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Characterization of Leukoencephalopathy and Association With Later Neurocognitive Performance in Pediatric Acute Lymphoblastic Leukemia

Pryweller, Jennifer R. PhD; Glass, John O. MS; Sabin, Noah D. MD; Laningham, Fred H. MD; Li, Yimei PhD; Jacola, Lisa M. PhD§; Conklin, Heather M. PhD§; Reddick, Wilburn E. PhD

Author Information
doi: 10.1097/RLI.0000000000000715

Abstract

Acute lymphoblastic leukemia (ALL) is the most common pediatric cancer and accounts for more than 3000 new cases each year in the United States.1 Methotrexate (MTX) is an essential chemotherapeutic drug, which is frequently administered both intrathecally and at high doses (HDMTX) intravenously as consolidation treatment, and contributes to contemporary cure rates approaching 90% in pediatric ALL.2 However, MTX can cause clinical neurotoxicity by disrupting the central nervous system (CNS) folate homeostasis and/or direct neuronal damage, impeding the process of myelination and, subsequently, neuronal transmission.3–6 Magnetic resonance (MR) neuroimaging studies reveal leukoencephalopathy (LE) to be the most common MTX-related neurotoxic adverse effect, and it has been associated with increased MTX exposure.7,8

Leukoencephalopathy is seen as T2-weighted white matter (WM) hyperintensities and may be transient or chronic.9–11 Magnetic resonance image contrast reflects differences in T1 and T2 relaxation times in LE, resulting from a variety of biological underpinnings.12 Previous studies have shown increases in T1 and T2 relaxation rates in LE compared with that in normal-appearing white matter (NAWM), which were significantly correlated with each other and were dependent on the proportion of WM affected.13,14

Neurocognitive deficits in ALL patients can have a devastating effect on long-term quality of life.15 Most commonly reported sequelae are attention deficits and working memory.16–19 Longer-term disruptions in neurocognitive functioning can lead to lower academic performance and decreased social attainment, which are associated with clinical and demographic factors.15,20,21

A recent study identified clinical, pharmacokinetic, and genetic risk factors for treatment-induced neurotoxicity22 in a large cohort of children treated for ALL at a single institution on a uniform protocol-directed, risk-stratified treatment regimen that did not include irradiation. In addition, higher doses of MTX and younger age at diagnosis have previously been associated with decreased neurocognitive performance at the end of therapy15 and at a 2-year follow-up23 in this same cohort of patients. However, the impact of early imaging changes on long-term neurocognitive performance remains to be evaluated. The purpose of this study was to first characterize the prevalence, extent, and intensity of LE during therapy and then investigate the hypothesis that these early imaging markers will be associated with later neurocognitive performance.

MATERIALS AND METHODS

Patients and Treatment Protocol

Between June 2000 and October 2007, 498 pediatric ALL patients were enrolled on an institutional frontline treatment protocol (NCT00137111) and were approached for participation in this prospective, longitudinal MR imaging screening study. The study was approved by the institutional review board and conducted in accordance with guidelines from the National Cancer Institute and the Office for Human Research Protections. Research imaging was integrated into protocol-directed clinical care for all patients at least 1 year of age at diagnosis. Informed consent was obtained from each patient's parent or guardian. Among these patients, 391 volunteered to participate. Fourteen patients were excluded: 10 based on a Down syndrome diagnosis, and 1 each due to early withdrawal from the protocol, cerebral thrombosis, brain cyst, and brain hematoma. Data from 377 (207 male patients) children, ages 1 to 18 years (mean, 6.9 ± 4.6), were analyzed.

The institutional treatment protocol has previously been described24 and is demonstrated graphically in Figure 1. All patients received triple intrathecal chemotherapy with MTX, cytarabine, and hydrocortisone as CNS-directed therapy beginning with remission induction. High-dose MTX was given as 1 course at 1.0 g/m2 on day 1 of remission induction, and 4 additional courses were given upon achieving completion of remission induction as consolidation therapy, with doses adjusted to achieve targeted systematic exposure to eliminate individual differences due to variability in clearance. Patients with low-risk (LR) ALL were treated with an average dose of 2.5 g/m2, and those with standard- or high-risk (SHR) were treated with an average dose of 5.0 g/m2, with dosages adjusted to achieve a plasma steady-state concentration of 33 μM or 65 μM, respectively. Courses of HDMTX were followed by standardized leucovorin rescue.

