CT Number Stability of Plasticine Bolus Based on HU Measurements in 25-Fraction 6 MV Radiotherapy

This study investigates the Hounsfield Unit (HU) stability of plasticine bolus material during fractionated radiotherapy to evaluate its suitability for clinical application in external beam radiotherapy. A plasticine bolus (10×10 cm², 1 cm thickness) was subjected to 25 fractions of 6 MV photon irradiation using a linear accelerator (LINAC). HU values were measured at baseline and after fractions 5, 10, 15, 20, and 25 using a Canon Aquilion CT-simulator (120 kVp, 300 mAs, 2 mm slice thickness). Dose distribution parameters (minimum, maximum, and mean doses) were calculated using ARIA Treatment Planning System (TPS) at each measurement point. The plasticine bolus demonstrated a baseline HU of 120, increasing to 130 HU (maximum deviation of 10 HU or 8.3%) by fraction 10, with values remaining stable through fraction 25. The peak HU change occurred at fraction 15 (131 HU, representing 9.2% deviation). Corresponding dose variations were minimal: mean dose decreased by 0.11% (from 5035.5 to 5029.8 cGy), minimum dose by 0.43% (from 3751.0 to 3735.0 cGy), and maximum dose by 0.23% (from 5128.5 to 5116.8 cGy). All HU changes remained well below the clinical tolerance threshold of ±20 HU, and dose variations were substantially lower than the 1-2% clinical significance threshold. These findings demonstrate that plasticine bolus maintains adequate HU stability for short-to-medium fractionation protocols (≤25 fractions) when proper quality assurance is implemented, though it exhibited higher HU variability compared to silicone-based alternatives reported in literature. Keywords: Hounsfield Unit; plasticine bolus; fractionated radiotherapy; dose distribution; treatment planning system; clinical tolerance.