Abstract
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.
Highlights
Treatment plan development and evaluation in head and neck cancers are a challenge—in part because of the large number of radiosensitive normal structures in close proximity to the targets
The protocol specifies dose–volume objectives for planning target volume (PTV), but because clinical target volume (CTV) were the structures of primary clinical interest for us, our analysis considers the effect of set-up error and uncertainty on the CTVs
The EUD5% to the CTV66 in the Intensity modulated radiation therapy (IMRT) plan is slightly higher than the EUD5% to the CTV66 in the 3D-CRT plan, both have the same isocentre dose
Summary
Treatment plan development and evaluation in head and neck cancers are a challenge—in part because of the large number of radiosensitive normal structures in close proximity to the targets. The dose distribution using IMRT has been shown to be superior to that using three-dimensional conformal radiation therapy (3D-CRT) 2. Whether IMRT retains its superiority in the actual clinical situation, where set-up error and uncertainty are features of every treatment course, has not yet been established. To account for geometric uncertainties and organ motion, the clinical target volume (CTV), which encompasses the primary tumour and subclinical microscopic disease, is extended by a margin to form the planning target volume (PTV). For head and neck cancer, the expansion of the CTV to a PTV accommodates set-up error and geometric uncertainties, because organ motion and rotation are not significant for these sites 5
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