Intrafraction motion monitoring to determine PTV margins in early stage breast cancer patients receiving neoadjuvant partial breast SABR.

Abstract Background and Purpose To quantify intra-fraction tumor motion using imageguidance and implanted fiducial markers to determine if a 5mm planning-target-volume (PTV) margin is sufficient for early stage breast cancer patients receiving neoadjuvant stereotactic ablative radiotherapy (SABR). Materials and Methods A HydroMark© (Mammotome) fiducial was implanted at the time of biopsy adjacent to the tumor. Sixty-one patients with 62 tumours were treated prone using a 5mm PTV margin. Motion was quantified using two methods (separate patient groups): 1) difference in 3D fiducial position pre- and post-treatment cone-beam CTs (CBCTs) in 18 patients receiving 21Gy/1fraction (fx); 2) acquiring 2D triggered-kVimages to quantify 3D intra-fraction motion using a 2D-to-3D estimation method for 44 tumours receiving 21Gy/1fx (n=22) or 30Gy/3fx (n=22). For 2), motion was quantified by calculating the magnitude of intra-fraction positional deviation from the pretreatment CBCT. PTV margins were derived using van Herkian analysis. Results The average±standard deviation magnitude of motion across patients was 1.3±1.15mm Left/Right (L/R), 1.0±0.9mm Inferiorly/Superiorly (I/S), and 1.8±1.5mm Anteriorly/Posteriorly (A/P). 85/105 (81%) treatment fractions had dominant anterior motion. 6/62patients (9.7%) had mean intra-fraction motion during any fraction >5mm in any direction, with 4 in the anterior direction. Estimated PTV margins for single and three-fx patients in the L/R, I/S, and A/P directions were 6.0x4.1x5.9mm and 4.5x2.9x4.3mm, respectively. Conclusion Our results suggest that a 5mm PTV margin is sufficient for the I/S and A/P directions if a lateral kV image is acquired immediately before treatment. For the L/R direction, either further immobilization or a larger margin is required.