Comparison of three differently shaped ROIs in free breathing breast radiotherapy setup using surface guidance with AlignRT®

Optimal fractionation and timing of weekly cone-beam CT in daily surface-guided radiotherapy for breast cancer

PURPOSE

Surface-guided radiotherapy (SGRT) has been demonstrated to be a promising supplement to cone-beam computed tomography (CBCT) in adjuvant breast cancer radiotherapy, but a rational combination mode is lacking in clinical practice. The aim of this study was to explore this mode and investigate its impact on the setup and dose accuracy.

METHODS AND MATERIALS

Daily SGRT and weekly CBCT images were acquired for 23 patients with breast cancer who received conventional fractionated radiotherapy after lumpectomy. Sixteen modes were acquired by randomly selecting one (CBCT₁), two (CBCT_ij), three (CBCT_ijk), four (CBCT_ijkl), and five (CBCT₁₂₃₄₅) images from the CBCT images for fusion with the SGRT. The CTV-PTV margins, OAR doses, and dose coverage (V95%) of PTV and CTV was calculated based on SGRT setup errors with different regions of interest (ROIs). Dose correlations between these modalities were investigated using Pearson and Spearman's methods. Patient-specific parameters were recorded to assess their impact on dose.

RESULTS

The CTV-PTV margins decreased with increasing CBCT frequencies and were close to 5 mm for CBCT_ijkl and CBCT₁₂₃₄₅. For the ipsilateral breast ROI, SGRT errors were larger in the AP direction, and target doses were higher in all modes than in the whole breast ROI (P < 0.05). In the ipsilateral ROI, the target dose correlations between all modes increased with increasing CBCT time intervals, decreased, and then increased with increasing CBCT frequencies, with the inflection point being CBCT participation at week 5. The dose deviations in CBCT₁₂₃, CBCT₁₂₄, CBCT₁₂₅, CBCT_ijkl, and CBCT₁₂₃₄₅ were minimal and did not differ significantly (P > 0.05). There was excellent agreement between CBCT₁₂₄ and CBCT₁₂₃₄, and between (CBCT_ijkl, CBCT₁₂₃₄₅) and CBCT₁₂₅ in determining the classification for the percentage of PTV deviation (Kappa = 0.704-0.901). In addition, there were weak correlations between the patient's D_{ips_b} (ipsilateral breast diameter with bolus) and CTV doses in modes with CBCT participation at week 4 (R = 0.270 to 0.480).

CONCLUSIONS

Based on weekly CBCT, these modes with ipsilateral ROI and a combination of daily SGRT and a CBCT frequency of ≥ 3 were recommended, and CBCT was required at weeks 1 and 2 for CBCT_ijk.

Accuracy and potential improvements of surface-guided breast cancer radiotherapy in deep inspiration breath-hold with daily image-guidance

Objective. Radiotherapy of left-sided breast cancer in deep inspiration breath-hold (DIBH) reduces the heart dose. Surface guided radiotherapy (SGRT) can guide the DIBH, but the accuracy is subject to variations in the chest wall position relative to the patient surface. Approach. In this study, ten left-sided breast cancer patients received DIBH radiotherapy with tangential fields in 15–18 fractions. After initial SGRT setup in free breathing an orthogonal MV/kV image pair was acquired during SGRT-guided breath-hold. The couch was corrected to align the chest wall during another breath-hold, and a new SGRT reference surface was acquired for the gating. The chest wall position error during treatment was determined from continuous cine MV images in the imager direction perpendicular to the cranio-caudal direction. A treatment error budget was made with individual contributions from the online registration of the setup MV image, the difference in breath-hold level between setup imaging and SGRT reference surface acquisition, the SGRT level during treatment, and intra-fraction shifts of the chest wall relative to the SGRT reference surface. In addition to the original setup protocol (Scenario A), SGRT was also simulated with better integration of image-guidance by capturing either the new reference surface (Scenario B) or the SGRT positional signal (Scenario C) simultaneously with the setup MV image, and accounting for the image-guided couch correction by shifting the SGRT reference surface digitally. Main results. In general, the external SGRT signal correlated well with the internal chest wall position error (correlation coefficient >0.7 for 75% of field deliveries), but external-to-internal target position offsets above 2 mm occasionally occurred (13% of fractions). The PTV margin required to account for the treatment error was 3.5 mm (Scenario A), 3.4 mm (B), and 3.1 mm (C). Significance. Further integration of SGRT with image-guidance may improve treatment accuracy and workflow although the current study did not show large accuracy improvements of scenario B and C compared to scenario A.