Why avoid the hippocampus? A comprehensive review

Dosimetric factors associated with pituitary function after Gamma Knife Surgery (GKS) of pituitary adenomas

BACKGROUND AND PURPOSE

Gamma Knife Surgery (GKS) can be an adjunctive option to surgery in the case of pituitary adenomas. The effect of dosimetric variables on the incidence of new anterior pituitary deficits after GKS requires better definition.

MATERIALS AND METHODS

This retrospective study considered 130 patients with a follow up after GKS >6 months. The diagnosis was nonfunctioning pituitary adenoma (NFPA) in 68 patients and secreting pituitary adenoma (SPA) in 62 patients. Median margin dose was 15/25 Gy for NFPA and SPA, respectively. The endocrinological median follow-up was 60 months. Hypopituitarism was defined as a new pituitary deficit in (at least) one of the three hormonal axes (hypogonadism, hypothyroidism and hypoadrenalism). The predictive value of clinical/dosimetric parameters was tested by univariate/multivariate analyses.

RESULTS

Sixteen patients (12.3%) showed a new pituitary deficit in one or more axes. Multivariate analysis confirmed that the mean dose to the stalk/pituitary and the amount of healthy tissue within the high dose region were strong independent predictors of pituitary dysfunction; their best cut-off values were around 15.7 Gy, 7.3 Gy and 1.4 cm(3), respectively.

CONCLUSIONS

Our data showed a dose-dependent incidence of new hormonal deficits after GKS for pituitary adenoma. During planning definition, the risk of hypopituitarism could be reduced using the outlined safe dose-volume values.

Redox balance- and radiation-mediated alteration in hippocampal neurogenesis

Changes in the intracellular and extracellular redox balance have been correlated with cell fate decisions in terms of proliferation versus differentiation, entering versus existing cell cycle and survival versus cell death. Adult hippocampal neurogenesis has been correlated with neuronal plasticity of learning and memory; however, the process is exquisitely sensitive to changes in redox balance. Cranial irradiation is an effective modality in treating brain tumours but often leads to deficits in hippocampus-related learning and memory, which is most likely due to sustained elevation of oxygen free radical production and suppression of hippocampal neurogenesis. The subcellular redox environment affecting hippocampal neurogenesis is largely unknown. Using mutant mice deficient in each one of the three superoxide dismutase (SOD, EC 1.15.1.1) isoforms, we have begun to determine the consequences of SOD deficiency in hippocampal neurogenesis and the related functions of learning and memory under normal condition and following cranial irradiation.

Whole brain irradiation with hippocampal sparing and dose escalation on multiple brain metastases: a planning study on treatment concepts

PURPOSE

To develop a new treatment planning strategy in patients with multiple brain metastases. The goal was to perform whole brain irradiation (WBI) with hippocampal sparing and dose escalation on multiple brain metastases. Two treatment concepts were investigated: simultaneously integrated boost (SIB) and WBI followed by stereotactic fractionated radiation therapy sequential concept (SC).

METHODS AND MATERIALS

Treatment plans for both concepts were calculated for 10 patients with 2-8 brain metastases using volumetric modulated arc therapy. In the SIB concept, the prescribed dose was 30 Gy in 12 fractions to the whole brain and 51 Gy in 12 fractions to individual brain metastases. In the SC concept, the prescription was 30 Gy in 12 fractions to the whole brain followed by 18 Gy in 2 fractions to brain metastases. All plans were optimized for dose coverage of whole brain and lesions, simultaneously minimizing dose to the hippocampus. The treatment plans were evaluated on target coverage, homogeneity, and minimal dose to the hippocampus and organs at risk.

RESULTS

The SIB concept enabled more successful sparing of the hippocampus; the mean dose to the hippocampus was 7.55±0.62 Gy and 6.29±0.62 Gy, respectively, when 5-mm and 10-mm avoidance regions around the hippocampus were used, normalized to 2-Gy fractions. In the SC concept, the mean dose to hippocampus was 9.8±1.75 Gy. The mean dose to the whole brain (excluding metastases) was 33.2±0.7 Gy and 32.7±0.96 Gy, respectively, in the SIB concept, for 5-mm and 10-mm hippocampus avoidance regions, and 37.23±1.42 Gy in SC.

CONCLUSIONS

Both concepts, SIB and SC, were able to achieve adequate whole brain coverage and radiosurgery-equivalent dose distributions to individual brain metastases. The SIB technique achieved better sparing of the hippocampus, especially when a10-mm hippocampal avoidance region was used.