FIGURE 1
FIGURE 1:
Graphical representation of the timing of imaging and neurocognitive evaluations relative to the timeline of the therapy. Green circle represents start of therapy and red circle is end of therapy (EOT). Therapy is divided into 3 stages: induction, consolidation, and continuation (120 weeks for female patients and 146 weeks for male patients). Both male and female patients were imaged and underwent neurocognitive testing at week 120 for consistency. Orange rounded boxes represent courses of high-dose MTX (HDMTX) that were administered as 1 course at 1.0 g/m2 on day 1 of remission induction, and 4 additional courses given as 2.5 or 5.0 g/m2 depending on risk stratification. Blue boxes represent the 4 MR imaging time points: upon completion of 6 weeks of remission induction (MR1), after consolidation treatment on week 7 (MR2), and week 48 (MR3) and week 120 (MR4) of continuation treatment. Gold pentagons represent neurocognitive testing (Ψ) at EOT and 2-year follow-up.

Continuation treatment included weekly intravenous MTX at 40 mg/m2 together with daily mercaptopurine for 3 weeks, followed by pulse therapy with vincristine plus dexamethasone at week 4. Continuation therapy spanned 120 weeks for girls and 146 weeks for boys and was interrupted by 2 reinduction treatments. Both male and female patients were imaged at week 120 for consistency. Therefore, the last patient was imaged in June 2010.

Quantitative Magnetic Resonance Imaging and Diagnosis of Leukoencephalopathy

Longitudinal MR data were acquired at 4 time points: upon completion of 6 weeks of remission induction (MR1), week 7 (MR2) after consolidation treatment with 4 courses of HDMTX, and week 48 (MR3) and week 120 (MR4) of continuation treatment (end-therapy time point). Patients demonstrated no neurologic symptoms at the time of imaging regardless of the presence or absence of LE.

All MR images were 4-mm-thick contiguous axial imaging datasets collected on a 1.5-T Vision whole-body unit (Siemens Medical Systems, Iselin, NJ). T1-weighted images were acquired with a multiecho inversion recovery sequence (repetition time [TR], 8000 milliseconds; echo time [TE], 29 milliseconds; inversion time [TI], 300 milliseconds; 1 average, 7 echoes). Because LE is best detected by a T2-weighted sequence, preferably with cerebrospinal fluid (CSF) attenuation, a fluid-attenuated inversion recovery (FLAIR) image was collected with a multiecho sequence (TR, 9000 milliseconds; TE, 119 milliseconds; TI, 2470 milliseconds; 1 average, 7 echoes). A dual spin-echo sequence was used to acquire proton density and T2-weighted images simultaneously (TR, 3500 milliseconds; TE1, 17 milliseconds; TE2, 102 milliseconds; 1 average, 5 echoes). Diffusion-weighted imaging was not acquired on this protocol. Using an automated segmentation routine based on artificial neural networks, tissue maps were obtained for CSF, gray matter (GM), NAWM, and LE volumes.25 Variable inversion T1 (TR, 5000 milliseconds; TE, 600 milliseconds; 1 average; TI, 100/500/900/2324 milliseconds) and multiple echo T2 (TR, 2000 milliseconds; 1 average; 16 echoes sampled every 22.5 milliseconds) images were also acquired for 2 sections at the level of the basal ganglia and the centrum semiovale and fit with monoexponential models to produce quantitative relaxation maps. This is very limited coverage and was designed to minimize acquisition time and still sample the periventricular and centrum semiovale WM.

Datasets were read retrospectively by 2 neuroradiologists (F.H.L. and N.D.S.), each with over 20 years of experience. The neuroradiologists retrospectively assessed each patient's longitudinal dataset for the presence of LE. Leukoencephalopathy was diagnosed according to radiographic criteria of Common Terminology Criteria for Adverse Events Version 4.0. None of the patients exhibited any neurologic symptoms at the time of imaging. The areas of LE ranged from small focal or subtle diffuse T2/FLAIR hyperintensities involving primarily the periventricular WM (Fig. 2) to more extensive T2/FLAIR hyperintensities extending into the centrum semiovale and could occupy more than one third of the total WM (Fig. 3). Because the periventricular WM is frequently involved and given the young age of patients, care was taken to differentiate therapy-induced WM changes from normal terminal zones of myelination relying on the experience of the radiologists. Because of the large doses of corticosteroids used in therapy, increases in subarachnoid spaces or ventriculomegaly were not considered in the diagnosis of LE.

FIGURE 2
FIGURE 2:
T2-weighted images demonstrating appearance of more subtle leukoencephalopathy and its phenotypical expression over the course of therapy as either transient (top row) or chronic (lower row). The 2 images on the left are from early in therapy at the MR2 time point while the 2 images on the right are from the end of therapy at the MR4 time point.
FIGURE 3
FIGURE 3:
A transverse T2-weighted fluid-attenuated inversion recovery image demonstrating avid leukoencephalopathy within the WM of the centrum semiovale sparing the U-fibers.

