Based on the results obtained as well as the literature data, VMAT was found to be the most appropriate method in many treatment situations. However, this cannot be applied universally, and each case must be considered individually in order to get the best outcome. If we evaluate the effect of LDB, then VMAT has the largest impact. It is also higher in IMRT than in 3DCRT and, according to a study by Hall et al. (2003) is accompanied by almost twice the frequency of secondary radiation-induced cancers in successfully treated patients . However, these aspects for VMAT require further investigation. Additionally, if we consider the data obtained for the first time on the dose load on the cardiac conduction system, we have yet to study the role of VMAT in the occurrence of post-radiation arrhythmias.
An overwhelming majority of scientists agree that radiation treatment of left-sided breast cancer is associated with a higher risk of heart complications than treatment of right-sided tumors. Rehammar et al. (2017) reported that this association is especially true if we focus on ischemic complications or all cardiac complications ; however, this difference is practically nullified if arrhythmias and conduction disturbances can be distinguished from each other. In this case, the ratio between the left and right side in the localization of cancer will be 32:26, but in ischemia it will remain at the level widely described in the literature − 32:19. As mentioned earlier, the VMAT method is not universal. It has the highest LDB and, thus, gives the greatest dose load on the cardiac conduction system and on the contralateral lung. In addition to studying absorbed doses in different parts of the heart, we studied the features of the propagation of low doses of ionizing radiation at a distance from the target.
Many researchers, demonstrating the dose load on the left coronary artery, calculate it according to representations from static images of topometric CT scans, where the artery occupies a position corresponding to the moment in time when the CT scan occurred. Mobility is not taken into consideration. We specifically studied Internal target volume (ITV) using coronary angiograms. This revealed that the difference of their position depending on the phase of the cardiac cycle, can reach 1-1.5 cm. The data are presented in Figs. 4–6. Then, we analyzed the change in dose load on the LAD artery when shifted in the ventro-dorsal direction by at least 1 cm. It was found the range of 5–10% doses shifted to 10–20% and, overall, the dose load increased by 25–30% when compared to those established by static CT-cuts. If this is correlated with the data obtained on absorbed doses in individual volumes of the myocardium, then the dose, instead of those indicated in Table 2 (4.49–27.32 Gy), will become 5.82–35.41 Gy. Of course, the dose will depend on how long a particular section of the artery has been in a particular isodose area. However, the results obtained more closely represent true radiation dose absorbed by the coronary arteries. Coronary angiograms are not performed for radiotherapeutic purposes; therefore, it is impossible to assess the physiological mobility of blood vessels in three directions as is customary in radiation therapy. It should be noted, however, that the order of the numbers is quite indicative. If we extrapolate the displacements that we have discovered into the planning system, this changes the concept of the absorbed dose, which has not been described in the literature so far. The displacement of the LAD artery towards the target is particularly dangerous.
Our study of the dose load on the cardiac conduction system is, to the best of our knowledge, the first experiment in the history of radiotherapy. The importance of obtaining such knowledge is clear from the side effects of radiation therapy for breast cancer in the cited literature. This should be especially important when treating patients with heart disease, and doctors do not have any information about this at the time. We tried to delineate in our patients the most functionally important part of the cardiac conduction system: the atrioventricular node, the sinoauricular node and bundle bunch. Our sample size was small since in the conditions of native CT scans, the anatomical structures of the heart, acting as landmarks, are visualized indistinctly when searching for them or are not differentiated due to the lack of contrast and artifacts from respiratory movements and heartbeat.
Previously, we tried to estimate the dose load on the cardiac conduction system in 24 patients with unilateral disease in whom we were able to develop the corresponding Planning organ at risk volume (PRV) with the highest reliability on topometric CT scans [36–38]. We found that, regardless of the side and volume of irradiation, the lowest average doses to the conducting system were obtained using the 3DCRT method. IMRT and VMAT performed less effectively, mainly due to the effect of LDB. We identified that, even with the most advanced irradiation methods in the context of the dose to the conduction system, LDB is not as low a dose as expected. The data obtained are presented in Figs. 7 and 8. We have shown that the greater the side effects of ionizing radiation during radiation treatment of breast cancer on the main elements of the cardiac conduction system, the larger the clinical target volume (CTV). The sinoauricular node receives the highest doses with right-sided irradiation (according to Dmean, up to 33.6% of the prescribed dose for irradiation of the right breast and all areas of regional metastasis), the atrioventricular node and especially the bundle bunch receive from 12–22.4% of the prescribed dose, with a similar left-sided exposure from 12–17.6%. However, these data are subject to large individual fluctuations and, in practice, require separate monitoring during dosimetric planning for each patient individually. In addition, the effect of the LDB must also be taken into account.
We concluded that VMAT is the best treatment for SBBC with any irradiation volume as it shows the lowest dose load on the lung, myocardium, and LADA as compared to 3DCRT and IMRT. IMRT is generally intermediate between VMAT and 3DCRT in terms of effectiveness in the irradiation of mammary glands or mammary glands with lymphatic drainage areas. In contrast, IMRT showed the least effective results in relation to the myocardium and LADA with post-mastectomy irradiation. Our study showed that the highest dose load on the myocardium and LADA in patients with SBBC was achieved with 3DCRT. However, this technique is not recommended for use in the treatment of patients with concomitant cardiovascular disease. IMRT gives better indicators of conformity and homogeneity when more beams are used, but this is accompanied by a pronounced increase in LDB, which is almost absent in 3DCRT.