Oxidative stress and adult neurogenesis--effects of radiation and superoxide dismutase deficiency

Hippocampus plays an important role in learning and memory and in spatial navigation. Production of new neurons that are functionally integrated into the hippocampal neuronal network is important for the maintenance of functional plasticity. In adults, production of new neurons in the hippocampus takes place in the subgranular zone (SGZ) of dentate gyrus. Neural progenitor/stem cells go through processes of proliferation, differentiation, migration, and maturation. This process is exquisitely sensitive to oxidative stress, and perturbation in the redox balance in the neurogenic microenvironment can lead to reduced neurogenesis. Cranial irradiation is an effective treatment for primary and secondary brain tumors. However, even low doses of irradiation can lead to persistent elevation of oxidative stress and sustained suppression of hippocampal neurogenesis. Superoxide dismutases (SODs) are major antioxidant enzymes for the removal of superoxide radicals in different subcellular compartments. To identify the subcellular location where reactive oxygen species (ROS) are continuously generated after cranial irradiation, different SOD deficient mice have been used to determine the effects of irradiation on hippocampal neurogenesis. The study results suggest that, regardless of the subcellular location, SOD deficiency leads to a significant reduction in the production of new neurons in the SGZ of hippocampal dentate gyrus. In exchange, the generation of new glial cells was significantly increased. The SOD deficient condition, however, altered the tissue response to irradiation, and SOD deficient mice were able to maintain a similar level of neurogenesis after irradiation while wild type mice showed a significant reduction in the production of new neurons.

FTY720, sphingosine 1-phosphate receptor modulator, selectively radioprotects hippocampal neural stem cells

Cranial irradiation is an effective treatment modality for both primary and metastatic brain tumors, yet it induces cognitive decline in a substantial number of patients. At present, there are no established methods for neuroprotection. Recent investigations have revealed a link between radiation-induced cognitive dysfunction and the loss of neural precursor cells in the hippocampus. Hence, identifying pharmacological agents, capable of protecting this cell population, is of interest. FTY720 (fingolimod), an FDA-approved oral drug for the treatment of multiple sclerosis, has been shown to promote the survival and differentiation of neural progenitors, as well as remyelination and repair after brain injury. In this study, we show that FTY720, used at nanomolar concentrations, is capable of increasing the viability and neurogenicity of irradiated neural stem cells from the hippocampus. In contrast, it does not provide radioprotection in a human breast cancer cell line and two glioma cell lines. These results suggest a potential therapeutic role for FTY720 as a neuroprotector during cranial irradiation. Further preclinical studies are warranted to evaluate this possibility.

Management of brain metastasis: past lessons, modern management, and future considerations

Brain metastasis is a major challenge for patients, physicians, and the broader health care system, with approximately 170,000 new cases per year. After a diagnosis of brain metastasis, patients have a poor prognosis, but modern management has made significant advances in the past two decades to improve palliative efficacy and patient survival through a multidisciplinary approach. A number of factors must be taken into consideration in the treatment approach, including the number of intracranial lesions, the control of extracranial disease, and the patient's overall health, while weighing the benefits of treatment against the toxicities, both acute and chronic. With quality of life as an emphasis, emerging concepts for modern management of brain metastasis have sought to minimize long-term toxicities. The economic impact of such strategies for patients and the health care system has been demonstrated in some studies, but has not been a consistent area of focus. Each of these strategies, as well as novel therapeutics, has embraced the concept of personalized treatment. This review will discuss the current knowledge of modern multidisciplinary management of brain metastasis and look forward to emerging concepts.

Phase measurement of cognitive impairment specific to radiotherapy

PURPOSE

Memory impairment is an early-delayed effect of radiotherapy (RT). The prospective longitudinal measurement of the cognitive phase effects from RT was conducted on treated and untreated brain tumor patients. The study design investigated semantic vs. perceptual and visual vs. verbal memory to determine the most disease-specific measure of RT-related changes and understanding of the neurotoxicity from RT to the brain.

METHODS AND MATERIALS

Tests of memory that had previously shown RT-related phasic changes were compared with experimental tests of memory to test hypotheses about cognition targeted to the neural toxicity of RT. The results from 41 irradiated and 29 nonirradiated patients with low-grade, supratentorial tumors were analyzed. The methods controlled for comorbid white matter risk, recurrence, interval after treatment, and age (18-69 years). The effects were examined before RT and at three points after RT to 1 year using a mixed effects model that included interval, group, surgical status, medication use, practice, and individual random effects. Four new tests of memory and other candidate cognitive tests were investigated, and a post hoc analysis of a comprehensive battery of tests was performed to identify the cognitive processes most specific to RT.

