Purpose: To use a treatment planning system (TPS)-based simulation to investigate the detectorâ€™s capability of real-time gross error detection in T&O HDR brachytherapy.
Methods: The scintillating nanoparticle-terminated fiber-optic detector (NanoFOD) generates a voltage signal proportional to dose in real-time. The expected voltage signal was calculated using the dose distribution from the TPS and a distance-based calibration curve for the NanoFOD. A representative clinical T&O plan with two extra needles was used for simulations to determine the limits of error detectability. The voltage values the NanoFOD should detect at each dwell position were calculated for all applicators for: 1) the clinical plan; 2) wrong treatment lengths (TL); 3) wrong connection to afterloader; 4) wrong digitization direction; and 5) wrong step size. The percent differences (PD) between clinical and incorrect plans voltages were compared to a previously determined threshold value of 20% that represents the limit of NanoFOD detection.
Results: PDs were > 20 % for differences in TL >12 mm for tandem, 5 mm for ovoids, and 8 mm for needles. PDs of 60%, 37% and 484% were captured at the first dwell when connections between two needles, left needle and left ovoid, left needle and tandem were swapped. Wrong connections between tandem and any of the ovoids was noticed at the tandemâ€™s 5th dwell (52%). Switching the ovoids did not change the voltage reading. Incorrect direction of digitization made the signal too low to be detected, an indication that treatment should be stopped. With wrong step size in tandem, ovoid and needle, the PD was 23%, 24% and 38%, respectively at 2nd, 2nd and 3rd dwell.
Conclusion: Using 20% error detection threshold, the NanoFOD can capture all simulated gross errors within the first few dwells into the treatment, indicating it is capable of real-time verification of T&O HDR brachytherapy.