Acute leukemia. Learning disabilities following CNS irradiation

Unraveling the fundamental molecular mechanisms of morphological and cognitive defects in the irradiated brain

Prenatal radiation exposure may have serious consequences for normal brain development. Results of epidemiological studies clearly pointed towards an increased risk of mental retardation in children of the surviving women of the Hiroshima/Nagasaki atomic bombing when in utero exposure had occurred between weeks 8 and 15 of pregnancy or, at a lower extend between weeks 15 and 25. The high sensitivity of the developing brain, in comparison to the adult brain is related to its higher number of non-differentiated, dividing neural precursor cells. Exposure of the developing brain to ionizing radiation can lead to three main outcomes in the developing brain, depending on the radiation dose and the elapsed period after irradiation. A first event occurs early after irradiation and triggers disturbances in cell proliferation, migration, differentiation, and cell death. A second event involves the generation of morphological abnormalities in the developing brain, if the radiation dose is sufficient. A third event involves cognitive dysfunctions that are a direct consequence from a disturbance in regional brain formation. The latter results from exposure to low doses and is usually only observed in the later period of development. In order to understand the mechanisms of radiation-induced cognitive dysfunctions, it is important to track back the underlying changes in specific molecular pathways. In this review, we present the possible relationships within and between molecular pathways potentially involved in cognitive dysfunctions induced by ionizing radiation in the developing brain.

Effect of low doses of ionising radiation in infancy on cognitive function in adulthood: Swedish population based cohort study

OBJECTIVE

To determine whether exposure to low doses of ionising radiation in infancy affects cognitive function in adulthood.

DESIGN

Population based cohort study.

SETTING

Sweden.

PARTICIPANTS

3094 men who had received radiation for cutaneous haemangioma before age 18 months during 1930-59.

MAIN OUTCOME MEASURES

Radiation dose to frontal and posterior parts of the brain, and association between dose and intellectual capacity at age 18 or 19 years based on cognitive tests (learning ability, logical reasoning, spatial recognition) and high school attendance.

RESULTS

The proportion of boys who attended high school decreased with increasing doses of radiation to both the frontal and the posterior parts of the brain from about 32% among those not exposed to around 17% in those who received > 250 mGy. For the frontal dose, the multivariate odds ratio was 0.47 (95% confidence interval 0.26 to 0.85, P for trend 0.0003) and for the posterior dose it was 0.59 (0.23 to 1.47, 0.0005). A negative dose-response relation was also evident for the three cognitive tests for learning ability and logical reasoning but not for the test of spatial recognition.

CONCLUSIONS

Low doses of ionising radiation to the brain in infancy influence cognitive abilities in adulthood.

Long-term cognitive outcome, brain computed tomography scan, and magnetic resonance imaging in children cured for acute lymphoblastic leukemia

BACKGROUND

Prevention of meningeal recurrence achieved by intrathecal methotrexate (MTX) and systemic chemotherapy is part of standard acute lymphoblastic leukemia (ALL) treatment. Cranial irradiation has been a routine part of past protocol treatment but is currently reserved only for select subsets of patients. Central nervous system (CNS) prophylaxis may cause brain abnormalities such as intracerebral calcifications, cerebral atrophy, and white matter alterations. In addition, long-term neuropsychologic sequelae following CNS prophylaxis have been investigated marginally in children cured for ALL.

METHODS

To explore possible correlations between neuroimaging findings and neuropsychologic outcome, we used detailed cognitive tests to evaluate 21 children with ALL who received cranial irradiation (range, 18-24 Gy) plus intrathecal MTX as CNS prophylaxis. All children were followed prospectively once a year by cerebral computed tomographic scan and magnetic resonance imaging. All patients had continuous complete disease remission for at least 4 years and cognitive tests were performed after neuroradiologic examinations.

RESULTS

White matter abnormalities were associated with poor performance only in a task exploring visual motor integration in about 50% of patients. Intracerebral calcifications correlate with the number of intrathecal MTX doses and with low scores in total intellectual quotient, performance intellectual quotient, and significant impairment in attention and visual motor integration tests. Girls are more vulnerable to the effects of CNS prophylaxis, whereas age at treatment and radiotherapy dose are not relevant to neuropsychologic outcome.

CONCLUSIONS

Our results indicate the need for careful follow-up of children's cognitive abilities because global intellectual measures often fail to detect specific disorders that may cause learning difficulties. Moreover, as the clinical implications of neuroimaging findings are often limited, periodic extensive evaluation by specific neurocognitive tests of mental abilities is recommended to detect early signs of cognitive impairment.

Effect of ionizing radiation on the human brain: white matter and gray matter T1 in pediatric brain tumor patients treated with conformal radiation therapy

OBJECTIVE

To test a hypothesis that fractionated radiation therapy (RT) to less than 60 Gy is associated with a dose-related change in the spin-lattice relaxation time (T1) of normal brain tissue, and that such changes are detectable by quantitative MRI (qMRI).

METHODS

Each of 21 patients received a qMRI examination before treatment, and at several time points during and after RT. A map of brain T1 was calculated and segmented into white matter and gray matter at each time point. The RT isodose contours were then superimposed upon the T1 map, and changes in brain tissue T1 were analyzed as a function of radiation dose and time following treatment. We used a mixed-model analysis to analyze the longitudinal trend in brain T1 from the start of RT to 1 year later. Predictive factors evaluated included patient age and clinical variables, such as RT dose, time since treatment, and the use of an imaging contrast agent.

