SU‐E‐T‐451: Is Rotational IMRT More Susceptible to Delivery Uncertainties than Dynamic IMRT?

Abstract

Purpose: To investigate whether rotational IMRT is susceptible to delivery uncertainties. Methods: Dosimetric effects of variation in dose rate, gantry position and MLC leaf positions were evaluated by incorporating their corresponding uncertainties into recalculated RapidArc treatment plans for four head‐and‐neck (HN) and four prostate test cases via modifications in gantry angles or MLC leaf positions. Total monitor units per aperture and therefore per plan were held fixed. The dose distributions and dose‐value histograms of the original and modified plans were compared using 3D Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR). Results reported below used 2%–2 mm Gamma test pass rate criterion. Results: For a 50% normally distributed random variation in dose rate (sigma = 1.3° in gantry angle), all HN plans had a 99.7% Gamma test pass rate. For HN PTVs, an increase in dose rate variation resulted in a decrease in minimum dose and an increase in maximum dose; for nearly all OARs, no more than a 0.4% dose difference was observed. For HN plans, a 5° rotational shift resulted in a 96.2% average pass rate. Dose distributions were sensitive to systematic variations especially above 1 mm; HN plans modified with a −2 mm shift in the X1 bank resulted in a 97.3% pass rate, and PTVs and OARs received a 3.8% higher dose on average. Uniformly distributed random shifts of +/−2 mm in aperture MLC leaf positions showed no significant change in mean dose (<0.5%) and resulted in a 100% pass rate Conclusions: Rotational IMRT deliveries were found to be very tolerant to random errors in dose rate, gantry position and MLC leaf position. Clinically acceptable treatments may be delivered successfully using RapidArc despite large fluctuations in dose rate and gantry position.