[Dosimetry analysis of intensity-modulated and conformal radiation therapy for head and neck tumors].

The aim of the study was to compare different treatment plans - intensity-modulated and conformal - for head and neck cancer patients. Treatment plans were developed for ten head and neck cancer patients by applying four different techniques: two conventional 3D conformal plans (forward treatment planning, with two opposing fields 90o-270o and one asymmetric anterior field, matching in isocenter /Conv/, conformal parotis sparing plans /ConPas/), 3D conformal plans with inverse treatment planning techniques /INVCRT/ and intensity-modulated radiation therapy plans /IMRT/. The plans were made for the same target volumes PTV50 (elective) and PTV66 (boost-16 Gy). The cumulative dose was 66 Gy, and the Philips Pinnacle3 v8.0m TPS was used for treatment planning. The organs at risk (OAR) were as follows: spinal cord, brain stem, left and right parotis and oral cavity. The dose constrains and conditions for optimization were determined for IMRT techniques with 7 fields. During the optimization we applied two different protocols: in one case the plans were made by 40 segments for "step and shoot" IMRT techniques and by 14 segments for INVCRT, which were converted into static fields. The homogeneity (HI) and conformity (COIN) indices were calculated for planning target volumes and the comparisons were assessed on several dosimetric parameters for OARs. The IMRT, INVCRT, Conv and ConPas techniques for PTV50 planning target volume gave the following values for homogeneity index: 0.13, 0.18, 0.22, 0.19, and for conformity index: 0.76, 0.68, 0.13, 0.09. The spinal cord received a maximum of 38 Gy, 42 Gy, 45 Gy and 44 Gy for the PTV66. Mean doses of the oral cavity outside the target volume were 33 Gy, 36 Gy, 30 Gy and 48 Gy. When the 16 Gy boost treatment was applied on one side only, the mean dose for the parotis on the contralateral side was 28 Gy, 31 Gy, 49 Gy and 43 Gy, while 39 Gy, 41 Gy, 59 Gy and 53 Gy on the same side. The objectives of adequate target coverage and sparing of critical structures were fulfilled only with IMRT technique. Although the sparing of the oral cavity was the most effectively provided by the traditional technique - due to the arrangement of the fields - it gave the worst results regarding the parotis and the target volume. The highest dose for the oral cavity was given by the ConPas technique, which can cause serious early and late side effects. By increasing the number of segments for IMRT at a reasonable level, the dose for OARs can be reduced.