm. Images were taken during the delivery of each segment of treatment using the linac’s electronic portal imager. The intra-fraction DIBH reproducibility was assessed by comparing the manually segmented lung area, using MATLAB, in images taken during the same fraction for similarly shaped fields. Setup reproducibility was assessed by comparing lung area in treatment images to the planned digitally reconstructed radiographs. The delivered dose to the planning target volume (PTV), lung and heart were estimated by comparing the average treated lung area to offset isocenter plans.

Results: The average difference in lung area for the 255 same fraction field pairs was 0.2cm² (StDev=3.1cm²). The average difference between the treated and planned lung area was -5.2cm² (StDev=6.8cm²). 89% of the 643 images were within the 5mm tolerance however two patients skewed the results. The ipsilateral and combined lung volumes receiving 20Gy were 33.3% and 15.4% (2.2% and 1.1% higher than planned). The mean heart dose was 3.0Gy (0.6Gy high). The maximum values were 37.6%, 18.2% and 4.5Gy respectively. The mean PTV dose was 0.02% higher than planned.

Conclusion: While the DIBH reproducibility images demonstrated small differences in treated lung area, larger differences resulted from the setup error. The delivered doses to the lung and heart were higher than planned but not unacceptable.

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