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Session: Multi-Disciplinary General ePoster Viewing [Return to Session]

Dosimetric Validation of An Intra-Fraction Stop-Scan-Adapt Protocol for 0.35T MRI-Guided Radiotherapy

J Lamb1*, R Lotey2, M Bellon2, S Tenn1, I Kawrakow2, J Dempsey2, (1) University of California, Los Angeles, Los Angeles, CA, (2) ViewRay Incorporated, Mountain View, CA


PO-GePV-M-223 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Magnetic resonance imaging-guided adaptive radiotherapy has demonstrated success in managing inter-fraction anatomic deformations for high-dose conformal radiotherapy. However, intra-fraction deformation remains a clinical challenge. Here we use thermo-luminescent dosimetry to evaluate a newly-developed “stop-scan-adapt” protocol that allows for pausing a treatment fraction midway through dose delivery, re-imaging, deforming dose delivered in the previous portion of the fraction, and adapting the radiation plan to achieve the originally intended target dose distribution.

Methods: Experiments were performed using a deformable phantom designed to mimic the rectum-bladder interface in a post-prostatectomy patient. The phantom was composed of a water bladder held adjacent to bags of crushed fruit. A target volume was defined as a 1 cm expansion about the bladder-fruit interface. Thermoluminescent dosimeters (TLDs) were placed at the center of that target volume. Experiments consisted of imaging the phantom, developing and partially delivering a treatment plan, voiding 100 milliliters of water from the phantom’s water bladder, and completing the treatment using the stop-scan-adapt (SSA) protocol. Control experiments were performed by delivering the full fraction without deformation and without SSA, and with deformation but without SSA.

Results: Without deformation, the mean dose calculated to the location of the TLDs was 737.4 cGy, and the measured value was on average 1% higher than the calculated dose. When the phantom was deformed half way through treatment, but adaptation was not performed, the mean calculated dose was 551.5 cGy, and the measured value was on average 8.3% lower. When the phantom was deformed half way through the treatment, and SSA was performed, the mean calculated dose to the TLDs was 775.3 cGy, and the measured value was on average 1.5% lower.

Conclusion: In this phantom test the stop-scan-adapt protocol maintains target dose in the face of large deformations, and the calculated dose agrees well with measured dose.

Funding Support, Disclosures, and Conflict of Interest: James Lamb reports consulting fees from ViewRay, Inc., outside the presented work. Rajiv Lotey and Maria Bellon are employees of ViewRay, Inc.


Not Applicable / None Entered.


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