Optimal Fractionation and Timing of Weekly Cone-Beam CT in Daily Surface-Guided Radiotherapy for Breast Cancer

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SGRT has been demonstrated as a promising supplement to 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 setup and dose accuracy. Both daily SGRT and weekly CBCT images were acquired for 23 breast cancer patients receiving conventional fractionated radiotherapy after lumpectomy. Based on the scanning frequencies and time intervals of CBCT, sixteen modes were acquired by randomly selecting one (CBCT1), two (CBCTij), three (CBCTijk), four (CBCTijkl), and five (CBCT12345) images from the patient's off-line CBCT images for fusion with SGRT. Due to TPS system limitations, the 6D SGRT setup errors were transformed into 3D deviations in the translational directions, and the CTV-PTV margins, the dose coverage (V95%) of PTV and CTV, and the OAR doses (the ipsilateral lung's V5, V10, V20, and Dmean, the contralateral breast's V3 and Dmean, and the heart's Dmean and Dmax) were calculated based on the 3D deviations with different regions of interest (ROIs). Dose correlations between these modalities were investigated using Pearson and Spearman's methods. For the ipsilateral breast ROI, the larger SGRT errors in the AP direction and higher target doses were found in all modes compared to the whole breast ROI (P < 0.05). In the ipsilateral ROI, the CTV-PTV margins decreased as CBCT frequencies increased, reaching approximately 5 mm for CBCTijkl and CBCT12345. The decline of Σ (systematic errors) was greater than the decline of σ (random errors), and it contributed more to the decline of margins. For the same frequency, the CBCT time intervals had essentially little influence on the errors and margins. Compared with other directions, the margins in the SI direction were the largest for all modes except for CBCT124 and CBCT125. The target dose correlations between all modes increased with increasing CBCT time intervals, decreased and then increased with increasing CBCT frequencies, with the turning point observed at week 5 of CBCT participation. The dose deviations in CBCT123, CBCT124, CBCT125, CBCTijkl, and CBCT12345 were minimal and not significantly different (P > 0.05). There was excellent agreement in CBCT124 vs. CBCT1234, and (CBCTijkl, CBCT12345) vs. CBCT125 in determining the classification for the percentage of PTV deviation (Kappa = 0.775-0.901). The frequencies and time intervals of CBCT had little effect on the OAR doses. For all modes, there were strong correlations (R > 0.9) in the OAR doses between different modes, except for the ipsilateral lung's V20 and Dmean (R = 0.734-0.987). Based on weekly CBCT, these modes with the ipsilateral breast ROI and a combination of daily SGRT and CBCT frequencies of ≥ 3 were recommended, and the CBCT was required at weeks 1 and 2. Considering the additional radiation, time consumption, and expensive cost of CBCT, modes CBCT124 and CBCT125 could replace the modes with higher CBCT frequencies.