Neurocognitive Assessments

Assessments were conducted at week 120 of continuation therapy (end-therapy time point) and 2 years later at follow-up using age-standardized measures with demonstrated reliability and validity. Analyses focused on performance at 2-year follow-up and change in performance from the end of therapy to follow-up. Therefore, the last patient was assessed in October 2012. Patients 6 years or older at the time of the evaluation completed the measures of estimated IQ, working memory, and processing speed,26,27 and a computerized sustained attention measure,28 which yields scores for omissions, reaction time, variability, vigilance, and risk taking (as measured by β). Caregivers of patients 3 years or older at the time of the evaluation completed standardized ratings of attention and behavior in daily life (conduct, learning, and impulsivity).29 Memory (as measured by the California Verbal Learning Test's list A total recall) and academic skills (reading, spelling, and mathematics) were assessed in patients 6 years or older at the time of the evaluation.30–32 Patients had to meet these age requirements to be evaluated with these measures. All assessments were administered by master's-level psychological examiners under the supervision of a licensed clinical psychologist. A previous report of these measures from this patient cohort identified that a significantly higher proportion of patients performed more than one standard deviation worse than normative expectations on omissions and risk-taking scores on a computerized measure of attention, total recall on the verbal list learning measure, processing speed on the intelligence measure, all 3 measures of academic performance, and all 3 indices on the caregiver report measure at the 2-year follow-up.23 Based on these findings, we restricted our analysis to these specific measures.

Statistical Analyses

Chi-squared (χ2) tests were used to test the association between risk group and sex. Wilcoxon rank sum tests were used to compare age at diagnosis between risk groups and sex. Two-sided P values were assessed. Generalized estimating equation (GEE) models with logit link were built to compare the prevalence of LE at different time points in all patients and in SHR and LR patients separately. χ2 tests were used to test whether the prevalence of LE between SHR and LR patients was different at each time point separately.

Neuroimaging measures of LE prevalence and the extent of WM affected were modeled and compared between consecutive MR time points using GEE. Age at diagnosis was treated as a categorical variable with a threshold of 5 years of age (median age at diagnosis, 5.35 years). This method fully utilized the longitudinal MR imaging data by considering the correlation between repeated measurements of individual patients. The individual GEE models for main effects of age at diagnosis, sex, and treatment risk arm were developed separately, and the interaction effects within the models were tested using χ2 tests based on the Wald statistics. A final comprehensive multivariable model was developed incorporating all main effects and their interactions. All models used backward model selection methods to eliminate insignificant factors (P > 0.10) by the Wald test for type 3 analysis. However, if an interaction term was marginally significant (P < 0.10), then the main effect variables were kept in the final model regardless of their individual significance.

Cerebrospinal fluid, GM, WM, and LE volumes were assessed longitudinally across the 4 time points by GEE modeling. Quantitative T1 and T2 relaxation rates of NAWM and LE from every MR imaging examination were plotted as a function of age at examination and assessed with a monoexponential fit. This assessment did not take into consideration any regional variations within the brain.

General linear models were used to investigate the association between neurocognitive functions at 2-year follow-up and neuroimaging metrics, including the prevalence of LE, the extent of LE at MR2, and the extent of LE at MR4. Similar models were developed to investigate how those neuroimaging metrics are associated with the change in neurocognitive performance from the end of therapy to the 2-year follow-up. Because of the exploratory nature of this study and given the very selective choosing of cognitive and imaging variables of interest a priori based on the existing literature (resulting in a significant data reduction related to available cognitive and imaging variables), there was no need to control for multiple comparisons in the presented analyses. All statistical analyses were performed using SAS (version 9.3; SAS Institute, Inc, Cary, NC).

RESULTS

Patient Characteristics

Because of clinical factors or MR image artifacts, we were unable to obtain all 4 time points from every patient. Table 1 details the number of examinations acquired at each time point and distribution by sex, age at diagnosis, and treatment risk arm. Because T-cell ALL, an SHR leukemia subtype, occurs more frequently in older male patients, and children 5 years or younger are more likely to have LR leukemia, there is a substantial correlation between age at diagnosis, sex, and treatment risk arm across the whole cohort of 377 children.2 Sex was associated with both risk arm and age at diagnosis. There was a significantly higher proportion of female patients in the LR group (59.2%) and male patients in the SHR group (57.7%, P = 0.001). The 208 male patients (mean, 7.31 ± 4.6 years) were significantly older than the 169 female patients (mean, 6.46 ± 4.7 years; P = 0.014) at diagnosis. Across both risk groups, the 189 total SHR patients (mean, 8.8 ± 5.0 years) were significantly older than the 188 LR patients (mean, 5.1 ± 3.2 years; P < 0.001) at diagnosis.