RESULTS

In white matter (WM), a dose level of greater than 20 Gy was associated with a dose-dependent decrease in T1 over time, which became significant about 3 months following treatment. In gray matter (GM), there was no significant change in T1 over time, as a function of RT doses < 60 Gy. However, GM in close proximity to the tumor had an inherently lower T1 before therapy. Neither use of a contrast agent nor a combination of chemotherapy plus steroids had a significant effect on brain T1.

CONCLUSION

Results suggest that T1 mapping may be sensitive to radiation-related changes in human brain tissue T1. WM T1 appears to be unaffected by RT at doses less than approximately 20 Gy; GM T1 does not change at doses less than 60 Gy. However, tumor appears to have an effect upon adjacent GM, even before treatment. Conformal RT may offer a substantial benefit to the patient, by minimizing the volume of normal brain exposed to greater than 20 Gy.

The radioresponse of the central nervous system: a dynamic process

Radiation continues to be a major treatment modality for tumors located within and close to the central nervous system (CNS). Consequently, alleviating or protecting against radiation-induced CNS injury would be of benefit in cancer treatment. However, the rational development of such interventional strategies will depend on a more complete understand-ing of the mechanisms responsible for the development of this form of normal tissue injury. Whereas the vasculature and the oligodendrocyte lineage have traditionally been considered the primary radiation targets in the CNS, in this review we suggest that other phenotypes as well as critical cellular interactions may also be involved in determining the radio-response of the CNS. Furthermore, based on the assumption that the CNS has a limited repertoire of responses to injury, the reaction of the CNS to other types of insults is used as a framework for modeling the pathogenesis of radiation-induced damage. Evidence is then provided suggesting that, in addition to acute cell death, radiation induces an intrinsic recovery/repair response in the form of specific cytokines and may

Pathogenesis of irradiation-induced cognitive dysfunction

Neurocognitive dysfunction is a common sequela of cranial irradiation that is especially severe in young children. The underlying mechanisms of this disorder have not been described. The present review describes the role of the hippocampus and the anatomically related cortex in memory function and its marked susceptibility to ischemic and hypoxic injury. Based on studies of animal models of human amnesia and histopathological findings in the irradiated brain, the neurocognitive sequela of cranial irradiation can be seen to be mediated through vascular injury, resulting in ischemia and hypoxia in the hippocampal region. Recognition of the site and mechanisms of this injury may lead to the development of techniques to minimize the risks.

Neuropsychological effects of cranial radiation: current knowledge and future directions

Radiation is an invaluable therapeutic tool in the treatment of cancer, with well-established palliative and curative efficacy. As patient survival has improved, attention has focused on long-range treatment side effects. One such adverse effect, neuropsychological impairment, is incompletely understood. Much of the extant research has been directed at childhood leukemia survivors treated with low-dose whole-brain radiation. Less is known about the effects of high-dose focal or whole-brain radiation used in the treatment of brain lesions. This article reviews the scientific literature in this area, with greatest emphasis on methodologically rigorous studies. Research design considerations are discussed. Review findings suggest that low-dose whole-brain radiation (18 to 24 Gy) in children is associated with mild delayed IQ decline, with more substantial deficits occurring in children treated at a young age. A high incidence of learning disabilities and academic failure is observed in this population and may be caused by poor attention and memory rather than low intellectual level. Children who receive higher dose radiation for treatment of brain tumors experience more pronounced cognitive decline. At higher doses, whole-brain radiation, in particular, is linked to deleterious cognitive outcomes. Remarkably little is known about cognitive outcomes in irradiated adults. Preliminary findings indicate that certain cognitive functions, including memory, may be more vulnerable to decline than others. Suggestions for future research are proposed.

Late sequelae in children treated for brain tumors and leukemia

Forty-two survivors treated at an age of 2-16 years for brain tumors or leukemia were, 4-21 years after treatment, subjected to an extensive follow-up investigation, including physical examination and interview; 35 of them also had endocrinological and 33 psychological evaluation. Hormonal deficiencies were found in about two-thirds of patients and were most common in those treated for brain tumors. The great majority had verbal intelligence quotient (VIQ) within normal range. Also, the performance intelligence quotients (PIQ) were normal in most patients. However, the results suggested that the primary intellectual capacity in children treated for cancer was not being fully utilized, their PIQ being on the average higher than their VIQ; this tendency was especially pronounced in the leukemia patients.

The effect of scheduling in children undergoing prophylactic cranial irradiation for acute lymphoblastic leukaemia

Control of central nervous system (CNS) disease and overall survival have been analysed in a group of 829 children with leukaemia entered into the UKALL VIII trial to determine whether scheduling of the cranial irradiation is of prognostic significance. We show that short gaps in treatment do not influence prognosis and that current radiotherapy practice need not be modified.

CNS prophylaxis of childhood leukemia: what are the long-term neurological, neuropsychological, and behavioral effects?

Current medical treatments for childhood acute lymphoblastic leukemia (ALL) have improved the outlook to where more than 50% can be expected to survive five years or more. The use of CNS prophylaxis has contributed in a significant way to these improved survival statistics by reducing the likelihood of CNS relapses. The literature relating to the potential adverse psychological consequences of CNS prophylaxis, which include cranial radiation therapy (CRT), is reviewed and analyzed. The majority of published papers of children in first remission report that CNS prophylaxis, which include both CRT and intrathecal methotrexate, results in a variety of learning problems in many children who were younger than age 5 when treated. The available literature on the social, emotional, and educational sequelae of childhood ALL is also reviewed.