Author: Jingxi Weng, Jian Wu 👨🔬
Affiliation: University of Florida 🌍
Purpose: Modern brachytherapy treatment planning software includes pre-defined applicator models. However, previous studies have shown significant discrepancies between actual and model-based first dwell positions. Beyond dwell position offsets, this study quantifies the limitations of Elekta Oncentra intracavitary tandem and ring applicator models by measuring offsets between model-based and physical applicators.
Methods: The experiment used Elekta Oncentra interstitial intracavitary tandem and ring applicators with ring diameters of 26mm, 30mm, and 34mm and tandem lengths of 20mm, 40mm, and 60mm. Measurements were conducted in Oncentra, analyzing 13 combinations of tandem and ring applicators (4 rings, 6 tandems). Before measurement, the model-based ring was aligned with the physical ring. Ideally, the tandem should also align, but offsets were observed. Four key measurements were taken: axial offset at the ring plane (distance between the model and physical tandem centers), axial offset at the tandem tip plane, vertical offset between the model and physical tandem (using the tandem tip as a reference), and offset between model-based first dwell position and first marker.
Results: The tandem offsets at the ring plane ranged from 0.2mm to 1.2mm (average 0.49mm), while at the tip plane, they ranged from 0.1mm to 1.2mm (average 0.55mm). The tandem tip vertical offsets were larger, ranging from 0.8mm to 2.7mm (average 1.36mm). The first dwell position offsets were 5.8, 4.4, and 6.2mm for 26mm, 30mm, and 34mm ring, respectively. Conversely, our previous measurement of the offset between first dwell position and first marker is 1.3mm for all ring applicators.
Conclusion: These findings highlight the importance of decoupling tandem and ring applicators in brachytherapy modeling, which is still not available in current applicator models. Significant offsets exist in the current model, which may impact treatment accuracy. Thus, current clinical use of the applicator model should account for these offsets to ensure precise dose delivery.