2756 - Can Surface-Guided Radiation Therapy (SGRT) be Used to Monitor Intrafraction Motion of Patients Receiving Accelerated Partial Breast Irradiation (APBI)?

J. Zhou¹, S. Chen¹, B. Zhang¹, G. Lasio¹, H. Xu¹, J. Xu¹, Q. Chen¹, and R. Benda²; ¹Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, ²Department of Radiation Oncology, University Of Maryland School of Medicine, Baltimore, MD

Purpose/Objective(s): We investigated if the patients receiving accelerated partial breast irradiation (APBI) can be monitored with surface-guided radiation therapy (SGRT) to track the intrafraction motion. Materials/

Methods: The data were collected from 7 APBI patients with total 35 fractions treated in our institution. Three fiducial markers (superior, middle, and inferior markers) had been implanted outside the lumpectomy cavity and evenly in the target; and during each fraction of treatment, the patients were aligned to the fiducial markers with kV/kV images. Prior to the kV/kV images, patients were aligned based on the motion tracking system (VisionRT, Somerset, NJ). During the treatment, the intrafraction motion was monitored using the motion tracking system as well as triggered kV imaging of fiducial markers which represent the location of target. The t-tests were performed to compare the means of magnitude in the motion tracking system and distances between fiducial markers in triggered kV imaging to those in DRRs of planning CT.
Results: The average of magnitude in the motion tracking system and distances between fiducial markers in triggered kV imaging to those in DRRs of planning CT for each patient are shown in table 1. The average of magnitude in the motion tracking system is 0.097 cm, the average distances between superior, middle, and inferior fiducial markers in triggered kV imaging to those in DRRs of planning CT are 0.209, 0.187, and 0.200 cm. There are significant differences between the magnitude in the motion tracking system and the distances of each marker to its DRR of planning CT, such as superior marker (p=0.031), middle marker (p=0.005), inferior marker(p=0.011).
Conclusion: Although the motion tracking system is widely used in breast setup, it does not accurately represent the infraction motion to allow for smaller PTV margin in APBI.

Abstract 2756 – Table 1

		Mean (cm)	SD (cm)	p-value
Fiducial marker	superior	0.209	0.126	0.031
The motion tracking system	magnitude	0.097	0.060
Fiducial marker	middle	0.187	0.048	0.005
The motion tracking system	magnitude	0.097	0.060
Fiducial marker	inferior	0.200	0.082	0.011
The motion tracking system	magnitude	0.097	0.060