Improving intra-fractional target position accuracy using a 3D surface surrogate for left breast irradiation using the respiratory-gated deep-inspiration breath-hold technique

Abstract

Purpose: To evaluate the use of 3D optical surface imaging as a surrogate for respiratory gated deep-inspiration breath-hold (DIBH) for left breast irradiation.

Material and methods: Patients with left-sided breast cancer treated with lumpectomy or mastectomy were selected as candidates for DIBH treatment for their external beam radiation therapy. Treatment plans were created on both free breathing (FB) and DIBH computed tomography (CT) simulation scans to determine dosimetric benefits from DIBH. The Real-time Position Management (RPM) system was used to acquire patient's breathing trace during DIBH CT acquisition and treatment delivery. The reference 3D surface models from FB and DIBH CT scans were generated and transferred to the "AlignRT" system for patient positioning and real-time treatment monitoring. MV Cine images were acquired during treatment for each beam as quality assurance for intra-fractional position verification. The chest wall excursions measured on these images were used to define the actual target position during treatment, and to investigate the accuracy and reproducibility of RPM and AlignRT.

Results: Reduction in heart dose can be achieved using DIBH for left breast/chest wall radiation. RPM was shown to have inferior correlation with the actual target position, as determined by the MV Cine imaging. Therefore, RPM alone may not be an adequate surrogate in defining the breath-hold level. Alternatively, the AlignRT surface imaging demonstrated a superior correlation with the actual target positioning during DIBH. Both the vertical and magnitude real-time deltas (RTDs) reported by AlignRT can be used as the gating parameter, with a recommended threshold of ±3 mm and 5 mm, respectively.

Conclusion: The RPM system alone may not be sufficient for the required level of accuracy in left-sided breast/CW DIBH treatments. The 3D surface imaging can be used to ensure patient setup and monitor inter- and intra- fractional motions. Furthermore, the target position accuracy during DIBH treatment can be improved by AlignRT as a superior surrogate, in addition to the RPM system.

Figure 1. Displays of AlignRT system, MV Cine imaging, DRR, and the RPM system for treatments.

(a) AlignRT screen layout showing the RTDs in translational and rotational directions, the overlay of the body rendering from the RSM (pink) and reconstructed surface imaging (green), and the display of the magnitude of all three translational RTDs in mm as a function of time; (b) MV cine image for a left medial field, compared to (c) the DRR of the corresponding field; (d) Patient's RPM trace, as a function of time.

Figure 2. Anatomy and isodose comparisons for (a) the left breast and (b) the left CW patients (left: FB CT; right: DIBH CT).

The CW expansion increased from 5.58

Figure 3. The DRRs of a left medial field on the FB CT (a) and DIBH CT (b).

The CW excursion measured from one leaf end to the interior CW surface was increased from 1.51

Figure 4. CW excursions offsets of the lateral and medial beams for 24 treatment fractions for the breast patient (a) and CW patient (b), respectively.

Dash lines indicate ±3 mm region. The average offset was −1.8 mm with a standard deviation of 2.1 mm for the breast patient. The average offset was −1.6 mm with a standard deviation of 2.2 mm for the CW patient.

Figure 5. The correlation of RPM displacement (horizontal axis) and CW excursion offset (vertical axis) for the breast and CW patient.

Figure 6. Five segments of breath-holds, corresponding to five beams of one treatment fraction, represented by AlignRT RTDs as a function of time for vertical (red dash line), lateral (purple dotted line), longitudinal (green dash dot line), and magnitude (black solid line).

Dash lines indicate ±3 mm and ±5 mm thresholds.

Figure 7. AlignRT RTDs as a function of time for vertical (red dash line), lateral (purple dotted line), longitudinal (green dash dot line), and magnitude (black solid line).

Panel (a)–(d) show four different representations of surface motion represented by RTDs.

Figure 8. The correlation of the CW excursion offsets with (a) the vertical RTDs and (b) the magnitude RTDs, respectively.

A linear correlation was observed with correlation coefficient of 0.47 for the vertical RTDs and 0.52 for the magnitude RTDs. A threshold of ±3 mm for the vertical and 5 mm for the magnitude RTDs can ensure CW excursion offsets to be within ±3 mm.

Figure 9. The CW excursion offsets for 24 fractions for the subsequent two patients, one breast and one CW, who was treated with adopting the AlignRT as the secondary surrogate for gating.

The dash lines indicate a ±3 mm region. The average offset and its standard deviation were −0.05 mm, 2.5 mm for the breast patient and 0.6 mm, 2.1 mm for the CW patient.

Figure 10. The CW excursion offsets for an additional nine breast patients and five CW patients, who were treated with the new approach combining both RPM and AlignRT as dual surrogates.