Quantifying the Spatial Sensitivity of Helical Diode Arrays In SBRT Patient-Specific QA
Abstract
Purpose
To quantify Gamma Passing Rate (GPR) volatility in SBRT QA caused by the spatial coincidence of helical diode arrays (10 mm pitch) with high-gradient fields. While 3%/2mm criteria reduce alignment artifacts, more sensitive 3%/1mm criteria are recommended for SBRT. This study evaluates GPR instability driven solely by stochastic diode alignment relative to the stereotactic treatment fields.
Methods
QA was performed for one SBRT plan per site (Lung, Liver, Prostate, Spine, and Seminal Vesicles) on an Elekta Infinity (3%/1mm, 10% threshold, global normalization). To isolate sampling effects, plans were shifted longitudinally in the treatment planning system (0.0 to 17.5 mm) and physically on the couch during measurements. This isolated significant GPR changes to discrete sampling effects. GPR fluctuations were quantified with two main drivers: the Diode Inclusion Effect (change in GPR due to the number of diodes exceeding the 10% threshold) and the Local Agreement Effect (change in GPR due to agreement at specific diode coordinates as they navigate areas of low and high dose gradients).
Results
A periodic fluctuation of GPR was observed to coincide with the 10 mm detector pitch across all cases with a mean GPR range (Max–Min) of 6.1%. Volatility was inversely related to target size with lung (20.2 cc) and seminal vesicles (8.6 cc) showing the highest swings (8.8% and 7.9%, respectively) while the prostate (122.4 cc) was 3.5%. Decomposition indicated this was driven by the Local Agreement Effect as diodes shifted to more favorable regions of better agreement. Conversely, the T4 Spine case instability was masked by the Diode Inclusion Effect, where shifts "pruned" failing edge diodes from the analysis.
Conclusion
SBRT pass rates at 3%/1mm are highly dependent on the sub-millimeter spatial coincidence of the 10 mm -spaced detector grid. QA results may reflect geometric alignment rather than accurate treatment delivery.