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Optimal Planning Target Volume Margin for Lung SBRT with Minimal Immobilization

C Wilcken1, T Chow2, M Wierzbicki2*, (1) McMaster University, Hamilton, ON, CA (2) Juravinski Cancer Center, Hamilton, ON, CA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Patients undergoing lung SBRT are commonly positioned supine within a vacuum formed mould to ensure a stable setup. However, significant resources are necessary for mould preparation and maintenance while frail patients may struggle to enter the apparatus. Our goal is to determine the minimal planning target volume (PTV) margin necessary for lung SBRT without such a mould. Further, we aim to evaluate the suitability of optical skin surface tracking for real-time monitoring of patient setup during treatment.

Methods: 15 patients were treated over 69 fractions without the immobilization mould. Pre-treatment CBCT images were used to align targets. Patient position during treatment was monitored using optical surface tracking and post-treatment CBCTs were acquired. A previously validated method that relies on deformable image registration was used to localize the target on each CBCT. This was used to quantify the percentage of the target within a given PTV margin.

Results: For a 3mm PTV margin, the percentage of fractions where >95% of the target was within the PTV was 32, 100, and 83 for patient setup, corrected patient position based on the pre-treatment CBCT, and post-treatment position based on the final CBCT, respectively. For a 4mm margin, the percentages were 48, 100, and 93, while for a 5mm margin, they were 61, 100, and 95. The mean drift of the patient surface centroid from its starting location as detected using optical surface tracking was 1.5 and 2.5mm during the first and fifth minute of irradiation. Patients’ surfaces were within 5mm of their starting positions >95% of the time.

Conclusion: This analysis demonstrates a 5mm PTV margin is likely achievable in patients treated without a vacuum immobilization mould. Routine pre-treatment CBCT with target matching combined with optical surface tracking during treatment are sufficient tools to ensure accurate delivery of lung SBRT.


Deformation, Image-guided Therapy, Immobilization


TH- External Beam- Photons: onboard imaging (development and applications)

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