RESULTS

The RT effects on memory were identified in the treated group only; among the new tests of memory and the complete neurocognitive battery, the RT effects were significant only for delayed recall (p < 0.009) and interval to recognize (p < 0.002). Tumor location was not related to the treatment effect. Memory decline was specific to retrieval of semantic memories; a double dissociation of semantic from perceptual visual memory was demonstrated in the RT group.

CONCLUSIONS

These results implicate memory dependent on the semantic cortex and the hippocampal memory system. A cognitive measurement that is brief but specific to neural mechanisms is effective and feasible for studies of RT damage.

Concise review: stem cell effects in radiation risk

Stem cells of normal mammalian tissues are defined as nonspecialized cells that have two critical properties: (a) the ability to renew themselves through cell division and (b) the potency to differentiate into other cell types. Therefore, they play a crucial role in development and in tissue homeostasis during adult life. Being long-lived, they can be the targets of environmental carcinogens leading to the accumulation of consecutive genetic changes. Hence, the genome of stem cells must be exceptionally well protected, and several protective mechanisms have evolved to ensure the genetic integrity of the stem cell compartment in any given tissue. Ionizing radiation exposure can disrupt tissue homeostasis both through the induction of cell killing/depletion of radiosensitive stem cells, leading to loss of tissue functionality and by genotoxic damage, increasing overall risk of cancer. We will review the current knowledge about radiation effects in adult stem cells of specific normal tissues, including skin, breast, and brain, examine parallels, as well as differences with cancer stem cells, and discuss the relevance of stem cell effects to radiation risk and radiotherapy.

Head and neck region consolidation radiotherapy and prophylactic cranial irradiation with hippocampal avoidance delivered with helical tomotherapy after induction chemotherapy for non-sinonasal neuroendocrine carcinoma of the upper airways

Background

Non-sinonasal neuroendocrine carcinomas (NSNECs) of the head and neck are considered an unfrequent clinico-pathological entity. Combined modality treatment represents an established therapeutic option for undifferentiated forms where distant metastasis is a common pattern of failure.

Methods

We report on a case of NSNEC treated with sequential chemo-radiation consisting of 6 cycles of cisplatin and etoposide followed by loco-regional radiation to the head and neck and simultaneous prophylactic cranial irradiation to prevent from intracranial spread, delivered with helical tomotherapy with the 'hippocampal avoidance' technique in order to reduce neuro-cognitive late effects.

Results

One year after the end of the whole combined modality approach, the patient achieved complete remission, with no treatment-related sub-acute and late effects.

Conclusions

The present report highlights the importance of multidisciplinary management for NSNECs of the head and neck, as the possibility to achieve substantial cure rates with mild side effects with modern radiotherapy techniques.

Cognitive performance after postoperative pituitary radiotherapy: a dosimetric study of the hippocampus and the prefrontal cortex

OBJECTIVE

The hippocampus and prefrontal cortex (PFC) are important for memory and executive functioning and are known to be sensitive to radiotherapy (RT). Radiation dosimetry relates radiation exposure to specific brain areas. The effects of various pituitary RT techniques were studied by relating detailed dosimetry of the hippocampus and PFC to cognitive performance.

METHODS

In this cross-sectional design, 75 non-functioning pituitary macroadenoma (NFA) patients (61±10 years) participated and were divided into irradiated (RT+, n=30) and non-irradiated (RT-, n=45) groups. The RT+ group (who all received 25 fractions of 1.8 Gy; total dose: 45 Gy) consisted of three RT technique groups: three-field technique, n=10; four-field technique, n=15; and five-field technique, n=5. Memory and executive functioning were assessed by standardized neuropsychological tests. A reconstruction of the dose distributions for the three RT techniques was made. The RT doses on 30, 50, and 70% of the volume of the left and right hippocampus and PFC were calculated.

RESULTS

Cognitive test performance was not different between the four groups, despite differences in radiation doses applied to the hippocampi and PFC. Age at RT, time since RT, and the use of thyroid hormone varied significantly between the groups; however, they were not related to cognitive performance.

CONCLUSION

This study showed that there were no significant differences on cognitive performance between the three-, four-, and five-field RT groups and the non-irradiated patient group. A dose-response relationship could not be established, even with a radiation dose that was higher on most of the volume of the hippocampus and PFC in case of a four-field RT technique compared with the three- and five-field RT techniques.