TABLE 1 - No. MR Examinations Included in Study at Each Time Point
Data MR1 MR2 MR3 MR4
 All patients 284 324 305 306
Age at diagnosis
 ≤5 y 135 151 148 151
 >5 y 149 173 157 155
 Ratio >5:≤5 y 1.1 1.1 1.1 1.0
Sex
 Female 127 150 132 140
 Male 157 174 173 166
 Male-to-female ratio 1.2 1.2 1.3 1.2
Risk arm
 LR 150 169 164 166
 SHR 134 155 141 140
 LR/SHR ratio 1.1 1.1 1.2 1.2
No. examinations are also categorized by age at diagnosis, sex, and risk arm on the protocol. Within each category, the ratio is also reported.
MR, magnetic resonance; LR, low risk; SHR, standard/high risk.

Prevalence of Leukoencephalopathy

The prevalence of LE was defined as the presence of LE at any of the 4 imaging time points. Similar to previous reports,7 the LE prevalence significantly increased to its maximum at MR2 (74/324, 22.84%; P < 0.001) compared with that at MR1 (22/284, 7.75%). Subsequently, the LE prevalence significantly decreased during continuation therapy at MR3 (49/305, 16.07%; P = 0.003) and MR4 (39/306, 12.7%; P < 0.001) compared with that at MR2, regardless of risk arm, age at diagnosis, or sex.

The prevalence of LE was higher in SHR patients relative to LR patients at MR2, MR3, and MR4 but was only significant at MR4 (8.4% vs 17.9%, P = 0.016) as demonstrated in Figure 4A. For both SHR and LR patients, the prevalence of LE increased at MR2 compared with that at MR1 (SHR, 6.7%–25.8%; LR, 8.7%–20.1%; both, P < 0.001). The prevalence then significantly decreased in the LR patients at MR3 (LR, 20.1%–13.4%; P < 0.001) and in both groups at MR4 (SHR, 25.8%–17.9%; P = 0.035 and LR, 20.1%–8.4%; P < 0.001) compared with that at MR2.

FIGURE 4
FIGURE 4:
The prevalence of leukoencephalopathy (LE) at each of the 4 imaging time points throughout therapy (MR1–MR4). The prevalence of LE shown as a function of (A) treatment risk arm (bars: black = standard/high risk [SHR] patients, hatched fill = low risk [LR] patients), (B) age at diagnosis (bars: black = >5 years old [older], hatched fill = ≤5 years old [younger]), and (C) sex (bars: black = male, hatched fill = female). P values represent significant differences in LE prevalence in groups between time points.

The prevalence of LE was substantially higher in younger patients (≤5 years old at diagnosis) relative to older patients (>5 years old at diagnosis) at MR2 but decreased at a faster rate such that, by MR4, younger patients had a lower prevalence of LE than older patients as demonstrated in Figure 4B. In both age groups, the LE prevalence significantly increased at MR2 compared with that at MR1 (older, 4.0%–20.2%; younger, 11.9%–25.8%; both, P < 0.001). The prevalence then significantly decreased in younger patients at both MR3 (25.8%–15.5%, P < 0.001) and MR4 (25.8%–9.9%, P < 0.001), compared with that at MR2. The prevalence in older patients did not significantly change from MR2 to the end of therapy.

Male patients had a substantially but not significantly higher prevalence of LE relative to female patients at MR2, MR3, and MR4, which resolved at a slower rate than in female patients as shown in Figure 4C. For both sexes, the LE prevalence signficantly increased at MR2 compared with that at MR1 (male, 5.1%–26.4%; P < 0.001 and female, 11.0%–18.7%; P = 0.013), then significantly decreased at MR3 (male, 26.4%–18.5%; P = 0.003 and female, 18.7%–12.9%; P = 0.032) and MR4 (male, 26.4%–15.1%; P < 0.001 and female, 18.7%–10.0%; P = 0.003), compared with that at MR2.

Furthermore, a final comprehensive multivariable model (see Table, Supplemental Digital Content 1, http://links.lww.com/RLI/A559) was developed including time point, risk, sex, age at diagnosis, and their interactions. The Wald statistics for type 3 analysis of this model showed that time point (P < 0.001), age at diagnosis (P = 0.021), treatment risk arm (P = 0.001), sex (P = 0.087), interaction of age and time point (P = 0.028), and interaction of treatment risk arm with both sex (P < 0.001) and age (P = 0.001) were influential factors associated with probability of developing LE.

