Can We Ditch the Air Gap with Shielded Dosimeters?: Investigation of Backscatter Mitigation In Shielded Solid-State Radiation Detectors
Abstract
Purpose
The accurate estimation of patient entrance skin dose (ESD) is an important part of diagnostic radiology quality assurance. ESD is commonly calculated by measuring incident (scatter‑free) air kerma on a PMMA phantom and applying an appropriate backscatter factor (BSF) dependent on field size and beam quality. Scatter‑free measurement conditions are traditionally achieved using a large air gap between dosimeter and phantom surface, which can be inconvenient. Several commercially available solid‑state dosimeters include backscatter shielding and are promoted for contact measurements, but independent experimental verification of their ability to eliminate backscatter signal is limited. We quantified backscatter mitigation for two widely used shielded dosimeters (IBA Radcal AGMS‑DM+ and RaySafe X2).
Methods
BSFs were measured on a 25.8‑cm thick PMMA phantom across 40–125 kV for two beam filtrations and field sizes: 2.7 mm Al (30x30 cm2) and 3.1 mm Al + 0.9 mm Cu (15x15 cm2). An unshielded ionisation chamber (IBA Radcal 10X6‑60) provided reference BSFs for comparison with published values. Measurement uncertainty was estimated from positioning sensitivity and repeated‑exposure reproducibility.
Results
Ion‑chamber BSFs agreed with recent experimental literature, but were 6–8% lower than TRS‑457 values. For the shielded dosimeters, BSFs showed no significant energy dependence. Mean BSFs (±2 SD) were 1.04±0.03 (2.7 mm Al) and 1.01±0.02 (3.1 mm Al + 0.9 mm Cu) for the AGMS‑DM+, and 0.98±0.05 and 1.03±0.04, respectively, for the X2. Averaged over all beam qualities, BSFs were 1.03±0.04 (AGMS‑DM+) and 1.00±0.07 (X2). Estimated measurement uncertainty was 1–2%. Incident air‑kerma measurements from the ion chamber, AGMS‑DM+, and X2 all agreed within 4% on average.
Conclusion
Backscatter contribution for AGMS‑DM+ and RaySafe X2 contact measurements on a PMMA phantom was within the manufacturers’ stated dose uncertainty (5%), suggesting that these detectors can be used to measure incident (i.e. backscatter‑free) air kerma without requiring an air gap.