Hippocampal dosimetry predicts neurocognitive function impairment after fractionated stereotactic radiotherapy for benign or low-grade adult brain tumors

PURPOSE

To prospectively evaluate the association between hippocampal dose and long-term neurocognitive function (NCF) impairment for benign or low-grade adult brain tumors treated with fractionated stereotactic radiotherapy (FSRT).

METHODS AND MATERIALS

Adult patients with benign or low-grade adult brain tumors were treated with FSRT per institutional practice. No attempt was made to spare the hippocampus. NCF testing was conducted at baseline and 18 months follow-up, on a prospective clinical trial. Regression-based standardized z scores were calculated by using similar healthy control individuals evaluated at the same test-retest interval. NCF impairment was defined as a z score ≤-1.5. After delineation of the bilateral hippocampi according to the Radiation Therapy Oncology Group contouring atlas, dose-volume histograms were generated for the left and right hippocampi and for the composite pair. Biologically equivalent doses in 2-Gy fractions (EQD(2)) assuming an α/β ratio of 2 Gy were computed. Fisher's exact test and binary logistic regression were used for univariate and multivariate analyses, respectively. Dose-response data were fit to a nonlinear model.

RESULTS

Of 29 patients enrolled in this trial, 18 completed both baseline and 18-month NCF testing. An EQD(2) to 40% of the bilateral hippocampi >7.3 Gy was associated with impairment in Wechsler Memory Scale-III Word List (WMS-WL) delayed recall (odds ratio [OR] 19.3; p = 0.043). The association between WMS-WL delayed recall and EQD(2) to 100% of the bilateral hippocampi >0.0 Gy trended to significance (OR 14.8; p = 0.068).

CONCLUSION

EQD(2) to 40% of the bilateral hippocampi greater than 7.3 Gy is associated with long-term impairment in list-learning delayed recall after FSRT for benign or low-grade adult brain tumors. Given that modern intensity-modulated radiotherapy techniques can reduce the dose to the bilateral hippocampi below this dosimetric threshold, patients should be enrolled in ongoing prospective trials of hippocampal sparing during cranial irradiation to confirm these preliminary results.

Treatment of brain metastases: review of phase III randomized controlled trials

The optimal management of brain metastases remains controversial. Both whole brain radiotherapy (WBRT) and local treatment [surgery (S) or radiosurgery (RS)] are the cornerstones of treatment. The role of systemic therapy is also being explored. Randomized controlled trials (RCT) have tried to assess the individual and combined effects of different therapeutic strategies. (1) RCT in oligometastatic patients: WBRT alone vs. local treatment+WBRT. Combined treatment may improve both overall survival and local control in patients with a single metastasis, but it also leads to a local control benefit in patients with two to four lesions. Exclusive local treatment vs. WBRT plus local treatment. The addition of WBRT to local treatment may result in improved local control, improved freedom from new brain metastases and improved overall brain control. S+WBRT vs. RS+WBRT. There is no evidence of superiority of a combined treatment over the other one. (2) RCT addressing the point of improving WBRT outcome: differences in WBRT fractionation do not significantly alter outcome of treatments. Only a few systemic drugs may cause some significant advantages. (3) RCT that assessed neurocognitive impairment and quality of life: the baseline cognitive performance of most patients is significantly impaired. Intracranial tumor control is an essential factor in stabilizing neurocognitive function. The data on neurocognitive toxicity related to WBRT are still contradictory. Impairment of both neurocognitive function and quality of life of patients with brain metastases needs to be further addressed in RCT.

Photon and proton therapy planning comparison for malignant glioma based on CT, FDG-PET, DTI-MRI and fiber tracking

PURPOSE

The purpose of this study was to compare treatment plans generated using fixed beam Intensity Modulated photon Radiation Therapy (IMRT), inversely optimized arc therapy (RapidArc(R), RA) with spot-scanned Intensity Modulated Proton Therapy (IMPT) for high-grade glioma patients. Plans were compared with respect to target coverage and sparing of organs at risk (OARs), with special attention to the possibility of hippocampus sparing.

