Intrafraction Motion in Surface-Guided Breast Radiation Therapy and its Implications on a Single Planning Target Volume Margin Strategy.

Purpose: This study quantifies intrafraction motion in surface-guided radiation therapy (SGRT) for breast cancer and considers the need for individualized intrafraction motion measures when calculating planning target volume (PTV) margins.

Methods and materials: SGRT was used to assess intrafraction motion in consecutive patients according to (1) site irradiated (whole-breast/chest wall vs whole-breast/chest wall + regional lymph nodes) and (2) the use of deep inspiration breath hold versus free breathing. Intrafraction motion variation was evaluated throughout the treatment course for all cases. Associations between intrafraction motion and patient-specific characteristics were explored. The usefulness of individualized intrafraction motion measures for PTV margin determination was considered.

Results: One hundred two patients undergoing 1360 fractions were included. On a population level, average intrafraction motion was less than 0.4 mm and 0.2 degrees for translational and rotational directions, respectively, with 95th percentiles <1.2 mm and 0.6 degrees, respectively. No clinically meaningful differences in intrafraction motion were observed according to the site irradiated or the use of deep inspiration breath hold. Consistency in intrafraction motion was noted for all patients throughout the treatment course. No clinically meaningful associations were found between intrafraction motion and patient-specific characteristics such as age, seroma volume, PTV volume, and mean body volume.

Conclusions: Intrafractional deviations with SGRT, using manufacturer-recommended regions of interest, are minimal, do not vary substantially for different treatment techniques or patient-specific characteristics, and remain constant throughout the treatment course. A universal intrafraction motion measure may be sufficient for calculating PTV margins. Further validation studies are needed to evaluate the impact of region of interest size and coverage.