Balancing the risk of cardiac implantable electronic device (CIED) malfunction with oncological needs is crucial during RT. Guidelines like the DEGRO/DGK aim to optimize this balance [7]. Often, guidelines for rare diseases and clinical issues are based more on expert opinions than on controlled studies [8]. This explains differences between guidelines, for example between the DEGRO/DKS and the 2022 ESC guidelines [17]. For example, the surveillance recommendations of the ESC guideline in the high-risk group are weekly device checks, electrocardiography (ECG) or pulse oximetry monitoring during RT, and the presence of an external pacemaker [9]. In contrast, the DEGRO/DGK guideline recommends daily readouts in case of high-risk indications. Given the low number of events as well the absence of fatal events in almost all reported series, longer surveillance intervals, as recommended by the ESC guidelines, might be acceptable [10]. It is important to note that in the years following the publication of the German guidelines, substantial evidence was generated regarding the occurrence of malfunctions, which justifies a less stringent monitoring of patients.

A dose–effect relationship is documented in the literature. Mouton et al. (2002) reported a cumulative dose of less than 2 Gy leading to 11.5% of the devices experiencing “relevant malfunction.” The study concludes that there is no safe dose threshold for irradiating an implanted device [11]. Potential malfunctions described by Hurkmans et al. (2005) include unintended changes in pulse amplitude, frequency, and threshold; loss of telemetry capability; complete functional failure; and other issues (battery depletion, lead impedance change). These malfunctions occurred with direct irradiation of pacemakers at 5 Gy or higher [12]. Furthermore, ICDs appear to be more sensitive to low cumulative doses due to more complex circuits [13]. Controversially, in vivo studies often yield different results: Grant et al. (2015) conducted a retrospective analysis of 249 irradiations in patients with CIEDs. No correlation between the cumulative dose and malfunctions was found for a device up to 30 Gy [14].

In the present study, the maximum cumulative dose to the device for both malfunctions was below 1 Gy. Devices with significantly higher doses showed no malfunction. Although the cumulative dose to the device is the most extensively studied risk factor for malfunction, the threshold of 2 Gy in the current DEGRO/DGK risk stratification seems relatively restrictive. In the Heart Rhythm Society (HRS) Expert Consensus Statement of 2017, involving American, European, Asian, and Latin American societies, it is recommended to consider relocating the device above a cumulative dose of 5 Gy. Regular CIED checks are recommended, especially in cases of neutron-producing irradiation and pacemaker dependency of the patient [15].

The DEGRO/DGK guideline mentions the energy level of the radiation as one of the key risk factors and recommends limiting it to 6 (10) MV. It is not mentioned that classification into risk groups outside of these limitations is not recommended; however, it should be noted that limiting the radiation energy to 6 MV might have potentially prevented both malfunctions [7]. In newer guidelines, including those of French, Italian, and European societies, neutron-producing irradiation with energies ≥ 10 MV is considered an independent risk factor (Table 4; [9, 16, 17]). In a study from 2020, Gauter-Fleckenstein et al. prospectively evaluated the implementation of the DEGRO/DGK guidelines and compared them with the 1994 APPM guidelines. In this context, the radiation energy used for CIED patients was limited to 6 MV. There were no events in 160 cases. The authors found no significant influence of cumulative dose, irradiated region, PTV or fraction dose, or CIED manufacturer. A tendency towards higher susceptibility to errors in ICDs compared to pacemakers was observed [18].

Zaremba et al. examined a total of 453 radiation cycles from Denmark in 2015. In all therapy-associated events (n = 14), the photon energy was ≥ 15 MV. The authors identified energy as the main risk factor, standing alone (odds ratio 5.73; 95% CI 1.58–20.76; [19]). Grant et al. found in a retrospective analysis of 249 treatment courses that RT with > 10 MV was the predominant risk factor for malfunction [14].

In a meta-analysis by Malawasi et al. in 2023, 32 studies with a total of 3121 patients were examined. In a cumulation of comparable studies, a significant association was found between event rate and higher energy > 10 MV, as well as increased susceptibility of ICDs. An event rate of 6.6% was described. A cumulative device dose of > 2 Gy had no relevant impact on the event rate [10]. Neutron-induced disturbances are primarily considered as the cause of these malfunctions. At high radiation energy (high linear energy transfer, LET), neutron production can lead to single-event upsets (SEU) in the circuits of the storage unit of an implanted device. These SEUs, as “soft errors,” do not physically damage the device, but they can lead to an electrical restart or loss of stored diagnostic data [20]. In the present study, there were no malfunctions or adverse events in patients treated with 6‑MV photons.

The alteration of aggregate parameters was examined by Bravo-James et al. in 2018, involving 109 patients. In two cases, a change in lead impedance (right atrium, right ventricle) occurred as a result of a “reset/restart.” These patients were treated with 6‑MV photons, with no consideration given to the dose to the device or lead insertion site. In two cases, a threshold increase in response to a restart was described (left ventricle, right ventricle). These events also involved radiation with 6 MV [21]. Overall, exceeding the predefined limits had no clinical relevance.

In the present study, even with a high cumulative dose of over 40 Gy at the lead insertion site and neutron-producing radiation, there was no significant change in lead impedance. The other relevant parameters such as threshold and perception were exceeded in 14.8% and 13.5%, respectively. In none of these cases was this change interpreted as problematic or result in a change in the course of therapy. While the limits of aggregate parameters have a fixed value in the consideration of the effects of radiological imaging (especially MRI imaging), a systematic consideration of these parameters in radiotherapy has not yet been firmly established in studies. The interpretation of a change in lead parameters must therefore be assessed by the attending cardiologist. A reliable proof of device malfunction cannot be derived. Notably, our data do not rule out late effects at the lead insertion site, as fibrosis usually takes months to years to develop.

Nevertheless, by collecting and analyzing aggregate parameters, in addition to secure malfunctions such as electrical restart and memory erasure, another objective control parameter is obtained. This may provide an opportunity to interpret an already collected and existing parameter and implement it into risk stratification. We were able to show that there is a considerable proportion of therapy courses with a temporary change in these parameters. Even though no correlation between this change and individual therapy plans was established in the small cohort, the question of the cause of this observation remains. In a prospective approach, these anomalies could be further investigated and also checked for differences between device types (pacemakers, ICDs). It should be noted that the change in parameters could not anticipate any adverse events or be directly related to them.

The statistical power of the present study is limited by both the heterogeneous patient population and the small sample size. A generally cautious approach to radiation planning in these patients (avoidance of high radiation energies, low cumulative dose at the device) also before the guideline was published further leads to low event rates in the limited patient collective. Similar retrospective studies have also shown this limitation [10, 22].

However, the considered cohort is capable of further specifying national and international recommendations. In particular, the focus on reducing neutron-producing radiation to increase the safety of patients with implanted devices could be further supported.