METHOD

Fifteen consecutive patients diagnosed with grade III and IV glioma were selected for this study. The target and OARs were delineated based on computed tomography (CT), FDG-positron emission tomography (PET) and T1-, T2-weigted, and Diffusion Tensor Imaging (DTI) magnetic resonance imaging (MRI) and fiber-tracking. In this study, a 6 MV photon beam on a linear accelerator with a multileaf collimator (MLC) with 2.5 mm leaves and a spot-scanning proton therapy machine were used. Two RA fields, using both a coplanar (clinical standard) and a non-coplanar, setup was compared to the IMRT and IMPT techniques. Three and three to four non-coplanar fields where used in the spot-scanned IMPT and IMRT plans, respectively. The same set of planning dose-volume optimizer objective values were used for the four techniques. The highest planning priority was given to the brainstem (maximum 54 Gy) followed by the PTV (prescription 60 Gy); the hippocampi, eyes, inner ears, brain and chiasm were given lower priority. Doses were recorded for the plans to targets and OARs and compared to our clinical standard technique using the Wilcoxon signed rank test.

RESULT

The PTV coverage was significantly more conform for IMPT than the coplanar RA technique, while RA plans tended to be more conform than the IMRT plans, as measured by the standard deviation of the PTV dose. In the cases where the tumor was confined in one cerebral hemisphere (eight patients), the non-coplanar RA and IMPT techniques yielded borderline significantly lower doses to the contralateral hippocampus compared to the standard (22% and 97% average reduction for non-coplanar RA and IMPT, respectively). The IMPT technique allowed for the largest healthy tissue sparing of the techniques in terms of whole brain doses and to the fiber tracts. The maximum doses to the chiasm and brainstem were comparable for all techniques.

CONCLUSION

The IMPT technique produced the most conform plans. For tumors located in the one of the cerebral hemispheres, the non-coplanar RA and the IMPT techniques were able to reduce doses to the contralateral hippocampus. The IMPT technique offered the largest sparing of the brain and fiber tracts. RA techniques tended to produce more conform target doses than IMRT.

Time course of hypothalamic-pituitary deficiency in adults receiving cranial radiotherapy for primary extrasellar brain tumors

BACKGROUND

No longitudinal data on hypothalamic-pituitary (HP) function are available in patients who had received cranial radiation therapy (CRT) for primary extrasellar brain tumors (PBT).

PURPOSE

To investigate the effects of CRT on HP function in adults with PBT.

PATIENTS AND METHODS

Twenty-six adults irradiated for PBT and six CRT naive controls were studied. CRT was delivered with 6 MV X-ray by a linear accelerator (2 Gy fraction schedule). Gross Tumor Volume (GTV) excluded the HP region that was contoured on the planning CT. Median dose to the HP region was 41.8 Gy (IQR: 30.7-49.8).

RESULTS

All controls maintained normal HP function. Hypopituitarism developed in 38% of CRT patients (GH deficiency 29%, ACTH 22%, TSH 14%, gonadotropin 4%, no abnormal prolactin level or diabetes insipidus). All HP failures occurred within 32 months after CRT.

CONCLUSIONS

Adults undergoing CRT for PBT are at increased risk for HP dysfunction within 3 years from CRT. Endocrine surveillance is recommended also in adults patients exposed to CRT for primary brain tumors distant from HP region.

Glycogen synthase kinase 3beta inhibition enhances repair of DNA double-strand breaks in irradiated hippocampal neurons

Prevention of cranial radiation-induced morbidity following the treatment of primary and metastatic brain cancers, including long-term neurocognitive deficiencies, remains challenging. Previously, we have shown that inhibition of glycogen synthase kinase 3β (GSK3β) results in protection of hippocampal neurons from radiation (IR)-induced apoptosis and attenuation of neurocognitive dysfunction resulting from cranial IR. In this study, we examined whether regulation of the repair of IR-induced DNA damage is one of the mechanisms involved in the radioprotective effects of neurons by inhibition of GSK3β. Specifically, this study showed that inhibition of GSK3β accelerated double strand-break (DSB) repair efficiency in irradiated mouse hippocampal neurons, as assessed by the neutral comet assay. This coincided with attenuation of IR-induced γ-H2AX foci, a well characterized in situ marker of DSBs. To confirm the effect of GSK3 activity on the efficacy of DSB repair, we further demonstrated that biochemical or genetic inhibition of GSK3 activity resulted in enhanced capacity in nonhomologous end-joining-mediated repair of DSBs in hippocampal neurons. Importantly, none of these effects were observed in malignant glioma cells. Taken together, these results suggested that enhanced repair of IR-induced DNA damage may be a novel mechanism by which inhibition of GSK3β specifically protects hippocampal neurons from IR-induced apoptosis. Furthermore, these findings warrant future investigations of the molecular mechanisms underlying the role of GSK3β in the DSB repair of normal neurons and the potential clinical application of neuroprotection with GSK3β inhibitors during cranial IR.