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Weekly Vs. Daily Online Adaptation for Head and Neck Intensity-Modulated Proton Therapy

M Bobic1,2*, A Lalonde1,3, G Sharp1,3, C Grassberger1,3, J Verburg1,3, B Winey1,3, A Lomax2,4, H Paganetti1,3, (1) Massachusetts General Hospital, Boston, USA (2) ETH Zurich, Zurich, CH, (3) Harvard Medical School, Boston, USA, (4) Paul Scherrer Institute, Villigen, CH

Presentations

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

Room: AAPM ePoster Library

Purpose: To investigate the impact of weekly and daily online intensity-modulated proton therapy (IMPT) plan adaptation using GPU Monte Carlo (gPMC) for dose calculation on scatter-corrected cone-beam CT (CBCT) images.

Methods: A representative cohort of six head and neck cancer patients with daily acquired CBCTs was evaluated in this retrospective study. For each patient, an IMPT plan was created based on the planning CT. Dose tracking of the IMPT treatment was performed for three scenarios: base plan with no adaptation (BP), weekly adaptation (WA) and daily adaptation (DA). WA was performed on the first day of each week and the adapted plan was then used for each fraction until the next weekly adaptation. Both adaptation schemes used an in-house developed online adaptation workflow performing gPMC dose calculation on CBCTs. The CBCT images were scatter-corrected using an algorithm previously validated for proton therapy. Online adaptation was achieved by combining isocenter shifts and energy modifications of individual beamlets with subsequent weight adjustments. Planning contours were propagated using deformable image registration from the CT to each daily CBCT. The same propagated contours were used for dose tracking in all three scenarios.

Results: Both WA and DA achieved an improved target coverage compared to BP, while DA outperformed WA. For the dose scored on each fraction individually, the mean values over all fractions for the CTV V95% were 95.0%, 97.9% and 99.6% of the target volume for BP, WA, and DA respectively. Scoring the accumulated dose over all fractions yields 95.9%, 99.4% and 99.7% for the same metrics, reducing the gap between WA and DA. The dose to organs at risk remained in a clinically acceptable range for both adaptation scenarios.

Conclusion: This study demonstrates considerably improved plan qualities for online IMPT plan adaptation, whereby weekly adaptation appears sufficient for the majority of patients.

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Funding Support, Disclosures, and Conflict of Interest: This work was supported by NCI R01 229178

Keywords

Not Applicable / None Entered.

Taxonomy

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

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