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

Abstract

Background and PurposeThis study quantifies intrafraction motion in surface-guided radiotherapy (SGRT) for breast cancer and considers the need for individualized intrafraction motion measures when calculating Planning Target Volume (PTV) margins. MethodsSGRT was used to assess intrafraction motion in consecutive patients according to (1) site irradiated (whole breast (WB)/chest wall (CW) versus WB/CW + regional lymph nodes) and (2) the use of deep inspiration breath hold (DIBH) versus free-breathing (FB). 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 individualised intrafraction motion measures for PTV margin determination was considered. Results102 patients undergoing 1360 fractions were included. On a population level, average intrafraction motion was less than 0.4mm and 0.2 degrees for translational and rotational directions, respectively, with 95th percentiles < 1.2mm and 0.6 degrees. No clinically meaningful differences in intrafraction motion were observed according to site irradiated or the use of DIBH. 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. ConclusionIntrafractional deviations with SGRT, using manufacturer-recommended ROIs, 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 ROI size and coverage.