A COMPARISON OF SETUP ERRORS AND SAFETY MARGINS BETWEEN 3-POINT AND 5-POINT IMMOBILIZATION SYSTEMS IN HEAD AND NECK RADIATION THERAPY AT MILITARY HOSPITAL 175
DOI:
https://doi.org/10.59354/ydth175.2026.500Keywords:
Head and neck cancer, IMRT, setup error, safety margin, 5-point immobilization, Van HerkAbstract
Objective: To compare setup errors and calculate safety margins (PTV margins) between 3-point and 5-point HeadSTEP immobilization systems in Intensity-Modulated Radiation Therapy (IMRT) for head and neck cancer at Military Hospital 175.
Methods: A total of 2539 iViewGT verification image sets from 101 treatment plans (51 with 3-point and 50 with 5-point masks) were analyzed. Systematic errors (∑), random errors (σ), and safety margins (A) were determined in three directions: Anterior-Posterior (A.P), Left-Right (L.R), and Superior-Inferior (S.I) using the Van Herk formula (A=2.5 ∑ₚₒₚ + 0.7 σₚₒₚ, A=2.5 ∑ₚₒₚ + 0.7 σₚₒₚ).
Results: The 5-point system demonstrated superior setup accuracy. Specifically, the safety margin (A) in the A.P direction was reduced from 4.3 mm to 2.8 mm, and the S.I direction from 4.1 mm to 3.45 mm. The L.R direction remained stable below 3 mm. Systematic error (∑) showed the most significant improvement in the A.P direction, decreasing from 1.15 mm to 0.74 mm.
Conclusions: Utilizing a 5-point immobilization system significantly enhances patient setup precision, allowing for a PTV margin reduction to below 3.5 mm. This optimization facilitates better healthy tissue sparing and treatment efficacy.
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