Volumetry of Cortical Atrophy and Extent of White Matter Affected by Leukoencephalopathy

Cerebrospinal fluid, GM, WM, and LE volumes were assessed longitudinally across the 4 time points. Cerebral atrophy was evident in both the size of the ventricles and widening of the sulci early in therapy. When analyzed across all patients, quantitative CSF volumes relative to intracranial volume significantly decreased between each of the 4 MR time points in this study ([CSF/intracranial volume]; MR1, 10.9%; MR2, 8.7%; MR3, 7.0%; MR4, 5.9%; χ2, 884.39; P < 0.001). These changes indicate decreasing cortical atrophy, which is likely associated with tapering of corticosteroids throughout the course of therapy.

The extent of LE was quantified by the volume of LE relative to the total WM volume, including both NAWM and LE, and was evaluated at all 4 time points in therapy. The extent of LE significantly increased at MR2 (P < 0.001), compared with that at MR1, and subsequently, significantly decreased during continuation therapy at MR3 (P < 0.001) and MR4 (P < 0.001), compared with that at MR2.

Patients receiving more intensive therapy on the SHR risk arm demonstrated significantly more extensive LE at MR2 (P = 0.040), MR3 (P < 0.001), and MR4 (P = 0.006) as can be observed in Figure 5A. For both SHR and LR patients, the extent of LE significantly increased at MR2 compared with that at MR1 (SHR, 2.9%–9.5%; P < 0.001 and LR, 3.9%–6.5%; P = 0.004). Subsequently, the extent of LE significantly decreased at MR3 (SHR, 9.5%–7.6%; P = 0.021 and LR, 6.5%–3.6%; P < 0.001) and MR4 (SHR, 9.5%–6.5%; P = 0.008 and LR, 6.5%–3.6%; P < 0.001) relative to MR2.

FIGURE 5
FIGURE 5:
The extent of WM affected by leukoencephalopathy (LE) relative to total WM at each of the 4 imaging time points (MR1–MR4) throughout therapy. Average extent shown as a function of (A) treatment risk arm (bars: black = standard/high risk [SHR] patients, hatched fill = low risk [LR] patients), (B) age at diagnosis (bars: black = >5 years old [older], hatched fill = ≤5 years old [younger]), (C) sex (bars: black = male, hatched fill = female), and (D) LE phenotype (bars: black = chronic LE patients, hatched fill = transient LE patients). P values represent significant differences in the extent of LE in groups between time points.

Regardless of age, the extent of WM affected was similar at MR2. However, younger patients rapidly recovered over the next 2 years, whereas older patients maintained a more extensive involvement even at MR4 as demonstrated in Figure 5B. In both age groups, LE extent significantly increased at MR2 compared with that at MR1 (older, 2.3%–8.0%; younger, 3.8%–8.2%; both, P < 0.001). The extent then significantly decreased in younger patients at both MR3 (8.2%–3.5%, P < 0.001) and MR4 (8.2%–3.0%, P < 0.001), compared with that at MR2. The extent of WM affected in older patients did not significantly change from MR2 to the end of therapy.

Although the proportions of WM affected by LE in male and female patients were approximately equal at MR2, the extent of LE decreased more rapidly in female patients leaving male patients with more extensive changes at MR4 as seen in Figure 5C. In both groups, the extent of LE significantly increased at MR2 compared with that at MR1 (male, 4.1%–8.2%; P < 0.001 and female, 3.0%–7.9%; P = 0.001). Thereafter, both groups significantly decreased at MR3 (male, 8.2%–6.7%; P = 0.007 and female, 7.9%–4.0%; P < 0.001) and MR4 (male, 8.2%–6.1%; P = 0.002 and female, 7.9%–4.4%; P = 0.020), compared with that at MR2.

Furthermore, a final comprehensive multivariable model (see Table, Supplemental Digital Content 2, http://links.lww.com/RLI/A560) was developed including time point, risk, sex, age at diagnosis, and their interactions. The Wald statistics for type 3 analysis of this model showed that time point (P = 0.004), treatment risk arm (P = 0.014), sex (P = 0.084), and interaction of age and time point (P = 0.092) were influential factors associated with the proportion of WM affected by LE.

The phenotypical expression of LE is characterized by its temporal evolution during therapy; it is either transient, defined as the presence of LE at any point in therapy but no LE on MR4, or chronic, defined as the presence of LE at MR4 regardless of when it first appeared. Only those patients that developed LE earlier in therapy and had imaging at MR4 (73 patients; 39 chronic/34 transient) could be evaluated. The extent of WM affected differed between chronic and transient phenotypes with more extensive WM involvement being associated with the chronic phenotype as appreciated in Figure 5D. For patients with the chronic LE phenotype, the extent of LE significantly increased at MR2 (3.7%–9.7%, P < 0.001), compared with that at MR1, and subsequently, significantly decreased at MR3 (9.7%–6.1%, P < 0.001), compared with that at MR2. At the end of therapy, MR4, patients with chronic LE still had 5.5% of their WM affected.

