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EPID 2D TRANSMISION IN VIVO DOSIMETRY: Can It Detect Clinically Relevant Differences in Lung SBRT?

N Jornet*, P Carrasco De Fez, A Latorre-mussoll, P Gallego, P Delgado, J Perea-alija, M Ribas Morales, Hospital Sant Pau, Barcelona


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

To study the sensitivity of a commercial 2D EPID transmission In Vivo Dosimetry (IVD) system to detect imposed anatomy and positioning differences under well controlled experimental conditions and in a series of patients for SBRT-lung techniques.
A sensitivity test using QUASAR phantom (Modus) mimicking a lung case was designed. One configuration was CT-scanned for treatment planning using 2-field 3DCRT, 6-field IMRT and 1- partial arc VMAT. These plans were then delivered on the original and 11 modified phantom settings. For each fraction CBCT, logfiles and EPID images were exported to PerFraction (Sun NuclearTM). 2D EPID-IVD measurements were compared to the predicted 2D dose. Different metrics where used: global gamma 3%-3mm, global dose differences 2%; th 10%). The sensitivity of the 2D transmission IVD was assessed. At the same time, by using the CBCT and logfiles, the impact on the imposed phantom changes on the DVHs for four structures were evaluated. The 2D IVD pass rates were compared to the 3D dose calculations on the CBCT. The same methodology was used for a series of 10 3DCRT SBRT lung patients.
The sensitivity of the 2D in vivo method to anatomy changes field per field using gamma 3%-3mm G pass rate 95% was 81%, 97% and 80% for 3DCRT, IMRT and VMAT while it was 94%, 97% and 100% if the comparison was done in terms of dose differences. If the mean for all fields was used the sensitivity increased to 100% for 3DCRT and IMRT. For the series of patients the correlations between the 2D IVD results and 3D CBCT dose recalculations were weaker.
Conclusion: sensitivity of the system to detect clinically relevant changes depends largely on the dose distribution with respect to the volumes of interest . Increasing the distribution of beam portals increases sensitivity.

Funding Support, Disclosures, and Conflict of Interest: Sun Nuclear advisory board member


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