Normalization of Ventilation Data From 4D-CT for Comparison Before and After Treatment
K Latifi1, T Huang2, V Feygelman3, E Moros4*, C Stevens5, T Dilling6, G Zhang7, (1) H. Lee Moffitt Cancer Center, Tampa, FL, (2) China Medical University, Taichung, ,(3) H. Lee Moffitt Cancer Center, Tampa, FL, (4) H. Lee Moffitt Cancer Center, Tampa, FL, (5) H. Lee Moffitt Cancer Center, Tampa, FL, (6) H. Lee Moffitt Cancer Center, Tampa, FL, (7) H. Lee Moffitt Cancer Center, Tampa, FLSU-E-J-69 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: If one were to evaluate ventilation distributions before and after radiotherapy derived from 4D-CT scans, changes in tidal volume (TV) might potentially bias the results. This is important because differences in TV were found to be large in some cases. To eliminate this uncertainty we proposed to normalize the ventilation to the TV.
Methods: Absolute ventilation (AV) data were generated for 6 SBRT cases before and after treatment, using the direct geometric algorithm and diffeomorphic morphons deformable image registration (DIR). Each pair of AV distributions was converted to TV-normalized, percentile ventilation (PV) and low-dose well-ventilated-normalized ventilation (LDWV) distributions. The latter two were described in previous publications. The ventilation change was calculated in various dose regions based on the treatment plans using the DIR-registered before and after treatment data sets. The ventilation change based on TV-normalized ventilation was compared with the AV as well as the data normalized by PV and LDWV.
Results: AV change may be misleading when the TV differs before and after treatment which was found to be up to 6.7% in the cases studied. All three normalization methods produced a similar trend in ventilation change: the higher the dose to a region of lung, the greater the degradation in ventilation. In low dose regions (<5 Gy), ventilation appears relatively improved after treatment due to the relative nature of the normalized ventilation. However, the LDWV may not be reliable when the ventilation in the low-dose regions varies. As an example, in one case LDWV appeared degraded regardless of dose. PV exhibited a similar ventilation change trend compared to the TV-normalized in all cases. However, by definition, the ventilation distribution in the PV is significantly different from the original distribution.
Conclusion: Normalizing ventilation distributions by the TV is a simple and reliable method for evaluation of ventilation changes.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by a research grant from Varian Medical Systems, Inc., Palo Alto, CA.