Relaxometry of Leukoencephalopathy

Measurements of T1 and T2 relaxation rates from all patients across all 4 MR imaging time points included 1187 examinations, of which 175 had measurable regions of LE. Quantitative T1 and T2 rates in NAWM demonstrated significant reductions with age at examination (both P's < 0.001). Quantitative relaxation rates were significantly longer in LE compared with that in NAWM in the same patient on the same examination (T1, P < 0.001; T2, P < 0.001) but were more easily separated from NAWM values on T2 than on T1 maps (Fig. 6). T1 and T2 intensity (elevation in LE relaxation times relative to NAWM) was not significantly different by age at diagnosis, sex, risk group, or LE phenotype.

FIGURE 6
FIGURE 6:
Relaxation times of normal-appearing WM (green dots) and leukoencephalopathy (gold dots) in all MR imaging examinations as a function of age at examination. Quantitative (A) T1 and (B) T2 relaxation times are shown.

Relating Neuroimaging During Therapy to Later Neurocognitive Performance

To investigate the relationships between neuroimaging metrics during therapy and neurocognitive performance after completion of therapy, we focused on early prevalence of LE (at either MR1 or MR2), extent of LE at MR2, and extent of LE at MR4 and evaluated the association of each of these imaging metrics with neurocognitive performance at the 2-year follow-up. The selections of the early imaging metrics were driven by the desire to associate the prevalence and extent of LE early in therapy with much later neurocognitive performance in the hope of identifying patients that may benefit from target interventions. The inclusion of the MR4 time point was an alternative if the early imaging markers did not reach significance. Neither early prevalence nor extent of LE at MR2 was significantly associated with any of the performance or rater-based measures. Of the 174 patients that had imaging at MR4 and completed the academic performance measure at the 2-year follow-up, only 19 (10.9%) had LE. However, even with this reduced sample size, a greater extent of LE at MR4 was significantly associated with performance at the 2-year follow-up on measures of reading (P = 0.004), spelling (P = 0.003), and mathematics (P = 0.019) as demonstrated in Figure 7.

FIGURE 7
FIGURE 7:
The extent of WM affected by leukoencephalopathy (LE) at the completion of therapy (imaging time point MR4) and its association with academic performance measures of (A) reading (P = 0.004), (B) spelling (P = 0.003), and (C) mathematics (P = 0.019) on Wechsler Individual Achievement Test at the 2-year follow-up.

Because decreasing neurocognitive performance from the end of therapy to a 2-year follow-up in these patients has previously been demonstrated,23 we next assessed the association of early prevalence of LE defined as LE at either MR1 or MR2, extent of LE at MR2, and extent of LE at MR4 with these differences. Parent reports at the end of therapy and at 2-year follow-up were available for 150 patients (114 with NAWM and 36 with early prevalence of LE). Early prevalence of LE was found to be significantly associated with increasing parent reports of conduct problems (NAWM: mean change, −2.8; early prevalence of LE: mean change, 1.1; P = 0.039) and learning difficulties (NAWM: mean change, −1.3; early prevalence of LE: mean change, 4.9; P = 0.036). Of the 81 patients that had imaging at MR2 and completed the processing speed measure at the end of therapy and at 2-year follow-up, only 18 (22.2%) had LE. A larger extent of LE at MR2 was significantly associated with decreasing processing speed (P = 0.003) after therapy as shown in Figure 8A. Similarly, of the patients that had imaging at MR4 and completed the attention measures (n = 106) and memory measure (n = 107) at the end of therapy and at 2-year follow-up, only 14 (13.2%) had LE. A larger extent of LE at MR4 was significantly associated with increasing problems with attention (omissions, P = 0.045; β, P = 0.015) and memory (list A total recall, P = 0.010) as shown in Figures 8B to D.

FIGURE 8
FIGURE 8:
The extent of WM affected by leukoencephalopathy (LE) and its association with differences in neurocognitive performance from the end of therapy to the 2-year follow-up evaluation. A, A greater extent of LE early in therapy, after consolidation treatment (imaging time point MR2), was significantly associated with decreasing processing speed (P = 0.003) on Wechsler Individual Achievement Test/Wechsler Adult Intelligence Scale. A greater extent of LE at the completion of therapy (imaging time point MR4) was significantly associated with increasing attention problems given by (B) omissions (P = 0.045) and (C) β (P = 0.015) scores on Conners Continuous Performance Test, and poorer memory given by (D) list A total recall (P = 0.010) scores on California Verbal Learning Test.

