Surface imaging-based analysis of intrafraction motion for breast radiotherapy patients

Abstract

Breast treatments are becoming increasingly complex as the use of modulated and partial breast therapies becomes more prevalent. These methods are predicated on accurate and precise positioning for treatment. However, the ability to quantify intrafraction motion has been limited by the excessive dose that would result from continuous X-ray imaging throughout treatment. Recently, surface imaging has offered the opportunity to obtain 3D measurements of patient position throughout breast treatments without radiation exposure. Thirty free-breathing breast patients were monitored with surface imaging for 831 monitoring sessions. Mean translations and rotations were calculated over each minute, each session, and over all sessions combined. The percentage of each session that the root mean squares (RMS) of the linear translations were outside of defined tolerances was determined for each patient. Correlations between mean translations per minute and time, and between standard deviation per minute and time, were evaluated using Pearson's r value. The mean RMS translation averaged over all patients was 2.39 mm ± 1.88 mm. The patients spent an average of 34%, 17%, 9%, and 5% of the monitoring time outside of 2 mm, 3 mm, 4 mm, and 5 mm RMS tolerances, respectively. The RMS values averaged over all patients were 2.71 mm ± 1.83 mm, 2.76 ± 2.27, and 2.98 mm ± 2.30 mm over the 5th, 10th, and 15th minutes of monitoring, respectively. The RMS values (r = 0.73, p = 0) and standard deviations (r = 0.88, p = 0) over all patients showed strong significant correlations with time. We see that the majority of patients' treatment time is spent within 5 mm of the isocenter and that patient position drifts with increasing treatment time. Treatment length should be consid- ered in the planning process. An 8 mm margin on a target volume would account for 2 SDs of motion for a treatment up to 15 minutes in length.

Figure 1

The striated pink surface represents a reference surface generated using the body structure from the treatment plan. The solid pink surface is the ROI used for AlignRT registration. The ROI includes the ipsilateral chest wall and the base of the breast.

Figure 2

An example of the AlignRT monitoring screen seen during treatment. The real‐time RMS offset along with linear translations and rotations are shown on the left. The bars are green if the suggested shifts are within a predefined tolerance (3 mm and 3° in this case). The reference surface and ROI are shown in pink on the right. The real time AlignRT surface is overlaid on the reference surface in green. At setup, the therapists attempt to minimize the shifts (by minimizing the length of the green bars). Note, the term RMS is used in the text to refer to the MAG shown in the figure.

Figure 3

Number of sessions and sampling time.

Figure 4

Example of raw RMS measurements (a) as a function of time for a monitoring session that shows therapist correction of patient position around the 600 s time. Example of “uncorrected” RMS data (b), where the data after the intervention shown in (a) have been rescaled to account for intrafraction repositioning (i.e., to give an estimate of patient position if no intrafraction repositioning had occurred).

Figure 5

Mean shift values over all patients and sessions.

Figure 6

(a) The total time spent outside a defined tolerance per session and (b) the consecutive time spent out of a defined tolerance per session normalized by the session time. Tolerances of 2 mm, 3 mm, 4 mm, and 5 mm are shown for each patient.

Figure 7

Mean RMS shifts per minute over all patients and sessions. A strong correlation can be seen between minutes in the session and RMS displacement.

Figure 8

Changes in the standard deviations over the group of all patients as a function of time.