DNA damage accumulation during fractionated low-dose radiation compromises hippocampal neurogenesis

Effect of different optimization parameters in single isocenter multiple brain metastases radiosurgery

PURPOSE

Automated treatment planning for multiple brain metastases differs from traditional planning approaches. It is therefore helpful to understand which parameters for optimization are available and how they affect the plan quality. This study aims to provide a reference for designing multi-metastases treatment plans and to define quality endpoints for benchmarking the technique from a scientific perspective.

METHODS

In all, 20 patients with a total of 183 lesions were retrospectively planned according to four optimization scenarios. Plan quality was evaluated using common plan quality parameters such as conformity index, gradient index and dose to normal tissue. Therefore, different scenarios with combinations of optimization parameters were evaluated, while taking into account dependence on the number of treated lesions as well as influence of different beams.

RESULTS

Different scenarios resulted in minor differences in plan quality. With increasing number of lesions, the number of monitor units increased, so did the dose to healthy tissue and the number of interlesional dose bridging in adjacent metastases. Highly modulated cases resulted in 4-10% higher V10% compared to less complex cases, while monitor units did not increase. Changing the energy to a flattening filter free (FFF) beam resulted in lower local V12Gy (whole brain-PTV) and even though the number of monitor units increased by 13-15%, on average 46% shorter treatment times were achieved.

CONCLUSION

Although no clinically relevant differences in parameters where found, we identified some variation in the dose distributions of the different scenarios. Less complex scenarios generated visually more dose overlap; therefore, a more complex scenario may be preferred although differences in the quality metrics appear minor.

Unraveling radiation-induced skeletal muscle damage: Insights from a 3D human skeletal muscle organoid model

BACKGROUND

Three-dimensional (3D) organoids derived from human pluripotent stem cells (hPSCs) have revolutionized in vitro tissue modeling, offering a unique opportunity to replicate physiological tissue organization and functionality. This study investigates the impact of radiation on skeletal muscle response using an innovative in vitro human 3D skeletal muscle organoids (hSMOs) model derived from hPSCs.

METHODS

The hSMOs model was established through a differentiation protocol faithfully recapitulating embryonic myogenesis and maturation via paraxial mesodermal differentiation of hPSCs. Key skeletal muscle characteristics were confirmed using immunofluorescent staining and RT-qPCR. Subsequently, the hSMOs were exposed to a clinically relevant dose of 2 Gy of radiation, and their response was analyzed using immunofluorescent staining and RNA-seq.

RESULTS

The hSMO model faithfully recapitulated embryonic myogenesis and maturation, maintaining key skeletal muscle characteristics. Following exposure to 2 Gy of radiation, histopathological analysis revealed deficits in hSMOs expansion, differentiation, and repair response across various cell types at early (30 min) and intermediate (18 h) time points post-radiation. Immunofluorescent staining targeting γH2AX and 53BP1 demonstrated elevated levels of foci per cell, particularly in PAX7+ cells, during early and intermediate time points, with a distinct kinetic pattern showing a decrease at 72 h. RNA-seq data provided comprehensive insights into the DNA damage response within the hSMOs.

CONCLUSIONS

Our findings highlight deficits in expansion, differentiation, and repair response in hSMOs following radiation exposure, enhancing our understanding of radiation effects on skeletal muscle and contributing to strategies for mitigating radiation-induced damage in this context.

Neurotoxicity from Old and New Radiation Treatments for Brain Tumors

Research regarding the mechanisms of brain damage following radiation treatments for brain tumors has increased over the years, thus providing a deeper insight into the pathobiological mechanisms and suggesting new approaches to minimize this damage. This review has discussed the different factors that are known to influence the risk of damage to the brain (mainly cognitive disturbances) from radiation. These include patient and tumor characteristics, the use of whole-brain radiotherapy versus particle therapy (protons, carbon ions), and stereotactic radiotherapy in various modalities. Additionally, biological mechanisms behind neuroprotection have been elucidated.

