PAROTID-GLAND TUMORS - A COMPARISON OF POSTOPERATIVE RADIOTHERAPY TECHNIQUES USING 3-DIMENSIONAL (3D) DOSE DISTRIBUTIONS AND DOSE-VOLUME HISTOGRAMS (DVHS)

Purpose: To compare different treatment techniques for unilateral trea tment of parotid gland tumors. Methods and Materials: The CT-scans of a representative parotid patient were used. The field size was 9 x 11 cm,the separation was 15.5 cm, and the prescription depth was 4.5 cm. Using 3D dose distributions, tissue inhomogeneity corrections, scatter integration (for photons) and pencil beam (for electrons) algorithms and dose-volume histogram (DVH), nine treatment techniques were compar ed. [1] unilateral 6 MV photons [2] unilateral 12 MeV electrons [3] un ilateral 16 MeV electrons [4] an ipsilateral wedge pair technique usin g 6 MV photons [5] a 3-field AP (wedged), PA (wedged) and lateral port al technique using 6 MV photons [6] a mixed beam technique using 6 MV photons and 12 MeV electrons (1:4 weighting) [7] a mixed beam techniqu e using 6 MV photons and 16 MeV electrons (1:4 weighting) [8] a mixed beam technique using 18 MV photons and 20 MeV electrons (2:3 weighting ) [9] a mixed beam technique using 18 MV photons and 20 MeV electrons (1:1 weighting). Results: Using dose-volume histograms to evaluate the dose to the contralateral parotid gland, the percentage of contralate ral parotid volume receiving greater than or equal to 30% of the presc ribed dose was 100% for techniques [I], [8] and [9], and < 5% for tech niques [2] through [7]. Evaluating the ''hottest'' 5 cc of the ipsilat eral mandible and temporal lobes, the hot spots were: 152% and 150% fo r technique [2], 132% and 130% for technique [6] Comparing the exit do ses, techniques [1], [8] and [9] contributed to greater than or equal to 50% of the prescribed dose to the contralateral mandible and the te mporal lobes, Only techniques [2], and [6] kept the highest point dose s to both the brain stem and the spinal cord below 50% of the prescrib ed dose. Conclusion: The single photon lateral field [1] and the mixed electron-photon beams [8] and [9] are not recommended treatment techn iques for unilateral parotid irradiation because of high doses deliver ed to the contralateral parotid gland and high exit doses which are as sociated with Xerostomia. The en face electron beam technique [2] and the mixed electron-photon beam technique [6] are unacceptable due to t he excessive dose heterogeneity to the contiguous normal structures. I n spite of optimal dose fall-off achieved using the en face technique [3], most patients cannot tolerate the resulting high skin doses. We c onclude that the ipsilateral wedge pair [4], the 3-field [5], and the mixed electron-photon beam [7] techniques are optimal techniques in pr oviding relatively homogeneous dose distributions within the target ar ea and for minimizing dose to the relevant normal structures. (C) 1998 Elsevier Science Inc.