Background
Radiotherapy (RT)-induced lymphopenia (i.e., a reduction in the total lymphocyte count [TLC]) has been reported in various types of tumors, such as glioblastomas, pancreatic cancer, and lung cancer [
1‐
7]. Although radiation has local effects, RT to peripheral organs can result in irradiation of a substantial proportion of circulating lymphocytes during multifraction treatments [
8]. Recently, several studies have demonstrated that partial brain RT can contribute to systemic lymphopenia [
5‐
8]. RT-induced lymphopenia is associated with poor survival in patients with high-grade gliomas who underwent standard therapy with RT and temozolomide [
5,
6].
Despite multimodal treatment involving surgery, RT, and temozolomide, glioblastoma has a poor prognosis and almost all patients with glioblastoma eventually experience disease relapse [
9]. Although repeat surgery, re-irradiation, and pharmacological treatment have been performed in the recurrent setting, evidence that any therapeutic intervention has a major effect on survival is lacking [
10,
11]. Accordingly, different immunotherapy modalities for glioblastoma are being actively investigated, spurred on by advances in immuno-oncology for other tumor types [
12]. However, conventional temozolomide-based chemoradiation has immunosuppressive effects, including lymphopenia [
5,
6,
13]. As lymphocytes are important mediators of the immune response to cancer, such iatrogenic immunosuppression can limit the administration of immunotherapy.
In this context, identifying and modifying the factors associated with RT-induced lymphopenia can help maintain an optimal TLC, which may facilitate a synergistic effect between RT and immunotherapy. Accordingly, maintaining an optimal TLC may effectively improve treatment outcomes in patients with glioblastoma. Therefore, this study aimed to examine the potential clinical predictors of treatment-related lymphopenia in patients with glioblastoma treated with RT plus temozolomide.
Discussion
In this study, we confirmed previous evidence regarding treatment-induced lymphopenia by using a relatively large population of patients with glioblastoma who received standard RT and chemotherapy [
5‐
7]. The present study showed that cell counts of various blood cell types change over time after chemoradiation. Among them, the decrease in the lymphocyte count after chemoradiation persisted for a year, while other blood cell types, such as white blood cells, platelets, and neutrophils, showed recovery patterns and the count of red blood cells did not decrease after chemoradiation. Grossman et al. [
5] showed that the CD4 count had the lowest value at 2 months after RT and remained persistently low after temozolomide-based chemoradiation in 96 patients with high-grade glioma. Moreover, this chronicity of lymphopenia was consistent with the lymphopenia pattern in other types of cancers such as pancreatic cancer and lung cancer [
1]. Our findings confirmed such a pattern of treatment-related lymphopenia in 323 patients with glioblastoma and showed that this pattern was unique to changes in the lymphocyte count.
The clinical significance of treatment-related lymphopenia in patients with glioblastoma has drawn more attention in the modern immuno-oncology era. The therapeutic options for patients with recurrent glioblastoma are currently limited, but repeat surgery, RT, and pharmacological treatment with alkylating agents or bevacizumab have been performed [
10,
11]. Nevertheless, a substantial proportion of patients do not receive any second-line anticancer therapy [
18,
19]. Accordingly, different types of immunotherapy are being actively examined as novel approaches for the treatment of recurrent glioblastoma, including the glioblastoma vaccine, oncolytic viral therapy, chimeric antigen receptor T-cell therapy, and immune-checkpoint inhibitors [
12]. For these immunotherapies, preserving the optimal lymphocyte count is essential, as lymphocytes play a fundamental role in cell-mediated immunologic destruction of cancers [
20]. However, conventional first-line therapies for glioblastoma, including RT and temozolomide, have immunosuppressive effects. Moreover, as shown in the TLC patterns after chemoradiation in our study, the immunosuppressive effects can become chronic, i.e., the effects can even limit the applicability of immunotherapy as a second-line treatment option after relapse that mostly occurs after a few months or years. Therefore, efforts to identify the risk factors for ASL are important for establishing optimal chemoradiation strategies to preserve the TLC, which may have a potential synergistic effect with immunotherapy as a second-line treatment.
On examining various clinical and therapeutic factors, we found that female sex, subtotal/partial resection, increased PTV, and the use of 3D-CRT were independently associated with the development of ASL. Interestingly, the association between female sex and ASL has also been shown in previous studies on temozolomide-based chemoradiation for high-grade glioma [
6,
7]. Although this could be attributed to differences in the pharmacokinetics and pharmacodynamics between sexes, the exact mechanism remains unknown [
21].