The role of particle radiotherapy in the treatment of skull base tumors

The skull base is an anatomically and functionally critical area surrounded by vital structures such as the brainstem, the spinal cord, blood vessels, and cranial nerves. Due to this complexity, management of skull base tumors requires a multidisciplinary approach involving a team of specialists such as neurosurgeons, otorhinolaryngologists, radiation oncologists, endocrinologists, and medical oncologists. In the case of pediatric patients, cancer management should be performed by a team of pediatric-trained specialists. Radiation therapy may be used alone or in combination with surgery to treat skull base tumors. There are two main types of radiation therapy: photon therapy and particle therapy. Particle radiotherapy uses charged particles (protons or carbon ions) that, due to their peculiar physical properties, permit precise targeting of the tumor with minimal healthy tissue exposure. These characteristics allow for minimizing the potential long-term effects of radiation exposure in terms of neurocognitive impairments, preserving quality of life, and reducing the risk of radio-induced cancer. For these reasons, in children, adolescents, and young adults, proton therapy should be an elective option when available. In radioresistant tumors such as chordomas and sarcomas and previously irradiated recurrent tumors, particle therapy permits the delivery of high biologically effective doses with low, or however acceptable, toxicity. Carbon ion therapy has peculiar and favorable radiobiological characteristics to overcome radioresistance features. In low-grade tumors, proton therapy should be considered in challenging cases due to tumor volume and involvement of critical neural structures. However, particle radiotherapy is still relatively new, and more research is needed to fully understand its effects. Additionally, the availability of particle therapy is limited as it requires specialized equipment and expertise. The purpose of this manuscript is to review the available literature regarding the role of particle radiotherapy in the treatment of skull base tumors.

Radiation-Induced Brain Injury: Age Dependency of Neurocognitive Dysfunction Following Radiotherapy

Cranial radiotherapy is a known risk factor for neurocognitive impairment in cancer survivors. Although radiation-induced cognitive dysfunction is observed in patients of all ages, children seem to be more vulnerable than adults to suffering age-related deficits in neurocognitive skills. So far, the underlying mechanisms by which IR negatively influences brain functions as well as the reasons for the profound age dependency are still insufficiently known. We performed a comprehensive Pubmed-based literature search to identify original research articles that reported on age dependency of neurocognitive dysfunction following cranial IR exposure. Numerous clinical trials in childhood cancer survivors indicate that the severity of radiation-induced cognitive dysfunction is clearly dependent on age at IR exposure. These clinical findings were related to the current state of experimental research providing important insights into the age dependency of radiation-induced brain injury and the development of neurocognitive impairment. Research in pre-clinical rodent models demonstrates age-dependent effects of IR exposure on hippocampal neurogenesis, radiation-induced neurovascular damage and neuroinflammation.

The Effects of Extrinsic and Intrinsic Factors on Neurogenesis

In the mammalian brain, neurogenesis is maintained throughout adulthood primarily in two typical niches, the subgranular zone (SGZ) of the dentate gyrus and the subventricular zone (SVZ) of the lateral ventricles and in other nonclassic neurogenic areas (e.g., the amygdala and striatum). During prenatal and early postnatal development, neural stem cells (NSCs) differentiate into neurons and migrate to appropriate areas such as the olfactory bulb where they integrate into existing neural networks; these phenomena constitute the multistep process of neurogenesis. Alterations in any of these processes impair neurogenesis and may even lead to brain dysfunction, including cognitive impairment and neurodegeneration. Here, we first summarize the main properties of mammalian neurogenic niches to describe the cellular and molecular mechanisms of neurogenesis. Accumulating evidence indicates that neurogenesis plays an integral role in neuronal plasticity in the brain and cognition in the postnatal period. Given that neurogenesis can be highly modulated by a number of extrinsic and intrinsic factors, we discuss the impact of extrinsic (e.g., alcohol) and intrinsic (e.g., hormones) modulators on neurogenesis. Additionally, we provide an overview of the contribution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection to persistent neurological sequelae such as neurodegeneration, neurogenic defects and accelerated neuronal cell death. Together, our review provides a link between extrinsic/intrinsic factors and neurogenesis and explains the possible mechanisms of abnormal neurogenesis underlying neurological disorders.

Imaging doses for different CBCT protocols on the Halcyon 3.0 linear accelerator - TLD measurements in an anthropomorphic phantom

INTRODUCTION

Image guided radiotherapy allows for particularly conformal tumour irradiation through precise patient positioning. Becoming the standard for radiotherapy, this increases imaging doses to the patient. The Halcyon 3.0 linear accelerator (Varian Medical Systems, Palo Alto, CA) requires daily imaging due to its geometry. For this reason, the accelerator is equipped with on-line kV and MV imaging. However, daily CBCT images required for irradiation apply additional radiation, which increases the dose to normal tissue and therefore can affect the patient's secondary cancer risk. In this study, actual organ doses were measured for the kV system, and a comparison of normal tissue doses for all available kV CBCT protocols was presented to demonstrate differences in imaging doses across entities and protocols. In addition, effective dose and secondary cancer risk from imaging are evaluated.

