Considering Lumpectomy Cavity PTV Expansions: Characterization of Intrafraction Lumpectomy Cavity Motion

Purpose: Accelerated partial breast irradiation and lumpectomy cavity boost radiation therapy plans generally use volumetric expansions from the lumpectomy cavity clinical target volume to the planning target volume (PTV) of 1 to 1.5 cm, substantially increasing the volume of irradiated breast tissue. The purpose of this study was to quantify intrafraction lumpectomy cavity motion during external beam radiation therapy to inform the indicated clinical target volume to PTV expansion. Methods and Materials: Forty-four patients were treated with a whole breast irradiation using traditional linear accelerator-based radiation therapy followed by lumpectomy cavity boost using magnetic resonance (MR)-guided radiation therapy on a prospective registry study. Two-dimensional cine-MR images through the center of the surgical cavity were acquired during each boost treatment to define the treatment position of the lumpectomy cavity. This was compared with the reference position to quantify intrafraction cavity motion. Free-breathing technique was used during treatment. Clinical outcomes including toxicity, cosmesis, and rates of local control were additionally analyzed. Results: The mean maximum displacement per fraction in the anterior-posterior (AP) direction was 1.4 mm. Per frame, AP motion was <5 mm in 92% of frames. The mean maximum displacement per fraction in the superior-inferior (SI) direction was 1.2 mm. Per frame, SI motion was <5 mm in 94% of frames. Composite motion was <5 mm in 89% of frames. Three-year local control was 97%. Eight women (18%) developed acute G2 radiation dermatitis. With a median follow-up of 32.4 months, cosmetic outcomes were excellent (22/44, 50%), good (19/44, 43%), and fair (2/44, 5%). Conclusions: In approximately 90% of analyzed frames, intrafraction displacement of the lumpectomy cavity was <5 mm, with even less motion expected with deep inspiratory breath hold. Our results suggest reduced PTV expansions of 5 mm would be sufficient to account for lumpectomy cavity position, which may accordingly reduce late toxicity and improve cosmetic outcomes.