SU-E-T-164: Comparing Measurement Derived (3DVH) and Machine Log File Derived Dose Reconstruction Methods for VMAT QA in Heterogeneous Patient Geometries.
PURPOSE
To extend the 3DVH analysis to heterogeneous patient geometries for VMAT delivery and comparing its accuracy using machine log file derived dose reconstruction method Methods: A total of 10 patient plans were selected from a regular fractionation plan to complex SBRT plans. Treatment sites in the lung and abdomen were chosen to explore the effects of tissue heterogeneity on the respective dose reconstruction algorithms. Delivered plan in the patient geometry was reconstructed by using ArcCheck Planned Dose Perturbation (ACPDP) within 3DVH software, and by converting the machine logfile to Pinnacle3 9.0 treatment plan format. In addition, delivered gantry angles between machine logfile and 3DVH 4D measurement was also compared to evaluate the accuracy of the virtual inclinometer within the 3DVH.


RESULTS
Measured ion chamber and 3DVH derived isocenter dose agreed with planned dose within 0.3±0.7 Gy and -0.5±0.8 Gy respectively. Machine log file reconstructed doses and TPS dose agreed to within 2 Gy in PTV and OARs over the entire treatment course. 3DVH reconstructed dose showed a difference of up to 3.2 Gy in maximum PTV doses compared to planned dose in hypo lung patients due to plan heterogeneity. For majority of normal structures, dose differences were within 1 Gy except for few cases, where a maximum point dose difference of up to 2.2 Gy in proximal bronchial tree dose for a hypo lung patient was seen. The average Virtual Inclinometer Error (VIE) was - 0.65±1.6° for all patients, with a maximum error of -5.16±4.54° for an SRS case.


CONCLUSION
Both methods are capable of taking into account the plan delivery errors. 3DVH is more sensitive to these errors compared to machine log file.