Purpose: To evaluate different dose verification modes of an EPID-based hybrid dose verification system for SBRT treatments of cervical, thoracic and lumbar spine regions.
Methods: An EPID-based hybrid system (PERFRACTION) was commissioned at our institution with 2D (Fraction-Zero Absolute Dose, FZAD) and two different 3D dosimetry modes using EPID or log-files. The EPID 3D mode extracts the MLC positions from the measured EPID images while the log-file mode extracts MLC positions from the log-files. Both 3D dosimetry modes reconstruct volumetric dose on the patient specific CT images while FZAD mode generate a 2D dose map in the water-equivalent phantom comparing with measured EPID images. Twenty nine spine SBRT plans with the prescription dose of 18 Gy in one fraction were delivered on EPID panel in both dosimetry and cine modes on a Varian Edge machine. The treatment plans, measured EPID images and log-files were transferred to PerFRACTION system for calculations and 2D/3D global gamma analysis at 2%/2mm, 3%/2mm and 3%/3mm levels with 10% low dose threshold. The relative point dose differences (%ΔD) were compared against the TPS calculated dose at the high dose low gradient regions.
Results: The gamma passing rates from all modes were in a good agreement with the predicted dose. The average passing rates at 2%/2mm were 96.3%±2.1%, 96.9±4.0% and 98.7±1.1% for FZAD, EPID and log-file modes respectively. For 3%/2mm and 3%/3mm, the average passing rates were observed >98% for all modes. The average %ΔD for all modes were <0.4%.
Conclusion: EPID-based dose verification provides a high resolution for SBRT QA. Gamma passing rates from all three modes were comparable while 2D-FZAD and 3D-EPID modes were more sensitive than the log-file based mode at a stricter gamma metric.
Not Applicable / None Entered.