• Bram van Asselen, Homan Dehnad, Cornelis P J Raaijmakers, Judith M Roesink, Jan J W Lagendijk, and Chris H J Terhaard.
• Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan 100, Utrecht, The Netherlands.
• Radiother Oncol. 2002 Aug 1; 64 (2): 197-204.

PurposeThe aim of this paper is to quantify the importance of the reduction of positioning margins applied to the clinical target volume (CTV) on the dose distribution of the parotid gland for different intensity-modulated radiotherapy (IMRT) strategies for the treatment of oropharyngeal cancer.Methods And MaterialsCTVs and organs at risk were delineated in the planning computed tomographic (CT) scans of three patients. Margins of 0, 3, 6 and 9mm were applied to the CTVs in order to obtain the planning target volumes (PTVs). Three IMRT strategies were used to optimize the dose distribution.ResultsThe analysis of the three IMRT strategies resulted in: (1) an optimal dose distribution in the PTV; (2) optimal dose distribution in the PTV while sparing the parotid gland and (3) more parotid gland sparing but at expense of the dose homogeneity in the PTV. The mean parotid dose increased linearly with increasing margin by approximately 1.3Gy per mm. As a result, the normal complication probability (NTCP) for xerostomia decreased when smaller margins were applied. Reducing the margin from 6 to 3mm resulted in an NTCP reduction of approximately 20%.ConclusionReducing the CTV-PTV margin by improving the patient position accuracy may lead to a significant reduction of NTCP for the IMRT treatment of the oropharyngeal tumors and lymph nodes level II.

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