MATERIAL AND METHODS

Measurements were performed with thermoluminescent dosimeters in an anthropomorphic phantom positioned according to each entity (brain, head and neck, breast, lung, pelvis). CBCT images were obtained, using all available pre-set protocols without further adjustment of the parameters. Measured doses for each position and each protocol were then compared and secondary cancer risk of relevant and specifically radiosensitive organs was calculated.

RESULTS

It was found that imaging doses for protocols such as Pelvis and Head could be reduced by up to half using the corresponding Fast and Low Dose modes, respectively. On the other hand, larger field sizes or the Large mode yielded higher doses than their initial protocols. Image Gently was found to spare normal tissue best, however it is not suitable for certain entities due to low image quality or insufficient projection data.

DISCUSSION

By using appropriate kV-CBCT protocols, it is possible to reduce imaging doses to a significant extent and therefore spare healthy tissue. Combined with studies of image quality, the results of this study could lead to adjustments in workflow regarding the choice of protocols used in daily routine. This could prevent unnecessary radiation exposure and reduce secondary cancer risk.

Measuring out-of-field dose to the hippocampus in common radiotherapy indications

BACKGROUND

The high susceptibility of the hippocampus region to radiation injury is likely the causal factor of neurocognitive dysfunctions after exposure to ionizing radiation. Repetitive exposures with even low doses have been shown to impact adult neurogenesis and induce neuroinflammation. We address the question whether the out-of-field doses during radiotherapy of common tumour entities may pose a risk for the neuronal stem cell compartment in the hippocampus.

METHODS

The dose to the hippocampus was determined for a single fraction according to different treatment plans for the selected tumor entities: Point dose measurements were performed in an anthropomorphic Alderson phantom and the out-of-field dose to the hippocampus was measured using thermoluminescence dosimeters.

RESULTS

For carcinomas in the head and neck region the dose exposure to the hippocampal region for a single fraction ranged from to 37.4 to 154.8 mGy. The hippocampal dose was clearly different for naso-, oro- and hypopharynx, with maximal values for nasopharynx carcinoma. In contrast, hippocampal dose levels for breast and prostate cancer ranged between 2.7 and 4.1 mGy, and therefore significantly exceeded the background irradiation level.

CONCLUSION

The mean dose to hippocampus for treatment of carcinomas in the head and neck region is high enough to reduce neurocognitive functions. In addition, care must be taken regarding the out of field doses. The mean dose is mainly related to scattering effects, as is confirmed by the data from breast or prostate treatments, with a very different geometrical set-up but similar dosimetric results.

T cell-mediated additive cytotoxicity - death by multiple bullets

Immune effector cells, including cytotoxic T cells (CTLs), induce apoptosis and eliminate target cells by direct cell-cell contacts. In vivo, CTLs fail to efficiently kill solid tumor cells by individual contacts but rely upon multihit interactions by many CTLs (swarming). Recent evidence has indicated that multihit interactions by CTLs induce a series of sublethal damage events in target cells, including perforin-mediated membrane damage, induction of reactive oxygen species (ROS), nuclear envelope rupture, and DNA damage. Individual damage can be repaired, but when induced in rapid sequence, sublethal damage can accumulate and induce target cell death. Here, we summarize the sublethal damage and additive cytotoxicity concepts for CTL-induced and other cell stresses and discuss the implications for improving immunotherapy and multitargeted anticancer therapies.

Region-Specific Effects of Fractionated Low-Dose Versus Single-Dose Radiation on Hippocampal Neurogenesis and Neuroinflammation

BACKGROUND

Despite technical advances in hippocampus-sparing radiotherapy, radiation-induced injury to neural stem cell compartments may affect neurocognitive functions. In pre-clinical mouse models with fractionated low-dose radiation (FLDR) and single-dose radiation (SDR), the accurate response to radiation-induced injury was analyzed in different hippocampal subregions.

METHODS

Adult and juvenile C57BL/6NCrl mice were exposed to FLDR (20 × 0.1 Gy, daily exposure from Monday to Friday for 4 weeks) or SDR (1 × 2 Gy). In addition, 72 h after the last exposure, neuroglia (astrocytes and microglia) and neuroprogenitor cells were characterized and quantified in the hippocampal cornu ammonis (CA) and dentate gyrus (DG) by immunofluorescence studies.

RESULTS

After analyzing different hippocampal subregions, it was observed that radiation responses varied between non-neurogenic CA, with no detectable inflammatory alterations, and neurogenic DG, characterized by impaired neurogenesis and subsequent neuroinflammation. Age-dependent differences in radiosensitivity appeared to depend on the varying proliferative potential of neural stem cell niches. Using the same overall dose for FLDR and SDR (2 Gy), both the cumulative dose over time and also the single dose fraction have decisive impacts on hippocampal damage.

