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Dose Reconstruction for DMLC Tracking and Gating in Adaptive Prostate Radiotherapy

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E Colvill

E Colvill1*, J Ng1, R O'Brien1, P Poulsen2, J Booth3, P Keall1, (1) the University of Sydney, Sydney, Australia (2) Aarhus University Hospital, Aarhus, Denmark (3) Royal North Shore Hospital, Sydney, Australia

TU-E-141-4 Tuesday 2:00PM - 3:50PM Room: 141

Purpose: To assess the accuracy of dynamic multileaf collimator (DMLC) tracking delivery and the comparison to gating as an intervention strategy for adaptive prostate radiotherapy.

Methods: An experimentally benchmarked isocenter shift dose reconstruction method was used to retrospectively assess the dose distributions delivered during previously observed intensity modulated arc therapy (IMAT)(RapidArc) fractions and compared to the dose distributions which would have been delivered had DMLC tracking or gating intervention strategies been implemented. Delivered dose distributions were constructed by incorporating the monitored prostate motions with the patients original treatment plan, to mimic the treatment delivery. These were compared with dose distributions calculated for DMLC tracking delivery created using the same dose reconstruction method with the addition of MLC positions from dynalog files obtained during tracking simulation with the original motion input. The tracking dose distributions were then compared to the gated dose distributions, constructed using altered motion files created to mimic the application of a gating threshold of 3mm for 5 seconds.

Results: The delivered dose distributions showed that dosimetric effects of intrafraction prostate motion were substantial for some fractions with a decrease of more than 5% from the planned PTV D₉₅ value. Evaluation of dose distributions for DMLC tracking delivery showed that tracking was effective in improving the PTV D₉₅ to within 2% of planned value for all fractions, while gating improved the PTV D₉₅ to within 4% of the planned values.

Conclusion: Intrafraction prostate motion can result in a decrease in target coverage. The results of our study suggest that the implementation of DMLC tracking would allow for higher accuracy in the delivery of prostate cancer radiotherapy treatments beyond the accuracy achievable through gating intervention.

Funding Support, Disclosures, and Conflict of Interest: This work is supported by an NHMRC Australia Fellowship

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