Background: One of the most specific effects of high-density dental restorative materials on head & neck cancer radiotherapy is generating variations on isodose distributions. These variations might have an impact on the accuracy and effectiveness of the radiation treatment. The aim of this study is investigating the possible dosimetric effect of six different restorative materials on isodose distributions in head & neck radiotherapy planning process. Materials and Methods: A special phantom was developed and twenty-one caries-free human third molars (a control group + six different restorative materials) were used for the measurements. After acquiring the computed tomography (CT) images, seven treatment plans were created. Hounsfield Unit (HU) numbers, horizontal line dose profile (HLDP) and vertical line dose profiles (VLDPs) were compared with the control group. Results: The amalgam sample deformed the HU numbers in CT images. The median HU value for the S4 material was considerably different than the other samples. The median values were quite close for the remaining samples. For the amalgam sample, the mean of the calculated median isodose values for HLDP and VLDP at 3.5 cm away from the isocenter line were lower than the mean of the control group 4.03% and 6.94%, respectively (for HLDP with tooth numbers of 36 and 38 P = 0.025 and P < 0.001, respectively; for VLDP P < 0.001). In C-S1 comparison results, the statistically significant differences were found for the measurement point at 1 cm away from the isocenter (P = 0.037, P = 0.002, and P = 0.018 for the tooth numbers 36, 37, and 38, respectively). In C-S2 and C-S6 comparisons, there was a statistically significant difference for tooth number 36 (P = 0.035 and P = 0.003, respectively). Conclusions: The findings of the present study showed that amalgam should not be used in head & neck cancer patients who are planned to have radiation therapy. A high viscosity glass ionomer cement (GIC) and a ceramic reinforced GIC sample can be used instead of amalgam to minimize the distorting effect on isodose distributions.