CONCLUSION

Region-specific effects of radiation-induced hippocampal injury relies primarily on cell deaths of proliferating neuroprogenitors. Dose per fraction defines the extent of neuronal injury, and subsequently activated microglia and reactive astrocytes modulate dynamic processes of neuroinflammation. Thus, limiting both cumulative doses and dose fractions to hippocampal DG is an important issue of clinical radiotherapy to preserve neurocognitive functions.

Cultured Human Foreskin as a Model System for Evaluating Ionizing Radiation-Induced Skin Injury

Purpose: Precise molecular and cellular mechanisms of radiation-induced dermatitis are incompletely understood. Histone variant H2A.J is associated with cellular senescence and modulates senescence-associated secretory phenotype (SASP) after DNA-damaging insults, such as ionizing radiation (IR). Using ex vivo irradiated cultured foreskin, H2A.J was analyzed as a biomarker of radiation-induced senescence, potentially initiating the inflammatory cascade of radiation-induced skin injury. Methods: Human foreskin explants were collected from young donors, irradiated ex vivo with 10 Gy, and cultured in air-liquid interphase for up to 72 h. At different time-points after ex vivo IR exposure, the foreskin epidermis was analyzed for proliferation and senescence markers by immunofluorescence and immunohistochemical staining of sectioned tissue. Secretion of cytokines was measured in supernatants by ELISA. Using our mouse model with fractionated in vivo irradiation, H2A.J expression was analyzed in epidermal stem/progenitor cell populations localized in different regions of murine hair follicles (HF). Results: Non-vascularized foreskin explants preserved their tissue homeostasis up to 72 h (even after IR exposure), but already non-irradiated foreskin epithelium expressed high levels of H2A.J in all epidermal layers and secreted high amounts of cytokines. Unexpectedly, no further increase in H2A.J expression and no obvious upregulation of cytokine secretion was observed in the foreskin epidermis after ex vivo IR. Undifferentiated keratinocytes in murine HF regions, by contrast, revealed low H2A.J expression in non-irradiated skin and significant radiation-induced H2A.J upregulations at different time-points after IR exposure. Based on its staining characteristics, we presume that H2A.J may have previously underestimated the importance of the epigenetic regulation of keratinocyte maturation. Conclusions: Cultured foreskin characterized by highly keratinized epithelium and specific immunological features is not an appropriate model for studying H2A.J-associated tissue reactions during radiation-induced dermatitis.

An Adaptive Role for DNA Double-Strand Breaks in Hippocampus-Dependent Learning and Memory

DNA double-strand breaks (DSBs), classified as the most harmful type of DNA damage based on the complexity of repair, lead to apoptosis or tumorigenesis. In aging, DNA damage increases and DNA repair decreases. This is exacerbated in disease, as post-mortem tissue from patients diagnosed with mild cognitive impairment (MCI) or Alzheimer's disease (AD) show increased DSBs. A novel role for DSBs in immediate early gene (IEG) expression, learning, and memory has been suggested. Inducing neuronal activity leads to increases in DSBs and upregulation of IEGs, while increasing DSBs and inhibiting DSB repair impairs long-term memory and alters IEG expression. Consistent with this pattern, mice carrying dominant AD mutations have increased baseline DSBs, and impaired DSB repair is observed. These data suggest an adaptive role for DSBs in the central nervous system and dysregulation of DSBs and/or repair might drive age-related cognitive decline (ACD), MCI, and AD. In this review, we discuss the adaptive role of DSBs in hippocampus-dependent learning, memory, and IEG expression. We summarize IEGs, the history of DSBs, and DSBs in synaptic plasticity, aging, and AD. DSBs likely have adaptive functions in the brain, and even subtle alterations in their formation and repair could alter IEGs, learning, and memory.

Neurogenesis in aging and age-related neurodegenerative diseases

Adult neurogenesis, the process by which neurons are generated in certain areas of the adult brain, declines in an age-dependent manner and is one potential target for extending cognitive healthspan. Aging is a major risk factor for neurodegenerative diseases and, as lifespans are increasing, these health challenges are becoming more prevalent. An age-associated loss in neural stem cell number and/or activity could cause this decline in brain function, so interventions that reverse aging in stem cells might increase the human cognitive healthspan. In this review, we describe the involvement of adult neurogenesis in neurodegenerative diseases and address the molecular mechanistic aspects of neurogenesis that involve some of the key aggregation-prone proteins in the brain (i.e., tau, Aβ, α-synuclein, …). We summarize the research pertaining to interventions that increase neurogenesis and regulate known targets in aging research, such as mTOR and sirtuins. Lastly, we share our outlook on restoring the levels of neurogenesis to physiological levels in elderly individuals and those with neurodegeneration. We suggest that modulating neurogenesis represents a potential target for interventions that could help in the fight against neurodegeneration and cognitive decline.

