Current guidelines [1] classify patients with cardiac implantable electronic devices (CIED) into risk groups based on the maximal CIED dose (Dmax) to avoid malfunctions. If a CIED is located outside the radiation field, dose calculation by treatment planning systems (TPS) is not sufficiently accurate for safe classification. In certain cases, a magnet is placed on the CIED for deactivation of antitachyarrhythmia (ATA) therapy during treatment. The magnet is, however, not included in the treatment planning process, which leads to further uncertainty in the dose determination at the CIED. The aim of the study was to investigate the influence of the magnet on CIED dose and to examine if the inclusion of the magnet in the TPS allows a more accurate dose prediction for safe classification of the patients. Treatment plans with and without the magnet were calculated for 15 patients with naso-/orpharyngeal or esophageal carcinoma and transferred to a phantom with integrated CIED. Treatment plan verification was performed with thermoluminescence detectors (TLDs) and radiochromic films. Our study showed an overestimation of the dose in the CIED by the TPS by 0.4 Gy (20 %) on average. The magnet used for ATA deactivation during radiotherapy increased the dose to the CIED slightly by 0.1 Gy on average. For Volumetric Modulated Arc Therapy (VMAT) irradiations of patients with esophageal or oro-/nasopharyngeal carcinoma, placement of the magnet had no clinically relevant impact on the CIED dose in most of the cases. The inclusion of the magnet in the TPS had no influence on calculated CIED dose and did not change the classification into risk groups. Despite uncertainties in the dose calculation, risk classification by the TPS Monaco was safe in 97 % of the cases, even if a magnet was used for ATA deactivation and not included in the treatment planning process.