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Intrafraction Motion Monitoring to Determine PTV Margins in Early Stage Breast Cancer Patients Receiving Neoadjuvant Partial Breast SABR

M Mouawad1*, O Lailey1, M O'Neil1, P Poulsen2, P Keall3, H Biernaski4, M Brackstone5, M Lock5, B Yaremko5, S Karnas1, A Kornecki4, O Shmuilovich4, G Muscedere4, I Nachum4, F Prato4, R Thompson4, N Gelman4, S Gaede1, (1) London Regional Cancer Program, London, ON, CA, (2) Aarhus University Hospital, Aarhus, 82, DK, (3) University of Sydney, Camperdown, AU, (4) Lawson Health Research Institute, London, ON, CA, (5) London Health Sciences Center,London, ON, CA


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

Room: AAPM ePoster Library

Purpose: To quantify intra-fraction tumor motion using image-guidance and an implanted fiducial marker to determine if a 5mm planning target volume (PTV) margin is sufficient for treatment of early stage breast cancer patients receiving neoadjuvant stereotactic ablative radiotherapy (SABR).

Methods: At biopsy, a HydroMark© (Mammotome) fiducial was implanted adjacent to the tumor. Patients were treated prone (Aktina) using a 5mm PTV margin. Motion was quantified using three methods (separate patient groups): 1) Calculating the difference in 3D fiducial position between pre- and post-treatment cone-beam CTs (CBCTs) for 18 patients receiving 21Gy/1 fraction (fx); 2) Acquiring 2D triggered-kV images to quantify 3D intra-fraction motion using a 2D-to-3D estimation method for 18 patients receiving 21Gy/1fx; 3) Similar to 2) but also using pre-treatment CBCT projection data and automatic fiducial identification for 13 patients receiving either 21Gy/1fx (4) or 30Gy/3fx (9). For 2) and 3), motion was quantified by calculating the magnitude of intra-fraction positional deviation (µ(intra)) from the pre-treatment CBCT in each cardinal direction and averaging across patients (µ(pat)). The intra-fraction positional variability (s(pat)) is the root-mean-square of the intra-fraction positional standard deviation (s(intra)).

Results: The average±standard deviation magnitude of motion (µ(pat)) across all methods was: 1.5±1.1mm Left/Right (L/R), 1.1±1.0mm Inferiorly/Superiorly (I/S), and 1.8±1.6mm Anteriorly/Posteriorly (A/P). L/R motion was 0.6±2.1mm leftwards and 0.3±2.0mm rightwards for left and right sided patients, respectively. 53/62 (85%) treatment fractions had dominant anterior motion. 5/62fx (8%) had mean intra-fraction motion >5mm in any direction. 5/49 patients (10%) had motion >5mm (4/5 anterior direction). Finally, the intra-fraction positional variability (s(pat)) was: 1.4mm L/R, 0.4mm I/S, and 0.8mm A/P.

Conclusion: 5mm PTV margin is sufficient to account for treatment uncertainties for most patients, though anterior motion is a concern. This requires either intervening during treatment using triggered imaging, a larger anterior PTV margin, or better immobilization.

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