Optical surface guidance for submillimeter monitoring of patient position during frameless stereotactic radiotherapy

Abstract

Purpose: To evaluate the accuracy of monitoring intrafraction motion during stereotactic radiotherapy with the optical surface monitoring system. Prior studies showing a false increase in the magnitude of translational offsets at non-coplanar couch positions prompted the vendor to implement software changes. This study evaluated two software improvements intended to address false offsets.

Methods: The vendor implemented two software improvements: a volumetric (ACO) rather than planar calibration and, approximately 6 months later, an improved calibration workflow (CIB) designed to better compensate for thermal drift. Offsets relative to the reference position, obtained at table angle 0 following image-guided setup, were recorded before beam-on at each table position and at the end of treatment the table returned to 0° for patients receiving SRT.

Results: Prior to ACO, between ACO and CIB, and after CIB, 223, 155, and 436 fractions were observed respectively. The median magnitude of translational offsets at the end of treatment was similar for all three intervals: 0.29, 0.33, and 0.27 mm. Prior to ACO, the offset magnitude for non-zero table positions had a median of 0.79 mm and was found to increase with increasing distance from isocenter to the anterior patient surface. After ACO, the median magnitude was 0.74 mm, but the dependence on surface-to-isocenter distance was eliminated. After CIB, the median magnitude for non-zero table positions was reduced to 0.57 mm.

Conclusion: Ongoing improvements in software and calibration procedures have decreased reporting of false offsets at non-zero table angles. However, the median magnitude for non-zero table angles is larger than that observed at the end of treatment, indicating that accuracy remains better when the table is not rotated.

Keywords: frameless radiosurgery; intrafraction motion; optical surface imaging; stereotactic radiosurgery; surface guided radiotherapy.

Figure 1

(left) Reference surface (in blue) created in the treatment planning system by contouring the open portion of the mask and subtracting a 0.5 cm margin and removing the eye region. (right) The imported reference surface in optical surface monitoring system is saved as a region of interest.

Figure 2

Treatment workflow and report capture time points for radiosurgery. When motion limits are exceeded, the patient is moved back to the reference position. If the action limits are still exceeded, another CBCT is taken. Patients are returned to the reference position at the end of treatment to record the final positional offset

Figure 3

Drift of vertical real time delta over time for a stationary object (blue) and the change in the camera pod board temperature (red). The magnitude of the drift is mostly comprised of changes in the vertical direction and stabilizes after approximately 15 min.

Figure 4

Cumulative histogram of the magnitude of real time deltas (RTDs) for the three time periods: (red) before advanced camera optimization (ACO), (green) after ACO but before customer information bulletin (CIB), and (blue) after CIB before beam‐on at non‐zero table angles

Figure 5

Longitudinal real time delta increases as the distance between the centroid of the region‐of‐interest (ROI) and isocenter increases at non‐zero couch angles prior to advanced camera optimization (ACO). This dependence is eliminated after ACO. (Before ACO: slope = 0.077 mm/cm, intercept = −0.04 mm (R ² = 0.173); After ACO: slope = −0.006 mm/cm, intercept = −0.52 mm, R ² = 0.001).

Figure 6

A comparison of the two‐dimensional (2D) offset of a tungsten BB in an anthropomorphic phantom determined by either optical surface monitoring system (OSMS) (circles) or EPID (crosses). Anterior indicates the BB has zero vertical offset between the ROI surface and isocenter while the posterior BB has a 19 cm offset. After advanced camera optimization (ACO), the 2D offset for a posterior target decreases to that of an anterior target.

Figure 7

Difference between real time deltas (RTDs) and EPID measured shifts at various couch angles for an anthropomorphic phantom with bb at isocenter located 8 cm below region‐of‐interest (ROI). Customer information bulletin (CIB) calibration reduced the optical surface monitoring system (OSMS) reported shifts where (a) prior to CIB they exceed 1 mm to (b) <0.5 mm.