Infants and Newborns with Atypical Teratoid Rhabdoid Tumors (ATRT) and Extracranial Malignant Rhabdoid Tumors (eMRT) in the EU-RHAB Registry: A Unique and Challenging Population

Simple Summary

Malignant rhabdoid tumors (MRT) are deadly tumors that predominantly affect infants and young children. Even when considering the generally young age of these patients, the treatment of infants below the age of six months represents a particular challenge due to the vulnerability of this patient population. The aim of our retrospective study was to assess the available information on prognostic factors, genetics, toxicity of treatment and long-term outcomes of MRT. We confirmed that, in a cohort of homogenously treated infants with MRT, significant predictors of outcome were female sex, localized stage, absence of a GLM and maintenance therapy, and these significantly favorably influence prognosis. Stratification-based biomarker-driven tailored trials may be a key option to improve survival rates.

Abstract

Introduction: Malignant rhabdoid tumors (MRT) predominantly affect infants and young children. Patients below six months of age represent a particularly therapeutically challenging group. Toxicity to developing organ sites limits intensity of treatment. Information on prognostic factors, genetics, toxicity of treatment and long-term outcomes is sparse. Methods: Clinical, genetic, and treatment data of 100 patients (aged below 6 months at diagnosis) from 13 European countries were analyzed (2005–2020). Tumors and matching blood samples were examined for SMARCB1 mutations using FISH, MLPA and Sanger sequencing. DNA methylation subgroups (ATRT-TYR, ATRT-SHH, and ATRT-MYC) were determined using 450 k / 850 k-profiling. Results: A total of 45 patients presented with ATRT, 29 with extracranial, extrarenal (eMRT) and 9 with renal rhabdoid tumors (RTK). Seventeen patients demonstrated synchronous tumors (SYN). Metastases (M+) were present in 27% (26/97) at diagnosis. A germline mutation (GLM) was detected in 55% (47/86). DNA methylation subgrouping was available in 50% (31 / 62) with ATRT or SYN; for eMRT, methylation-based subgrouping was not performed. The 5-year overall (OS) and event free survival (EFS) rates were 23.5 ± 4.6% and 19 ± 4.1%, respectively. Male sex (11 ± 5% vs. 35.8 ± 7.4%), M+ stage (6.1 ± 5.4% vs. 36.2 ± 7.4%), presence of SYN (7.1 ± 6.9% vs. 26.6 ± 5.3%) and GLM (7.7 ± 4.2% vs. 45.7 ± 8.6%) were significant prognostic factors for 5-year OS. Molecular subgrouping and survival analyses confirm a previously described survival advantage for ATRT-TYR. In an adjusted multivariate model, clinical factors that favorably influence the prognosis were female sex, localized stage, absence of a GLM and maintenance therapy. Conclusions: In this cohort of homogenously treated infants with MRT, significant predictors of outcome were sex, M-stage, GLM and maintenance therapy. We confirm the need to stratify which patient groups benefit from multimodal treatment, and which need novel therapeutic strategies. Biomarker-driven tailored trials may be a key option.

Effects of low dose ionizing radiation on the brain- a functional, cellular, and molecular perspective

Over the years, the advancement of radio diagnostic imaging tools and techniques has radically improved the diagnosis of different pathophysiological conditions, accompanied by increased exposure to low-dose ionizing radiation. Though the consequences of high dose radiation exposure on humans are very well comprehended, the more publicly relevant effects of low dose radiation (LDR) (≤100 mGy) exposure on the biological system remain ambiguous. The central nervous system, predominantly the developing brain with more neuronal precursor cells, is exceptionally radiosensitive and thus more liable to neurological insult even at low doses, as shown through several rodent studies. Further molecular studies have unraveled the various inflammatory and signaling mechanisms involved in cellular damage and repair that drive these physiological alterations that lead to functional alterations. Interestingly, few studies also claim that LDR exerts therapeutic effects on the brain by initiating an adaptive response. The present review summarizes the current understanding of the effects of low dose radiation at functional, cellular, and molecular levels and the various risks and benefits associated with it based on the evidence available from in vitro, in vivo, and clinical studies. Although the consensus indicates minimum consequences, the overall evidence suggests that LDR can bring about considerable neurological effects in the exposed individual, and hence a re-evaluation of the LDR usage levels and frequency of exposure is required.

