Presentation of a 3D conformal radiotherapy technique for head-and-neck tumors resulting in substantial protection of the parotid glands

Purpose: The aim of this study was to improve the irradiation technique for the treatment of head-and-neck tumors and, in particular, to make use of the advantages found in modern 3D planning to protect the parotid glands. Patients and Methods: For this investigation the 3D dataset of a standard patient with oropharyngeal carcinoma of UICC stage IVA was used. In the CT scans (slice thickness 5 mm) the planning target volume (PTV), the boost volume and both parotids were delineated. Three different techniques were calculated for two different dose Levels (50 Gy for PTV and 64 Gy for boost volume, using single doses of 2 Gy). For technique 1 (T1) a parallel opposed field photon/electron irradiation was designed, for technique 2 (T2) an opposed/arc field irradiation was employed, and for technique 3 (T3) a combination of a static coplanar and arc field irradiation was designed. The sum doses D-min' D-max and D-mean for PTV, boost volume, and ipsilateral and contralateral parotid gland were evaluated, and the time needed for calculation of the plans was also determined. Results: For all techniques used, the calculated doses in the PTV (D-min 5.6 +/- 0.1 Gy, D-max 73.7 +/- 0.1 Gy, and D-mean 57.9 +/- 0.5 Gy) and in the boost volume (D-min 46.9 +/- 1.5 Gy, D-max 73.8 +/- 0.12 Gy, and D-mean 65.8 +/- 0.9 Gy) were equal. Significant differences were found regarding the three different techniques, e.g., for the ipsilaterall parotid gland D-min (T1 = 47.4, T2 = 50.6, and T3 = 38.4 Gy) as well as for the contralateral parotid gland D-min (T1 = 42.1, T2 = 44.2, and T3 = 17.8 Gy) and D-mean (T1 = 51.3, T2 = 52.8, and T3 = 32.6 Gy). Regarding the three different techniques, significant differences were found in favor of T3. The determined planning times were as follows: T1 = 90, T2 = 60, and T3 = 90 min. Conclusion: The combination of static coplanar and arc field technique (T3) resulted in a substantially better protection as compared to both other techniques. This was especially the case with regard to the contralateral parotid gland, when the dose distributions were calculated equally for PTV and boost volume. In this study, the D-man dose of the contralateral parotid gland was Lower than the TD50 of 37 Gy (95% confidence interval 32-43 Gy) previously assumed by the authors. Therefore, it can be concluded that in the present study a more intensive protection of this gland and a reduction in xerostomia were possibly obtained.