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 (CBCT(1)), two (CBCT(ij)), three (CBCT(ijk)), four (CBCT(ijkl)), and five (CBCT(12345)) 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 CBCT(12345). 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 CBCT(123), CBCT(124), CBCT(125), CBCT(ijkl), and CBCT(12345) were minimal and did not differ significantly (P > 0.05). There was excellent agreement between CBCT(124) and CBCT(1234), and between (CBCT(ijkl), CBCT(12345)) and CBCT(125) 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).