Partial-Brain Radiation-Induced Microvascular Cognitive Impairment in Juvenile Murine Unilateral Hippocampal Synaptic Plasticity

PURPOSE

Radiation-induced cognitive deficits have a severe negative impact on pediatric brain tumor patients. The severity of cognitive symptoms is related to the age of the child when radiation was applied, with the most severe effects seen in the youngest. Previous studies using whole-brain irradiation in mice confirmed these findings. To understand ipsilateral and contralateral changes in the hippocampus after partial-brain radiation therapy (PBRT) of the left hemisphere, we assessed the neuroplasticity and changes in the microvasculature of the irradiated and nonirradiated hippocampus in juvenile mice.

METHODS AND MATERIALS

The left hemispheres of 5-week-old mice were irradiated with 2, 8, and 20 Gy and a fractionated dose of 8 Gy in 2 fractions using a computed tomography image guided small animal radiation research platform. Long-term potentiation (LTP) has been monitored ex vivo in the hippocampal cornu ammonis 1 (CA1) region and was assessed 3 days and 5 and 10 weeks after PBRT in both hemispheres and compared to a sham group. Irradiation effects on the hippocampus microvasculature were quantified by efficient tissue clearing and multiorgan volumetric imaging.

RESULTS

LTP in irradiated hippocampal slices of juvenile mice declines 3 days after radiation, lasts up to 10 weeks in the irradiated part of the hippocampus, and correlates with a significantly reduced microvasculature length. Specifically, LTP inhibition is sustained in the irradiated (20 Gy, 8 Gy in 2 fractions, 8 Gy, 2 Gy) hippocampus, whereas the contralateral hippocampus remains unaffected after PBRT. LTP inhibition in the irradiated hemisphere after PBRT might be associated with an impaired microvascular network.

CONCLUSION

PBRT induces a long-lasting impairment in neuroplasticity and the microvessel network of the irradiated hippocampus, whereas the contralateral hippocampus remains unaffected. These findings provide insight into the design of PBRT strategies to better protect the young developing brain from cognitive decline.

Cytotoxic T cells are able to efficiently eliminate cancer cells by additive cytotoxicity

Lethal hit delivery by cytotoxic T lymphocytes (CTL) towards B lymphoma cells occurs as a binary, "yes/no" process. In non-hematologic solid tumors, however, CTL often fail to kill target cells during 1:1 conjugation. Here we describe a mechanism of "additive cytotoxicity" by which time-dependent integration of sublethal damage events, delivered by multiple CTL transiting between individual tumor cells, mediates effective elimination. Reversible sublethal damage includes perforin-dependent membrane pore formation, nuclear envelope rupture and DNA damage. Statistical modeling reveals that 3 serial hits delivered with decay intervals below 50 min discriminate between tumor cell death or survival after recovery. In live melanoma lesions in vivo, sublethal multi-hit delivery is most effective in interstitial tissue where high CTL densities and swarming support frequent serial CTL-tumor cell encounters. This identifies CTL-mediated cytotoxicity by multi-hit delivery as an incremental and tunable process, whereby accelerating damage magnitude and frequency may improve immune efficacy.

Fractionated Low-Dose Radiation Induces Long-Lasting Inflammatory Responses in the Hippocampal Stem Cell Niche

PURPOSE

Despite major technical advances in hippocampus-sparing radiation therapy, radiation-induced injury to the neural stem cell compartment may affect neurocognitive functions. In the brain, glial cells modulate neuronal functions and are major mediators of neuroinflammation. In a preclinical mouse model with fractionated low-dose radiation (LDR), the complex response to radiation-induced injury was analyzed in the hippocampal stem cell compartment over a period of 6 months.

METHODS AND MATERIALS

Adult and juvenile C57BL/6NCrl mice were exposed to low doses of ionizing radiation (IR; 20 fractions of 0.1 Gy, for up to 4 weeks) daily. At 72 hours and 1, 3, and 6 months after fractionated LDR, magnetic resonance imaging (9.4 T) was conducted to detect structural and functional abnormalities in the hippocampal region. Using immunofluorescence and histologic studies, neuroglia cells (astrocytes, microglia, oligodendrocytes) were quantified and neuroinflammatory responses were characterized in the hippocampal dentate gyrus. Using in vivo bromodeoxyuridine labeling, the cell fate of newly generated progenitor cells was tracked in the subgranular zone of the dentate gyrus during fractionated LDR.

