We compared the effect of set-up error and uncertainty on two radiation therapy treatment plans for head and neck cancer: one using intensity modulated radiation therapy (
IMRT) and one using conventional three-dimensional conformal radiation therapy (3D-
CRT). We used a Pinnacle3 (Philips Medical Systems, Markham, Ontario) system to create the two treatment plans (7-beam
IMRT and 5-beam 3D-
CRT) for the same volumetric data set, based on the objectives and constraints defined in the Radiation Therapy Oncology Group H-0022 protocol. In both plans, the dose–volume constraints for the targets and the organs at risk (
OARS) were met as closely as the beam geometries would allow. Monte Carlo–based simulations of set-up error and uncertainty were performed in three orthogonal directions for 840 simulated “courses of treatment” for each plan. A systematic error (chosen from distributions characterized by standard deviations ranging from 0 mm to 6 mm) and random uncertainties (2 mm standard deviation) were incorporated. We used a probability approach to compare the sensitivities of the
IMRT and the 3D-
CRT plans to set-up error and uncertainty in terms of equivalent uniform dose (
EUD) to targets and
OARS. Based on the
EUD analysis, the targets and
OARS showed considerably greater sensitivity to set-up error with the
IMRT plan than with the 3D-
CRT plan. For the
IMRT plan, target
EUDS were reduced by 4%, 7.5%, and 10% for 2-mm, 4-mm, and 6-mm set-up errors respectively. However, even with set-up error, the mandible, spinal cord, and parotid
EUDS always remained lower with the
IMRT plan than with the 3D-
CRT plan. We conclude that, when quantified by
EUD,
IMRT plan doses to
OARS and targets are more sensitive to set-up error than are 3D-
CRT-plan doses. However, as judged by the differences between target and
OAR doses,
IMRT retains its superiority over 3D-
CRT, even in the presence of set-up error.
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