Room: AAPM ePoster Library
Purpose: investigate the correlation between transit dosimetry and tumor volume change for patients undergoing radiation therapy for lung cancer.
Methods: lung cancer patients treated with IMRT or VMAT were retrospectively studied. All patients were treated while freely breathing. One patient received SBRT in five fractions, while others had conventional fractionation ranging from 10 to 35 fractions. Daily kVCBCT images and transit dosimetry were acquired at each fraction. The gross tumor volume (GTV) on the planning CT was deformed to the first fraction’s CBCT, which was then used as a baseline. GTVs in following fractions were compared to the baseline GTV. Transit doses acquired at each fraction were compared to the first fraction dose using mean gamma and gamma passing rate (1%/0.5 mm). The variation in transit dosimetry was evaluated as a function of tumor volume change.
Results: median initial GTV volume was 114.6cc (range: 8.8 to 178.5cc). The overall median GTV regression was 20% (2 to 56%). In general, mean gamma increased as tumor volume change increased while gamma passing rate decreased with an increasing tumor volume change. The median slope and R2 value from linear regression fitting were 4.1 (2.6 to 13.6) and 0.55 (0.009 to 0.8) for mean gamma, and -107.5 (-53.3 to -191.9) and 0.49 (0.1 to 0.7) for gamma passing rate. One patient’s data showed weak correlation due to limited GTV volume change.
Conclusion: mean gamma values and gamma passing rates correlated with tumor volume change (although in opposite ways), which indicates that transit dosimetry could be used as an indicator for tumor volume change. Since this work involved both IMRT/VMAT, conventional/SBRT fractionation and large/small lung lesions, future work will investigate the roles of those factors on the correlation using larger cohort and evaluating the impact of tumor volume change on dose distributions.
TH- External Beam- Photons: portal dosimetry, in-vivo dosimetry and dose reconstruction