RESULTS

Low doses of IR induced long-lasting inflammatory responses with local increases of activated microglia and reactive astrocytes, which were most pronounced in the juvenile hippocampus within the first months after LDR. Glial activation with the consequent release of proinflammatory mediators increased local blood flow and vascular permeability in the hippocampal region. Cell fate mapping of progenitors located in the subgranular zone revealed a transient shift from neurogenesis to gliogenesis.

CONCLUSIONS

Glial cell activation and transient neuroinflammation may reflect radiation-induced neuronal damage in the hippocampal stem cell niche. The increased proliferative capacity of the developing brain may explain the enhanced hippocampal radiosensitivity, with stronger inflammatory reactions in the juvenile hippocampus. Thus, limiting the radiation dose to the hippocampal region is an important issue of clinical radiation therapy at all ages to preserve neurocognitive functions.

Nasal Delivery of AntagomiR-741 Protects Against the Radiation-Induced Brain Injury in Mice

Radiation-induced brain injury (RBI) is a serious complication in patients who have received radiotherapy for head and neck tumors. Currently, there is a scarcity of information on early diagnostic and preventive methods of RBI. Accumulating evidence suggests that microRNAs are involved in the regulation of radiation injury, but the molecular biological mechanism of miRNAs in RBI is largely unknown. Therefore, in our study, microRNA sequencing was used to discover differential miRNAs in the hippocampus of RBI-modeled mice, which suggested that miR-741-3p was most significantly upregulated. To clarify the underlying mechanism of miR-741-3p in RBI-modeled mice, an inhibitor of miR-741-3p (antagomiR-741) was delivered into the brain via the nasal passage before irradiation. The delivery of antagomiR-741 significantly reduced miR-741-3p levels in the hippocampus of RBI-modeled mice, and the cognitive dysfunction and neuronal apoptosis induced by radiation were also alleviated at 6 weeks postirradiation. Downregulation of miR-741-3p was found to improve the protrusion and branching status of microglia after irradiation and reduced the number of GFAP-positive astrocytes. Additionally, antagomiR-741 suppressed the radiation-induced production of pro-inflammatory cytokines IL-6 and TNF-α in the hippocampus and S100B in the serum. Furthermore, Ddr2, PKCα and St8sia1 were revealed as target genes of miR-741-3p and as potential regulatory targets for RBI. Overall, our study provides identification and functional evaluation of miRNA in RBI and lays the foundation for improving the prevention strategy for RBI based on the delivery of miRNA via the nose-brain pathway.

Optimizing Adjuvant Stereotactic Radiotherapy of Motor-Eloquent Brain Metastases: Sparing the nTMS-Defined Motor Cortex and the Hippocampus

Brain metastases can effectively be treated with surgical resection and adjuvant stereotactic radiotherapy (SRT). Navigated transcranial magnetic stimulation (nTMS) has been used to non-invasively map the motor cortex prior to surgery of motor eloquent brain lesions. To date, few studies have reported the integration of such motor maps into radiotherapy planning. The hippocampus has been identified as an additional critical structure of radiation-induced deficits. The aim of this study is to assess the feasibility of selective dose reduction to both the nTMS-based motor cortex and the hippocampi in SRT of motor-eloquent brain metastases. Patients with motor-eloquent brain metastases undergoing surgical resection and adjuvant SRT between 07/2014 and 12/2018 were retrospectively analyzed. The radiotherapy treatment plans were retrieved from the treatment planning system (“original” plan). For each case, two intensity-modulated treatment plans were created: the “motor” plan aimed to reduce the dose to the motor cortex, the “motor & hipp” plan additionally reduce the dose to the hippocampus. The optimized plans were compared with the “original” plan regarding plan quality, planning target volume (PTV) coverage, and sparing of organs at risk (OAR). 69 plans were analyzed, all of which were clinically acceptable with no significant differences for PTV coverage. All OAR were protected according to standard protocols. Sparing of the nTMS motor map was feasible: mean dose 9.66 ± 5.97 Gy (original) to 6.32 ± 3.60 Gy (motor) and 6.49 ± 3.78 Gy (motor & hipp), p<0.001. In the “motor & hipp” plan, dose to the ipsilateral hippocampi could be significantly reduced (max 1.78 ± 1.44 Gy vs 2.49 ± 1.87 Gy in “original”, p = 0.003; mean 1.01 ± 0.92 Gy vs. 1.32 ± 1.07 Gy in “original”, p = 0.007). The study confirms the results from previous studies that inclusion of nTMS motor information into radiotherapy treatment planning is possible with a relatively straightforward workflow and can achieve reduced doses to the nTMS-defined motor area without compromising PTV coverage. Furthermore, we demonstrate the feasibility of selective dose reduction to the hippocampus at the same time. The clinical significance of these optimized plans yet remains to be determined. However, with no apparent disadvantages these optimized plans call for further and broader exploration.

