Implementation and evaluation of a transit dosimetry system for treatment verification. Issue 5 (May 2016) - Ricketts, K.
Highlights: A commercially available system for patient dose verification was tested. The system performed well in a phantom study on the central axis and at depth. 91% of all patient delivered fields achieved the initial set tolerance level. The EPID verification model agreed well with TPS dose for 3D conformal fields. The system was sensitive to patient thickness changes but not SSD errors. Abstract: Purpose: To evaluate a formalism for transit dosimetry using a phantom study and prospectively evaluate the protocol on a patient population undergoing 3D conformal radiotherapy. Methods: Amorphous silicon EPIDs were calibrated for dose and used to acquire images of delivered fields. The measured EPID dose map was back-projected using the planning CT images to calculate dose at pre-specified points within the patient using commercially available software, EPIgray (DOSIsoft, France). This software compared computed back-projected dose with treatment planning system dose. A series of tests were performed on solid water phantoms (linearity, field size effects, off-axis effects). 37 patients were enrolled in the prospective study. Results: The EPID dose response was stable and linear with dose. For all tested field sizes the agreement was good between EPID-derived and treatment planning system dose in the central axis, with performance stability up to a measured depth of 18 cm (agreement within −0.5% at 10 cm depth on the central axis and within −1.4% at 2 cm off-axis). 126 transit images were analysed of 37 3D-conformal patients. Patient results demonstrated the potential of EPIgray with 91% of all delivered fields achieved the initial set tolerance level of Δ D of 0 ± 5-cGy or %Δ D of 0 ± 5%. Conclusions: The in vivo dose verification method was simple to implement, with very few commissioning measurements needed. The system required no extra dose to the patient, and importantly was able to detect patient position errors that impacted on dose delivery in two of cases.