Influence of different air CT numbers for IVDT on the dose distribution in TomoTherapy MVCT

This study aims to evaluate the effect of different air computed tomography (CT) numbers of the image value density table (IVDT) on the retrospective dose calculation of head-and-neck (HN) radiotherapy using TomoTherapy megavoltage CT (MVCT) images. The CT numbers of the inside and outside air and each tissue-equivalent plug of the "Cheese" phantom were obtained from TomoTherapy MVCT. Two IVDTs with different air CT numbers were created and applied to MVCT images of the HN anthropomorphic phantom and recalculated by Planned Adaptive to verify dose distribution. We defined the recalculation dose with MVCT images using both inside and outside air of the IVDT as IVDTMVCTinair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{inair}}$ and IVDTMVCToutair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{outair}}$, respectively. Treatment planning doses calculated on kVCT images were compared with those calculated on MVCT images using two different IVDT tables, namely, IVDTMVCTinair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{inair}}$ and IVDTMVCToutair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{outair}}$. The difference between average MVCT numbers +/- 1 standard deviation on inside and outside air of the calibration phantom was 65 +/- 36 HU. This difference in MVCT number of air exceeded the recommendation lung tolerance for dose calculation error of 2%. The dose differences between the planning target volume (PTV): D-98%, D-50%, D-2% and the organ at risk (OAR): D-max, D-mean recalculated by IVDTMVCTinair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{inair}}$ and IVDTMVCToutair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{outair}}$ using MVCT images were a maximum of 0.7% and 1.2%, respectively. Recalculated doses to the PTV and OAR with MVCT showed that IVDTMVCToutair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{outair}}$ was 0.5%-0.7% closer to the kVCT treatment planning dose than IVDTMVCTinair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{inair}}$. This study showed that IVDTMVCToutair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{outair}}$ was more accurate than IVDTMVCTinair${\mathrm{IVDT}}_{\mathrm{MVCT}}<^>{\mathrm{inair}}$ in recalculating the dose HN cases of MVCT using TomoTherapy.