小児頭部X線撮影における後方散乱係数（BSF）の実験的評価：新規院内不均質組織等価ファントムを用いた

Paediatric patients are particularly sensitive to ionizing radiation, and accurate surface dose estimation is crucial in diagnostic X-ray imaging. Standard homogeneous phantoms inadequately reproduce tissue heterogeneity, leading to errors in backscatter evaluation. This study aimed to characterize backscatter factors (BSFs) in a heterogeneous, tissue-equivalent paediatric head phantom under conventional radiography conditions. An in-house epoxy-based heterogeneous phantom was fabricated with five inserts simulating bone, brain, cerebrospinal fluid, eye lens and air cavities. Entrance surface dose (ESD) and BSF measurements were performed using an ionization chamber and a Fujifilm digital X-ray system (40-150 kVp, field sizes: 10 × 10, 20 × 20, 25 × 25 cm²). Additional aluminium filtrations of 2.5 mm and 3.0 mm were applied to evaluate beam hardening effects. Measured ESD increased from 0.82 mGy at 40 kVp to 18.93 mGy at 150 kVp (no filtration, 25 × 25 cm²). Applying 3.0 mm Al filtration reduced ESD by up to 57% at 60 kVp. Experimental BSFs ranged from 1.1 to 2.1, increasing with tube potential and reaching a maximum under small field size and hardened beam (120 kVp, 10 × 10 cm², 3.0 mm Al), reflecting enhanced cumulative backscatter energy from heterogeneous tissue interfaces. Compared with IAEA TRS-457 reference data, the heterogeneous phantom yielded higher BSFs than PMMA or water for 10 × 10 cm² fields, demonstrating enhanced scatter from realistic tissue interfaces, while lower BSFs were observed for larger fields. The study confirms that homogeneous phantoms may underestimate paediatric backscatter under specific conditions, while heterogeneous phantoms provide more representative dosimetric references, improving BSF estimation and dose optimization.