DISCUSSION

This study systematically examined the prevalence, extent, and intensity of LE during therapy and the association of these early imaging markers with later neurocognitive performance in a large single-institution cohort of risk-adapted, uniformly treated children with ALL. Leukoencephalopathy was most prevalent (22.8%) at MR2, shortly after the completion of upfront intrathecal therapy for CNS prophylaxis and consolidation therapy with 4 courses of HDMTX. Leukoencephalopathy occurred more frequently and involved a larger extent of the WM in male than in female patients and among patients treated with the more intensive SHR than the LR regimen. These findings likely reflect the known higher prevalence of the more aggressive T-cell leukemia subtype in male patients and the more intensive CNS-directed therapy SHR patients receive.2

Patients treated on the less intensive LR arm were significantly younger than those on the more intensive SHR arm, in agreement with previously published literature.2 The prevalence of LE was substantially higher in younger patients relative to older patients at MR2 but decreased at a faster rate such that, by MR4, younger patients had a lower prevalence of LE than older patients. Likewise, the proportion of WM affected at MR2 was similar regardless of age but resolved in younger patients over the next 2 years, whereas older patients maintained a more extensive involvement even at MR4. Taken together, these results demonstrated that younger patients treated with less intense therapy were more susceptible to developing LE early in treatment, but most returned to having no LE by the end of therapy, which is consistent with a greater degree of brain plasticity that facilitates repair of less intense damage.33 Conversely, older patients treated on the more intense SHR arm presumably have more severe WM damage and less ability to repair it, resulting in a higher rate of chronic LE.

For the first time, the development and extent of LE during therapy were associated with clinical outcomes in the neurocognitive performance of survivors 2 years after the completion of therapy. A greater extent of LE at the end of therapy (MR4) was significantly associated with decreased performance on measures of reading, spelling, and mathematics 2 years later. Although decreasing neurocognitive performance from the end of therapy to a 2-year follow-up in these patients has previously been demonstrated,23 this study showed multiple novel associations between this decline and the presentation of LE at specific time points during therapy. Early prevalence of LE, LE at either MR1 or MR2, was significantly associated with increasing parent reports of conduct problems and learning difficulties. Further, a larger extent of LE during early therapy (at MR2) was significantly associated with decreasing processing speed, whereas a larger extent of LE at the end of therapy (at MR4) was significantly associated with increasing deficits in attention and memory. These novel associations between LE and cognition assist in increasing the understanding of the etiology of cognitive late effects and may promote refinement in treatment and/or development of interventions that could reduce risk. Although the spatial localization of the LE is predominantly in the centrum semiovale and periventricular WM,14 the diffuse nature of the lesions makes association of individual lesions with specific neurocognitive performance infeasible.

These associations between the prevalence and extent of LE during therapy with later neurocognitive performance are consistent with the localization of the LE within the deep WM of the frontal, corona radiata, and periventricular regions.14 In long-term survivors of childhood ALL, LE has been associated with damage to the WM microstructure and neurocognitive impairment.34,35 A subset of 46 patients that had LE during therapy on the current study were imaged again with an average of 5 years after therapy and found that 78% (36/46) continued to have LE.19 Damage in these specific regions is likely to have a significant impact on frontal mediated cognitive functions in attention and working memory and lead to slower processing speed.

T1 and T2 relaxation times in LE were longer than in NAWM. However, T2 relaxation rates better differentiated NAWM from LE. Longer T1 and T2 relaxation times may reflect biological underpinnings of LE related to MTX. In long-term survivors of childhood ALL, higher plasma MTX concentrations have been associated with damage to the WM microstructure in the frontostriatal tracts and with increased rates of below-normal neurocognitive performance.36 Methotrexate-based deficiencies in iron levels and the folate–vitamin B12–methylation pathway cause hyperhomocysteinemia and result in deficient myelin synthesis.3–6,37–39 Leukoencephalopathy is identified by regional hyperintensities on T2-weighted images, which had longer T2 relaxation times, possibly resulting from iron deficiencies caused by decreased levels of folate. Iron is essential to myelin synthesis.40–42 A decrease in iron and deficient metabolism involving the methyl transfer pathway are known to cause a decrease in the production of myelin proteins and lipids, resulting in hypomyelination and inadequate myelin compaction.43,44 Longer T1 relaxation times in LE may be linked to a decrease in magnetic interactions of hydrogen nuclei resulting from decreased hydrophobic lipids in LE. Increased interstitial water content, reflecting less structured WM due to decreased myelin compaction, may also be associated with longer T1 relaxation times in LE. Longer T1 and T2 relaxation times in LE may, therefore, reflect causation, at least in part, by MTX-induced reduction in iron levels and disruption of the folate–vitamin B12–methylation pathway. Future work should investigate these biological underpinnings using diffusion tensor imaging, which is more sensitive and specific to WM microstructure.

