E Persson*, E Goodwin, B Eiben, A Wetscherek, S Nill, U Oelfke, Joint Department of Physics, The Royal Marsden Hospital and The Institute of Cancer Research, Sutton, UK

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  E Persson

SU-K-BRC-4 (Sunday, 7/10/2022) 5:00 PM - 6:00 PM [Eastern Time (GMT-4)]

Ballroom C

Purpose: Real-time dose estimation is a prerequisite for online intra-fraction treatment adaptation. Here we propose a method for online dose estimation of MR-guided adaptive treatments based on real-time datastreams of the linac delivery parameters and target positions.

Methods: Our in-house motion management software DynaTrack was used to simulate treatment delivery with and without MLC tracking for three prostate patients, previously treated on the Elekta Unity MR-linac (36.25Gy/5 fractions), using two motion traces (erratic and drifting). Accumulated MUs, target and MLC-positions, were forwarded from DynaTrack at 25Hz to a dose calculation engine which utilises the research GPUMCD-library (Elekta AB, Stockholm, Sweden). A rigid isocenter shift according to input motion was applied to a bulk density-assigned session MR-image, accessed through the Research Monaco TPS (v.5.59.02a, Elekta AB, Stockholm, Sweden). With an update frequency too high to facilitate real-time calculation, a subsampled calculation approach was implemented. Once a new sample was received, a dose calculation with a predefined fixed number of particles was started and samples received in the meantime were put into a queue. After each calculation, one new sample was randomly picked from the queue and assigned the MU of the last sample. Online dose estimations for the different motion scenarios were compared to offline dose estimations using all samples.

Results: On average, the online dose calculation used 19% (15-22%) of received samples. One calculation typically lasted 200-300ms. Median gamma pass rate (1%/1mm global, 15% cut-off) was 100.0% (99.8-100%) with MLC tracking and 99.8% (99.0-100.0%) without. Differences in PTVDmean, CTVDmean, RectumD2 and BladderD2 (offline-online, % of prescribed dose) ranged between -0.3%-0.1% with MLC tracking and -0.3%-0.3% without.

Conclusion: Real-time dose estimation was successfully performed for different prostate motion traces with and without MLC tracking. Negligible dosimetric differences were seen compared to offline estimations, paving the way for online intra-fraction treatment adaptation.

Funding Support, Disclosures, and Conflict of Interest: The Institute of Cancer Research is supported by Cancer Research UK Programme Grants C33589/A28284 and C7224/A28724. The ICR is a member of the Elekta MR Linac consortium and has a research agreement with Elekta.

Keywords

MLC, Organ Motion, Reconstruction

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking

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