T Tseng*, S Green, C Starrs, Y Lo, Mount Sinai Medical Center, New York, NY

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  T Tseng

PO-GePV-M-191 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To evaluate the feasibility of reducing target margin on patients receiving accelerated partial breast irradiation (APBI) by utilizing intra-fraction motion review (IMR) on TrueBeam and to determine the dosimetric benefits with smaller target volumes.

Methods: Four patients who received APBI (30Gy in 5 fractions) were treated with IMR. All patients had four to five clips implanted in the lumpectomy cavities. The targets were delineated following the guideline of a randomized Phase III trial of Livi et al. – CTV was contoured with 1cm margin from surgical clips and PTV was created with additional 1cm margin from CTV. The clips+0.5cm structure was used as intra-treatment motion threshold. IMR images were triggered every 200MU during static IMRT delivery. Clips alignment on IMR with clips+0.5cm structure was visually inspected by therapists and the treatment would be paused if the clips move out of threshold and patients would be repositioned with orthogonal kV images. The feasibility of reducing PTV margin from 1cm to 0.5cm was evaluated with the visibility of the clips on IMR. The dosimetric benefit with smaller PTV margin were also evaluated.

Results: Two to three intra-treatment images were taken for each IMRT field. Therapists reported the clips can be clearly visualized and with the assistance of clips structure overlay, patient movement outside of threshold can be effectively and timely corrected. By reducing PTV margin from 1cm to 0.5cm, the mean PTV volume was decrease by 47.93±18.94cc, the PTV to breast volume ratio was decrease by 7.05±1.45%, and as result reduced the uninvolved breast V15Gy by 8.08±3.72%.

Conclusion: Intra-treatment imaging with IMR can effectively monitor patient position during APBI and may enable reduction of PTV margin. Lowering PTV volume allows APBI as a treatment option for patients with larger lumpectomy cavities by lowering the dose spillage to the uninvolved breast.

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