Room: Stars at Night Ballroom 2-3
Purpose: In intensity-modulated proton therapy (IMPT) protons are used to deliver highly conformal dose distributions, targeting tumors and sparing organs-at-risk. However, due to uncertainties in both patient setup and relative stopping power (RSP), margins are added to the treatment volume during treatment planning, leading to higher doses to normal tissues. Cone-beam CT (CBCT) images are taken daily before treatment, however the poor image quality of CBCT limits the use of these images to position verification. In this work, we use a deep-learning based method to predict stopping power maps from daily CBCT images, allowing for online dose calculation in a step towards adaptive radiation therapy.
Methods: Patients were simulated using a Siemens TwinBeam dual-energy CT scanner, and the mixed-energy CT images were used. The Gammex RMI 467 electron density phantom is used to build a calibration curve that relates measured Hounsfield units (HU) to experimentally-measured RSP for each insert. To train the deep learning algorithm, CT simulation images from 20 head-and-neck cancer patients were converted to stopping power maps. RSP maps are then registered to daily CBCT images to create training pairs, and the algorithm learns a mapping from CBCT to RSP. Leave-one-out validation was used for evaluation, and mean absolute error (MAE), mean error (ME) and normalized cross-correlation (NCC) were used to quantify the differences between the CT-based and CBCT-based RSP maps.
Results: The CT HU-RSP calibration curve has an ME (and MAE) of 0.03Â±0.03 across 17 materials. The CBCT-based RSP generation method was evaluated on 20 head-and-neck cancer patients. The mean MAE between CT-based and CBCT-based RSP was 0.04Â±0.02, the mean ME was -0.01Â±0.03 and the mean NCC was 0.97Â±0.01 for all patients.
Conclusion: The proposed method provides sufficiently accurate RSP map generation from CBCT images, possibly allowing for CBCT-guided adaptive treatment planning for IMPT.
Funding Support, Disclosures, and Conflict of Interest: This research is supported in part by the National Cancer Institute of the National Institutes of Health under Award Number R01CA215718 (XY).
Not Applicable / None Entered.