B Zhang*, W Yao, N Biswal, J Zhou, J Xu, H Xu, S Chen, B Yi, University of Maryland School of Medicine, Columbia, MD

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  B Zhang

SU-H300-IePD-F5-5 (Sunday, 7/10/2022) 3:00 PM - 3:30 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 5

Purpose: : To develop an objective and evidence-based method by analyzing Bragg Peak positional variations in setup cone-beam CTs as a precursor to predict replanning of HN cancer intensity modulation proton treatments (IMPT).

Methods: We retrospectively analyzed Bragg Peak positions of CBCT images of 40 HN patients, 616 CBCT images (20 replanning cases and 20 no replanning cases). Dose distributions in the CBCT images were calculated after a 6-D translational/rotational clinical patient setup. Then the Bragg Peak position of each pencil beam spot was calculated. The Bragg Peak position variation between the planning CT and each following CBCT would be weighted by each pencil beam spot’s MU and be summed up to get a metrics called Weighted Bragg Peak Positional Variation (WBPPV). The weekly or bi-weekly QA-CT images for proton treatments were used to verify the need of replanning dosimetrically and geometrically. For a HN proton patient with replanning, we check the WBPPV values before and on the re-sim (or QA-CT) day. For a HN proton patient without replanning, the WBPPV trend during the whole treatment course was analyzed.

Results: Our data shows that a WBPPV larger than 0.5cm is correlated with 3% or more changes in the D98% of the CTV of the QACT, which is indicative of replanning or careful examination of patient setup. For a WBPPV value under 0.5cm, no significant anatomical change or patient setup errors are found in our analysis.

Conclusion: Even although the Bragg Peak positions in CBCT images are not reflective of actual Bragg Peak positions, WBPPV shows a strong correlation with anatomical change and patient setup error. Therefore, WBPPV can be used as a precursor for prediction of HN proton adaptive planning. WBPPV analysis of daily CBCT provides a potential objective solution for HN proton cancer treatment replanning.

Keywords

Cone-beam CT, Protons, Image-guided Therapy

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy - adaptive replanning

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