Purpose: To experimentally investigate the effects of variations in respiratory motion during breath-holding (BH) at end-exhalation (EE) on intensity-modulated radiotherapy (BH-IMRT) dose distribution using a motor-driven base, films, and an ionization chamber.
Methods: Measurements were performed on a linear accelerator, which has a 120-leaf independently moving multileaf collimator with 5-mm leaf width at the isocenter for the 20-cm central field. Polystyrene phantoms with dimensions of 40 × 40 × 10 cm were set on a motor-driven base. All gantry angles of seven IMRT plans (a total of 35 fields) were changed to zero, and doses were then delivered to a film placed at a depth of 4 cm and an ionization chamber at a depth of 5 cm in the phantom with a dose rate of 600 MU/min under the following conditions: pulsation from the abdominal aorta and baseline drift with speeds of 0.2 mm/s (BD(0.2mm/s)) and 0.4 mm/s (BD(0.4mm/s)). As a reference for comparison, doses were also delivered to the chamber and film under stationary conditions.
Results: In chamber measurements, means ± standard deviations of the dose deviations between stationary and moving conditions were -0.52% ± 1.03% (range: -3.41-1.05%), -0.07% ± 1.21% (range: -1.88-4.31%), and 0.03% ± 1.70% (range: -2.70-6.41%) for pulsation, BD(0.2mm/s), and BD(0.4mm/s), respectively. The γ passing rate ranged from 99.5% to 100.0%, even with the criterion of 2%/1 mm for pulsation pattern. In the case of BD(0.4mm/s), the γ passing rate for four of 35 fields (11.4%) did not reach 90% with a criterion of 3%/3 mm. The differences in γ passing rate between BD(0.2mm/s) and BD(0.4mm/s) were statistically significant for each criterion. Taking γ passing rates of > 90% as acceptable with a criterion of 3%/3 mm, large differences were observed in the γ passing rate between the baseline drift of ≤5 mm and that of >5 mm (minimum γ passing rate: 92.0% vs 82.7%; p < 0.01).
Conclusions: This study suggested that the baseline drift of >5 mm should be avoided in the BH-IMRT.