In this study, WBRT (in conjunction with SRS) was shown to increase the incidence of leukoencephalopathy compared to SRS alone, while reducing OS rates. These results were in line with those in most previous studies [5, 6]. Note however that the SRS integral dose to the skull presented a non-significant correlation to the development of leukoencephalopathy in both subgroups, which indicated that WBRT is a major factor affecting the onset of leukoencephalopathy. Note also that two relatively small retrospective studies (n = 103 and 92) reported that SRS was associated with a reduced risk of developing leukoencephalopathy [7, 8]. SRS provided focal high radiation energy without inducing changes in white matter. To the best of our knowledge, this is the first large-scale study to demonstrate that SRS is safe in terms of leukoencephalopathy development following radiation therapy for BMs.
Concerns related to the side effects of WBRT and radiation-induced neurocognitive dysfunction have prompted several research groups to identify the prognostic factors for radiation-induced leukoencephalopathy, including WBRT dose and fractionation, the number of tumors, tumor size, total tumor volume and location, SRS prescription and integral dose to the brain [17–19]. The most important factor in selecting WBRT or SRS has been the numbers of tumors, followed by tumor size, cumulative brain burden, extracranial disease status, and radiotherapeutic dosimetry [20]. Note however that the conclusions in previous studies have been inconsistent with regard to the cutoff number of tumors in selecting SRS or WBRT more likely due to the number of tumors which is not major factors for evaluation of radiation-induced neurocognitive dysfunction. Thus, most previous studies dealing with the complications of radiosurgery have focused on extracranial disease status, brain tumor burden, the number of tumors, neurocognitive dysfunction, or quality of life [9, 21, 22]. One recent study identified tumor volume as a prognostic factor in cases where multiple BMs were treated using SRS [22]. Total tumor volume has also been identified as a significant predictor of radiation toxicity; however, it has seldom been considered in studies on the complications of radiosurgery [18, 23]. In the current study, total tumor volume was positively correlated with the development of leukoencephalopathy, particularly in cases where the total tumor volume exceeded 28 cm3 (Table 2). Therefore, the total tumor volume could be regarded as an important prognostic factor to evaluate radiation-induced leukoencephalopathy.
Note that 33 patients in the SRS alone group developed leukoencephalopathy, 15 of whom developed leukoencephalopathy before undergoing SRS treatment (Fig. 2). Although the pathophysiology of leukoencephalopathy was not totally understood [24, 25], age was thought to be a prognostic factor related to the development leukoencephalopathy in this study (Table 2 and Fig. 3C). Leukoencephalopathy is not uncommon among cancer patients undergoing chemotherapy (e.g., methotrexate, fluorouracil, or fludarabine) [26]. Leukoencephalopathy secondary to endothelial damage has also been shown to induce the breakdown of the blood-brain barrier, vasculitis, or demyelination [24, 25, 27]. Above hypotheses had been clinically indicated by evidence demonstrating that impaired glycemic control, and uncontrolled hypertension were important risk factors for endothelial damage.24, 25, 27 Thus, it appears that these clinical factors cannot be omitted when we attempt to conclude the occurrence of leukoencephalopathy in cancer patients.