Proton therapy for selected low grade glioma patients in the Netherlands

Proton therapy offers an attractive alternative to conventional photon-based radiotherapy in low grade glioma patients, delivering radiotherapy with equivalent efficacy to the tumour with less radiation exposure to the brain. In the Netherlands, patients with favourable prognosis based on tumour and patient characteristics can be offered proton therapy. Radiation-induced neurocognitive function decline is a major concern in these long surviving patients. Although level 1 evidence of superior clinical outcome with proton therapy is lacking, the Dutch National Health Care Institute concluded that there is scientific evidence to assume that proton therapy can have clinical benefit by reducing radiation-induced brain damage. Based on this decision, proton therapy is standard insured care for selected low grade glioma patients. Patients with other intracranial tumours can also qualify for proton therapy, based on the same criteria. In this paper, the evidence and considerations that led to this decision are summarised. Additionally, the eligibility criteria for proton therapy and the steps taken to obtain high-quality data on treatment outcome are discussed.

Assessment of Nuclear and Mitochondrial DNA, Expression of Mitochondria-Related Genes in Different Brain Regions in Rats after Whole-Body X-ray Irradiation

Studies of molecular changes occurred in various brain regions after whole-body irradiation showed a significant increase in terms of the importance in gaining insight into how to slow down or prevent the development of long-term side effects such as carcinogenesis, cognitive impairment and other pathologies. We have analyzed nDNA damage and repair, changes in mitochondrial DNA (mtDNA) copy number and in the level of mtDNA heteroplasmy, and also examined changes in the expression of genes involved in the regulation of mitochondrial biogenesis and dynamics in three areas of the rat brain (hippocampus, cortex and cerebellum) after whole-body X-ray irradiation. Long amplicon quantitative polymerase chain reaction (LA-QPCR) was used to detect nDNA and mtDNA damage. The level of mtDNA heteroplasmy was estimated using Surveyor nuclease technology. The mtDNA copy numbers and expression levels of a number of genes were determined by real-time PCR. The results showed that the repair of nDNA damage in the rat brain regions occurs slowly within 24 h; in the hippocampus, this process runs much slower. The number of mtDNA copies in three regions of the rat brain increases with a simultaneous increase in mtDNA heteroplasmy. However, in the hippocampus, the copy number of mutant mtDNAs increases significantly by the time point of 24 h after radiation exposure. Our analysis shows that in the brain regions of irradiated rats, there is a decrease in the expression of genes (ND2, CytB, ATP5O) involved in ATP synthesis, although by the same time point after irradiation, an increase in transcripts of genes regulating mitochondrial biogenesis is observed. On the other hand, analysis of genes that control the dynamics of mitochondria (Mfn1, Fis1) revealed that sharp decrease in gene expression level occurred, only in the hippocampus. Consequently, the structural and functional characteristics of the hippocampus of rats exposed to whole-body radiation can be different, most significantly from those of the other brain regions.

Combined Treatment with Low-Dose Ionizing Radiation and Ketamine Induces Adverse Changes in CA1 Neuronal Structure in Male Murine Hippocampi

In children, ketamine sedation is often used during radiological procedures. Combined exposure of ketamine and radiation at doses that alone did not affect learning and memory induced permanent cognitive impairment in mice. The aim of this study was to elucidate the mechanism behind this adverse outcome. Neonatal male NMRI mice were administered ketamine (7.5 mg kg⁻¹) and irradiated (whole-body, 100 mGy or 200 mGy, ¹³⁷Cs) one hour after ketamine exposure on postnatal day 10. The control mice were injected with saline and sham-irradiated. The hippocampi were analyzed using label-free proteomics, immunoblotting, and Golgi staining of CA1 neurons six months after treatment. Mice co-exposed to ketamine and low-dose radiation showed alterations in hippocampal proteins related to neuronal shaping and synaptic plasticity. The expression of brain-derived neurotrophic factor, activity-regulated cytoskeleton-associated protein, and postsynaptic density protein 95 were significantly altered only after the combined treatment (100 mGy or 200 mGy combined with ketamine, respectively). Increased numbers of basal dendrites and branching were observed only after the co-exposure, thereby constituting a possible reason for the displayed alterations in behavior. These data suggest that the risk of radiation-induced neurotoxic effects in the pediatric population may be underestimated if based only on the radiation dose.