Daily dose evaluation based on synthetic CTs for breast cancer patients: accuracy of dose and complication risk assessment

Abstract

Abstract Objectives: The goal of this study is to propose a method for the generation of synthetic CTs from daily CBCTs that can be used for dose evaluation in breast cancer patients with large anatomical changes treated with photon irradiation. Materials and method: Seventy-six breast cancer patients treated with a partial VMAT photon technique (70% conformal, 30% VMAT) were included in this study. All patients showed at least a 5 mm variation (swelling or shrinkage) of the breast on the CBCT compared to the planning-CT (pCT) and had a repeat-CT (rCT) for dose evaluation acquired within 3 days of this CBCT. The original CBCT was corrected using four methods: 1) HU-override correction (CBCTHU), 2) analytical correction and conversion (CBCTCC), 3) deep learning (DL) correction (CTDL) and 4) virtual correction (CTV). Image quality evaluation consisted of calculating the mean absolute error (MAE) and mean error (ME) within the whole breast clinical target volume (CTV) and the field of view of the CBCT minus 2 cm (CBCT-ROI) with respect to the rCT. The dose was calculated on all image sets using the clinical treatment plan for dose and gamma passing rate analysis. Results: The MAE of the CBCT-ROI was below 66 HU for all synthetic CTs, except for the CBCTHU with a MAE of 142 HU. No significant dose differences were observed in the CTV regions in the CBCTCC, CTDL and CTv. Only the CBCTHUdeviated significantly (p<0.01) resulting in 1.7%(±1.1%)average dose deviation. Gamma passing rates were >95% for 2%/2mm for all synthetic CTs. Conclusion: The analytical correction and conversion, deep learning correction and virtual correction methods can be applied for an accurate synthetic CT generation that can be used for dose evaluation during the course of photon radiotherapy of breast cancer patients.