Lymphocytes, the white blood cells that include T cells, B cells, and natural killer cells, are the sentinels of the adaptive immune system. They are also among the most radiosensitive cells in the human body. Unlike most tissues, however, lymphocytes do not sit still in one place during a course of radiotherapy. They circulate continuously through the bloodstream, which means that during brain irradiation, a substantial fraction of the body’s entire lymphocyte pool passes through the irradiated volume and is destroyed as it transits the treatment field. This mechanism, first modeled in detail by researchers at Johns Hopkins more than a decade ago, explains why even highly conformal modern radiotherapy can inflict significant collateral damage on the immune system.

The clinical stakes are considerable. Multiple studies, including a 2024 systematic review and pooled analysis, have found that patients who develop severe lymphopenia during chemoradiotherapy for glioblastoma survive substantially shorter periods than those who maintain their lymphocyte counts. Some investigators have proposed that radiation-induced immune depletion may blunt any potential benefit of immunotherapy in these patients and may even impair the body’s intrinsic ability to control residual tumor cells. What has remained contentious is precisely which dosimetric parameters best predict the severity of lymphocyte loss, and whether the culprit is the volume of brain irradiated, the dose delivered, or simply the size of the tumor target itself.

To address this question, Paolo Tini and colleagues conducted a retrospective single-center cohort study of 102 adults with histologically confirmed IDH-wildtype glioblastoma, the most common and aggressive molecular subtype of the disease. All patients had paired blood counts taken before radiotherapy and at treatment completion, along with detailed dose-volume histogram data for the whole brain and precise measurements of their target volumes—the gross tumor volume, clinical target volume, and planning target volume. The researchers also accounted for a battery of potential confounders, including baseline lymphocyte count, corticosteroid exposure, temozolomide administration, infection, and the number of radiation fractions delivered.

The results were striking. Median absolute lymphocyte counts fell from 1,810 cells per microliter before treatment to 1,050 cells per microliter at the end of radiotherapy, a median relative decline of 42.6 percent. Clinically significant lymphopenia was common: 32.4 percent of patients developed grade 2 or worse lymphopenia according to the Common Terminology Criteria for Adverse Events, and 13.7 percent developed grade 3 or worse. These figures underscore that immune depletion is not a rare toxicity but a near-universal feature of standard glioblastoma chemoradiotherapy, occurring in roughly one in three patients at a severity level that clinicians would typically flag as clinically meaningful.

The dosimetric analysis revealed a coherent and consistent pattern. In multivariable robust linear regression models adjusted for baseline lymphocyte count and major confounders, several measures of cerebral dose burden independently predicted lower lymphocyte counts at the end of treatment. Mean brain dose showed a significant association, with each additional 5 Gy reducing the log-transformed lymphocyte count by a factor corresponding to a beta coefficient of −0.251. The volume of brain receiving 15, 20, and 25 Gy—often abbreviated V15Gy, V20Gy, and V25Gy—each showed significant associations, as did the integral dose, a metric that captures the total energy deposited in the brain. Notably, V10Gy was also significant, while V5Gy was not, suggesting that intermediate-dose exposure in the range of roughly 15 to 25 Gy represents the most consistent dosimetric signal.

Perhaps the most provocative finding concerns the relationship between target volume and lymphocyte depletion. The planning target volume was strongly correlated with V20Gy, with a Spearman correlation coefficient of 0.78, meaning that larger targets inevitably expose more brain to intermediate doses. Yet when the researchers examined the direct association between target volume and lymphocyte preservation, it was negligible—a correlation of just −0.06, with a p-value of 0.55 indicating no statistically meaningful relationship. In other words, target size alone does not explain the immune damage. Two patients with similarly sized tumors could experience very different degrees of lymphocyte depletion depending on how the radiation dose is distributed through the surrounding brain, a conclusion that carries immediate implications for treatment planning.

The prognostic implications were equally clear. Using a landmark analysis anchored at the completion of radiotherapy—a design that avoids the statistical pitfalls of immortal time bias—the researchers found that grade 2 or worse lymphopenia was independently associated with inferior overall survival, with a hazard ratio of 2.43, and inferior progression-free survival, with a hazard ratio of 2.42. Patients whose immune systems were depleted during treatment faced more than double the risk of death and disease progression compared with those who preserved their lymphocyte counts, even after adjustment for other clinical and molecular variables. While the retrospective design of the study means that causality cannot be definitively established—lymphopenia could theoretically be a marker of some other adverse factor—the consistency of the association across multiple independent cohorts lends considerable weight to the biological interpretation.

The findings arrive at a moment when the field of radiation oncology is increasingly attentive to the immunological consequences of treatment. Prior work has shown that reducing the radiation treatment volume can lessen lymphopenia, and a randomized phase II study has demonstrated that proton therapy, which deposits less exit dose in normal tissue, reduces the likelihood of high-grade radiation-induced lymphopenia compared with conventional photon therapy. Other investigations have explored the role of intensity-modulated radiotherapy techniques and even the dose delivered to brain vasculature through which lymphocytes traffic. The Siena study adds an important nuance to this conversation: it is not enough to shrink the target or switch particle types. Planners must actively attend to the dose bath delivered to the otherwise healthy brain, particularly the intermediate-dose region between 15 and 25 Gy.

The authors argue that their findings support the prospective evaluation of immune-sparing radiotherapy strategies that reduce unnecessary cerebral dose exposure without compromising coverage of the tumor target. Such strategies might include tighter conformality, dose-escalation constraints on the normal brain, proton therapy in selected cases, or modified fractionation schedules. Given that glioblastoma remains one of the deadliest human cancers, with median survival measured in months despite aggressive multimodal treatment, any intervention that could preserve immune function without sacrificing tumor control deserves rigorous testing. This study, conducted without external funding and approved by the local ethics committee, offers a compelling dosimetric roadmap for that effort—and a reminder that in the quest to kill tumor cells, the collateral cost to the immune system may be a modifiable determinant of how long patients live.

Subject of Research: The association between cerebral radiation dose burden, treatment-related lymphopenia, and survival in IDH-wildtype glioblastoma

Article Title: Brain dose burden is associated with treatment-related lymphopenia in IDH-wildtype glioblastoma: dosimetric determinants and prognostic impact

Article References: Tini, P., Donnini, F., Battaglia, G., Pastina, P., Rubino, G., Carfagno, T., Vannini, M., Didona, A., Vanzi, E., & Banci Buonamici, F. (2026). Brain dose burden is associated with treatment-related lymphopenia in IDH-wildtype glioblastoma: dosimetric determinants and prognostic impact. Journal of Neuro-Oncology, 179(2), Article 81. https://doi.org/10.1007/s11060-026-05801-2

Image Credits: AI Generated

DOI: 10.1007/s11060-026-05801-2

Keywords: glioblastoma, lymphopenia, radiotherapy, brain dosimetry, V20Gy, integral dose, mean brain dose, temozolomide, immune-sparing radiotherapy, proton therapy, overall survival, IDH-wildtype