Adequate margins for random setup uncertainties in head-and-neck IMRT

Purpose: To investigate the effect of random setup uncertainties on the highly conformal dose distributions produced by intensity-modulated radiotherapy (IMRT) for clinical head-and-neck cancer patients and to determine adequate margins to account for those uncertainties. Methods and Materials: We have implemented in our clinical treatment planning system the possibility of simulating normally distributed patient setup displacements, translations, and rotations. The planning CT data of 8 patients with Stage TI-T3NOMO oropharyngeal cancer were used. The clinical target volumes of the primary tumor (CTVPprimary) and of the lymph nodes (CTVelective) were expanded by 0.0, 1.5, 3.0, and 5.0 min in all directions, creating the planning target volumes (PTVs). We performed IMRT dose calculation using our class solution for each PTV margin, resulting in the conventional static plans. Then, the system recalculated the plan for each positioning displacement derived from a normal distribution with sigma = 2 min and sigma = 4 min (standard deviation) for translational deviations and sigma = 1degrees for rotational deviations. The dose distributions of the 30 fractions were summed, resulting in the actual plan. The CTV dose coverage of the actual plans was compared with that of the static plans. Results: Random translational deviations of or = 2 mm and rotational deviations of sigma = 1degrees did not affect the CTVprimary volume receiving 95% of the prescribed dose (V,,) regardless of the PTV margin used. A V,, reduction of 3% and 1% for a 0.0-mm and 1.5-mm PTV margin, respectively, was observed for or = 4 mm. The V-95 of the CTVelective contralateral was approximately 1% and 5% lower than that of the static plan for sigma = 2 min and or = 4 mm, respectively, and for PTV margins < 5.0 mm. An additional reduction of 1% was observed when rotational deviations were included. The same effect was observed for the CTVelective ipsilateral but with smaller dose differences than those for the contralateral side. The effect of the random uncertainties on the mean dose to the parotid glands was not significant. The maximal dose to the spinal cord increased by a maximum of 3 Gy. Conclusions: The margins to account for random setup uncertainties, in our clinical IMRT solution, should be 1.5 mm and 3.0 mm in the case of sigma = 2 mm and sigma = 4 mm, respectively, for the CTVprimary. Larger margins (5.0 mm), however, should be applied to the CTV.,,,ti,., if the goal of treatment is a V,, value of at least 99%. (C) 2005 Elsevier Inc.