A novel technique for the quality assurance of dwell positions for ovoid applicators using 2D optical imaging

We propose a new method for commissioning lunar ovoid applicators in high dose rate gynecology brachytherapy based on 2D optical scintillating imaging of the applicators.

Treatment plans were generated for 22 mm and 26 mm diameter Venezia applicators, using four to eight dwell positions in each ovoid. Images of the applicator dwell positions were acquired using a pinhole apparatus combined with scintillating material and optical camera. Images were acquired for individual dwell positions and processed to identify pixel locations with peak signal intensity. Catheter dwell positions were used to register pixel locations in the optical images and absolute coordinates of the applicator in the treatment planning system. Errors were calculated using the standard deviation in the Euclidean distance between measured and expected ovoid dwell positions. Measurements were repeated three times, including repositioning the applicator on the measurement system.

Imaging of each applicator required between 20 and 25 min for all dwell positions. Catheter registration errors were 0.14 ± 0.09 mm and 0.21 ± 0.04 mm for the 26 and 22 mm applicators, respectively. Average differences between the imaged and planned ovoid dwell positions were 0.48 ± 0.14 mm and 0.48 ± 0.16 mm for the 26 and 22 mm applicators, respectively. The maximum difference between the measured and planned ovoid positions was 0.7 mm and 1.3 mm measured for the 26 and 22 mm applicators, respectively. These uncertainties are lower than our clinical tolerance of 2.0 mm.

2D-Scintillating imaging of lunar ovoid applicator dwell positions is feasible, accurate and faster than previous methods used at our center.