A mathematical computation model has supported the association between local RT for body parts without the bone marrow or lymphatic tissue and systemic lymphopenia [
8]. This model demonstrated that the mean dose to circulating lymphocytes is approximately 2 Gy and that nearly all the circulating blood receives at least 0.5 Gy during a typical course of RT for glioblastoma. Considering that lymphocytes are the most radiosensitive cells among all blood cell types and that the LD50 (lethal dose required to reduce the surviving fraction of lymphocytes by 50%) is only 2 Gy, this effect of RT on the circulating blood volume can cause ASL [
22]. This model also examined two different PTVs (4.2 and 268 cm
3) for glioblastoma and showed a large difference in the percent of blood receiving at least 0.5 Gy according to the target volume size. Our study confirmed the finding of this theoretical model regarding the effect of conventionally fractionated RT and PTV size on treatment-related lymphopenia by using clinical data of patients with glioblastoma.
We demonstrated that IMRT significantly reduced the development of ASL in patients receiving a conventionally fractionated regimen for glioblastoma. Compared to 3D-CRT, IMRT improves the dose distribution to the target volume as well as reduces the dose to normal tissues, such as normal brain tissues, optic structures, and the brain stem [
23]. We collected data on the low-dose distribution to the brain and found that although V
0.5 Gy and V
3 Gy were not significantly different between the 3D-CRT and IMRT groups, V
5 Gy, V
10 Gy, and V
25 Gy were significantly lower in the IMRT groups (Additional file
1: Table S5). However, the irradiation volume of blood can become high in IMRT because IMRT usually takes a longer time than 3D-CRT. Accordingly, a precise mathematical method is needed to calculate the accurate dose for individual cases, which should be evaluated in further studies. IMRT has been adopted for the routine clinical treatment of many types of cancers including brain tumors [
24]. However, compared to the use of IMRT for head and neck cancer [
25], the clinical significance of IMRT for glioblastoma has not been well established to date. In this study, we showed the benefit of using IMRT for treating brain tumor in terms of preserving immunity. As immunotherapy is rapidly evolving and can be used in the future as a second-line treatment, IMRT can be applied more actively for glioblastoma for the purpose of preserving immunity.
Reduced TLC and reduced lymphocyte infiltration in pathologic specimens are associated with poor OS [
1,
3‐
5,
7,
26‐
29]. Several studies have also investigated the association between treatment-related lymphopenia and OS in patients with glioblastoma [
5‐
7], and have shown that radiation-induced reduction of circulating lymphocyte counts and subsequent lymphocyte infiltration of tumors may have a tangible impact on OS outcomes [
27,
30]. However, owing to the heterogeneous study population and small number of patients, it was difficult to draw a solid conclusion on the prognostic effect of RT-induced lymphopenia in glioblastoma. In our study, although patients with ASL had poorer OS than did those without ASL, ASL was not a significant factor on multivariate analysis. This inconsistency might be because our study included well-established biomarkers such as
IDH1 mutation, MGMT methylation, and subventricular zone involvement, while previous studies investigated none of these factors [
5] or only investigated MGMT methylation [
6]. The inclusion of strong prognostic factors [
31‐
33] in our study could have relatively diminished the prognostic significance of ASL. However, when we included grade ≥ 3 lymphopenia at the 3-month time point instead of ASL (grade ≥ 3 lymphopenia at any time point within 3 months) in the multivariate analysis, the results showed that this alternative definition of lymphopenia was a strong independent prognostic factor (Additional file
1: Table S3). This implies that recovery from ASL may be a more important finding than the presence or absence of ASL. However, owing to a lack of consensus and the use of different time points in defining treatment-related lymphopenia across studies, efforts to find the best definition of treatment-related lymphopenia should be continued [
30].
The main limitation of this study is that the margins used to define the PTV differed between the IMRT and 3D-CRT groups. Nevertheless, we used the same definition of the GTV and CTV for both groups, and standardized target volume delineation was performed for all patients. The incidence of ASL still showed a significant difference after the PTV was balanced between both groups by using propensity score matching. Because this was a single-center retrospective study, there might have been unrecognized biases that were not completely addressed by multivariate analysis or propensity score matching. Therefore, our findings should be interpreted keeping these limitations in mind.