The aim of this study was to evaluate changes in NCF and HRQoL at patient level 3 and 6 months after SRT, providing insight in the impact of treatment on the individual patient level. The overall results, in line with results at group level [
21] and several other studies in patients with limited brain metastases [
19,
40,
41], indicate that most patients with brain metastases treated with SRT maintained their pre-treatment levels of NCF for at least 6 months. Although NCF and HRQoL at the group level showed little variation, this is not necessarily translated into little variation at domain/scale and patient level. Indeed, changes in scores on the different HRQoL scales did vary substantially within patients, and most individual patients showed both a decline and improvement in separate HRQoL scales in the first 6 months after initial SRT. This finding is in contrast with the HRQoL findings at group level, in which patients who deteriorated and improved most likely cancelled each other out. When informing patients about the impact of a certain treatment or monitor their disease status, it is not sufficient to have information at group level only, nor at the scale level. Clinicians should also be aware that the large majority of patients will experience both deterioration and improvement in HRQoL.
An explanation for the relatively unaffected NCF in brain metastases patients may be that our study population represents a highly selected group of patients with good functioning. Indeed, patients in our sample had a higher KPS score and longer survival compared to our patients without sufficient NCF/HRQoL data. This is also supported by the finding that prior to SRT, only 50% of patients had an impairment in at least one neurocognitive domain, which is considerably lower than in previous studies in metastatic brain tumor patients (67–92%) [
40,
42]. Particularly for neurocognitive testing, compliance rates decreased substantially over 6 months’ time. Responsible for non-compliance, among other things, were poor neurological or physical functioning and assessment considered too burdensome. Another explanation is the operational definition of objective neurocognitive decline, for which different cut-offs have been suggested [
43]. Brown et al. [
44], using a ≥ 1.0 SD cut-off score, found considerably higher neurocognitive deterioration rates compared to our study, with most patients showing cognitive deterioration at 3 months after SRT. However, when using a ≥ 1.0 SD cut-off in our study, still the majority of patients showed no cognitive deterioration, meaning a different cut-off does only partially explains the difference in neurocognitive deterioration rates [
44]. Taking into account the aforementioned explanations for the relatively unaffected NCF, maintenance of NCF over 6 months’ time might have been overestimated in our biased sample and likely limits generalizability of the results to brain metastases patients with poor functioning.
Although average HRQoL remained stable at group level, except for physical functioning and fatigue, this did not hold true on scale level nor at patient level. On scale level, patients were relatively similarly distributed over the three different categories (deterioration; stable score; and improvement). At patient level, however, the majority of patients showed both deterioration and improvement in different HRQoL scales after radiotherapy, which has been previously reported in patients with brain metastases, but comparison is difficult because the majority of patients received WBRT instead of SRT [
45,
46]. Caissie et al. [
45] reported that upon follow-up 1 month after radiotherapy significant improvement was seen in several HRQoL scales, including communication deficit [
45]. On the contrary, Steinmann et al. [
46] reported that upon follow-up 3 months after the start of radiotherapy patients showed a significant and clinically relevant deterioration in several preselected HRQoL scales, including global health status, physical functioning, fatigue, motor dysfunction and communication deficit, while other scales remained unchanged [
46]. In our study, the majority of patients showed a clinically relevant deterioration between baseline and 3 months in physical functioning (46%), role functioning (54%) and fatigue (61%), reflecting the findings at group level [
21]. Nevertheless, considering the varying trajectories of changes in HRQoL after SRT, an important observation is that the majority of our patients showed both decline and improvement in separate HRQoL scales. An explanation for the varying trajectories of changes is that HRQoL measures vastly different concepts, encompassing physical, emotional, and social components, and that this outcome may be influenced by many factors, including comorbidity, marital status, heterogeneity of the primary tumor, SRT dose, total tumor volume, progression of the extracranial cancer and its corresponding supportive or anti-tumor treatment [
47]. Although, SRT dose received, total tumor volume, intracranial progression and active systemic disease did not differ significantly between the four different categories at patient level, this result must be interpreted with caution due to our small sample size. As pointed out by Wilson and Cleary [
48] in their model, more distal measures to the disease or the treatment (i.e. global health status and the functioning scales) are not only affected by health status but also by non-medical factors, as opposed to more proximal measures (i.e. symptoms) [
48]. NCF is a proximal measure, which is mainly influenced by the presence of brain metastases, or its treatment. Patients who deteriorated on at least one HRQoL scale did most often have decreased performance status, suggesting that especially the patients’ overall functioning influences HRQoL. Moreover, Caissie et al. [
49] found that fatigue and emotional functioning were the two strongest predictors of global health status in brain metastases patients, which is similar to the findings of our cluster analysis; deterioration in global health status clusters with increased fatigue and worse emotional functioning, suggesting fatigue may be a target for intervention to improve overall HRQoL [
49].
To conclude, in accordance with previous results at group level, this study showed that most patients with brain oligometastases treated with SRT maintained their pre-treatment NCF for at least 6 months. However, changes in scores for the various HRQoL scales differed considerably between and within patients, suggesting that overall functioning is determined by complex underlying mechanisms which should be further analysed.