N C Biswal*, B Zhang, E Nichols, M E Witek, W F Regine, B Yi, University of Maryland School of Medicine, Baltimore, MD

PO-GePV-M-80 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: This study tested the change in beam path length (BPL) on daily cone-beam (CBCT) images as an adaptive planning indicator in proton beam therapy (PBT) for extremity tumors.

Methods: In this IRB-approved study, we retrospectively analyzed 577 CBCT images from 59 patients, with extremity tumors treated with PBT. Sixteen patients underwent adaptive planning on their quality assurance CT scans (QACTs) for various reasons (e.g., anatomic changes, tumor growth, and swelling). Each beam was ray traced from isocenter to skin surface along the central axis on the daily CBCT image. Patterns of change in BPL on CBCT images were compared to those from adaptive planning using QACTs. Here, true positive is an outcome where the BPL change correctly predicts the adaptive replan, and true negative is an outcome where it predicts the non-replanned cases. Similarly, false negative misses to predict adaptive replan and false positive incorrectly predicts adaptive replans.

Results: We used a 4-mm threshold for BPL change, above which an adaptive plan was required. If at least 1 of the beams showed a BPL ≥ 4 mm on 3 consecutive CBCT sessions, a QACT was required for adaptive replanning. Fifteen of the 16 adaptively replanned patients showed BPL changes ≥ 4 mm, resulting in a sensitivity of 93.8% for BPL change as an indicator for adaptive planning. The false negative case was replanned due to change in anatomy. Similarly, 32 out of 43 non-adaptively planned patients had BPL changes <4 mm, resulting in a specificity of 74.4%. In 2 of the 16 replanned and 15 of the 43 no-replanned cases, BPL changes were negative.

Conclusion: BPL changes from isocenter to skin surface along the beam central axis on CBCT images can be used as an adaptive plan indicator for PBT of extremity tumors with 93.8% sensitivity and 74.4% specificity.

Keywords

Cone-beam CT, Protons

Taxonomy

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

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