Development of an Aperture Shape Displacement Score In Online Adaptive Radiation Therapy for Pancreatic Cancer
Abstract
Purpose
In online adaptive radiation therapy (oART), a new treatment plan is generated using three-dimensional images acquired before irradiation. However, no metric exists for evaluating the extent of day-to-day plan changes. This study aimed to develop a novel aperture shape displacement score (ASDS) to quantify aperture shape deviations in oART plans for pancreatic cancer and to evaluate its relationship with plan complexity and isocenter displacement relative to the CT-simulation-based plan (CTS plan).
Methods
A total of 135 oART plans from 15 patients with pancreatic cancer treated with IMRT using the ETHOS therapy system (Varian) from December 2022 to March 2024 were analyzed. Among the 15 patients, 12 received moderately hypofractionated radiotherapy (42–48 Gy in 15 fractions) and three received ultrahypofractionated radiotherapy (35 Gy in 5 fractions). The ASDS was quantified by calculating (1) the difference in corresponding leaf positions, (2) the difference in paired leaf widths, and (3) the Dice similarity coefficient of the corresponding aperture shape between the CTS and oART plans for each control point. The ASDS was defined on a scale of 0 to 1. Correlations between the ASDS and the modulation complexity score (MCS) and isocenter positional deviation from the CTS plan were evaluated using Pearson correlation coefficients (CCs).
Results
The mean ± standard deviation of the ASDS between the CTS and oART plans was 0.59 ± 0.06. The MCS and isocenter positional deviation were 0.22 ± 0.02 and 8.1 ± 3.1 mm. No correlation was observed between the ASDS and MCS (CC = 0.39); however, there was a moderate correlation between the ASDS and isocenter positional deviation (CC = 0.50).
Conclusion
We developed a novel metric to quantify aperture shape displacement between an oART plan and the CTS plan. This ASDS may serve as a new indicator for evaluating geometric variations in oART plans.