Diffusion-weighted imaging has been used to image ALL patients presenting with sudden onset of a central neurological syndrome within days of intrathecal MTX.45,46 This syndrome can include slurred speech, emotional lability, and hemiparesis and usually occurs within a few days of intrathecal MTX administration. Diffusion-weighted imaging in these cases displays a restricted diffusion pattern reflecting cytotoxic edema within cerebral WM. This finding is most consistent with a reversible metabolic derangement, rather than ischemia. In contrast, none of the patients in this study exhibited any neurologic symptoms at the time of imaging. Based on studies in ALL survivors, diffusion-weighted imaging in these patients would most likely exhibit elevated diffusion.34 However, this hypothesis would need to be tested prospectively.

There are limitations of this study. This study reports on patients treated from 2000 to 2010, and therapy has continued to evolve including the most recent changes, which now included risk stratification and treatment based on genetic phenotypes.47,48 However, high-dose intravenous MTX and triple intrathecal chemotherapy with MTX, cytarabine, and hydrocortisone remain the mainstay of CNS-directed prophylactic therapy in children treated for ALL. Time points in this study were chosen based on a previous study that included more frequent imaging and determined that the maximum prevalence occurred immediately after consolidation.7 Patients have a “break” of 6 weeks after consolidation to recover from this intense therapy before continuing to receive the first of 2 reinductions during continuation (see Fig. 1). There may exist a more optimal time to image but determining this timing must be made in consideration of the patients, their families, and the limited resources available. The present analysis did not control for multiple comparisons due to the exploratory nature of the study and given the very selective choosing of cognitive and imaging variables of interest a priori. It has been demonstrated previously that a strict adjustment across an entire body of work is less critical for exploratory studies as subsequent prospective studies with preplanned hypotheses may be conducted in the future to confirm the observed associations.49–51 Furthermore, bar charts between groups and time points were presented for each analysis so the reader can see how different the groups are and can use their own judgment about the relative weight of the conclusions rather than relying on an arbitrarily thresholded P value. The wide age range of patients limits the specificity of results for ALL patients of all ages; however, the enrollment of young patients was restricted to those who were 1 year of age and older to decrease the difficulty in distinguishing LE from neurodevelopmentally driven, partially myelinated or unmyelinated WM. The study's ability to investigate longitudinal changes across all 4 MR time points was affected by the missing data for some patients. Elongated T1 and T2 relaxation times in LE may reflect the effect of MTX, but the contribution of other agents used during the course of therapy should also be considered. Results related to LE phenotype outcomes were limited to 73 patients who developed LE during therapy and had an evaluable scan at the final imaging time point. Finally, the effect of LE on WM development is not fully understood. It is possible that remyelination of existing tracts or the development of aberrant tracts occurs as a compensatory mechanism in response to regional LE.

Given the results of the current study, a more contemporary imaging protocol can be designed to take advantage of more recent technological developments. Rather than collecting relatively lengthy independent sequences for T1, T2, PD, and FLAIR imaging, a QRAPMASTER (quantification of relaxation times and proton density by multiecho acquisition of a saturation recovery using turbo spin-echo readout) pulse sequence, which is a multislice, multiecho, and multisaturation delay acquisition sequence, could be acquired requiring only 5 minutes.52 Synthetic MR imaging can reconstruct the corresponding T1, T2, PD, and FLAIR imaging contrast as well as whole head T1 and T2 relaxation maps and myelin water maps all from this one acquisition. Unfortunately, there are some limitations such as inferior image quality (lower contrast-to-noise ratio) in the synthetic FLAIR images as well as fluid pulsation artifacts in synthetic T2-weighted images.53 These may necessitate the acquisition of a separate FLAIR-weighted image that could be acquired with a quite gradient acquisition requiring only 3 minutes and 38 seconds to be more compatible with imaging of young patients.54 Recent advances in imaging of other WM lesions have demonstrated the utility of double inversion recovery sequences, which attenuate the normal WM signal while maintaining the T2 weighting of the lesions. Use of compressed sensing means this sequence can be acquired in just over 3 minutes.55 Lastly, the segmentation of LE could be automated using a computational pipeline developed for the quantification of multiple sclerosis.56 Unfortunately, although this approach is highly correlated with manual segmentations, it still has some limitations such as a limited lesion detection rate of only 80% and a median false-positive rate of 33%.56

In summary, this study showed that LE was most prevalent at the completion of consolidation therapy. Patients that were older at diagnosis received more intense therapy, and male patients were at a greater risk for developing LE that was more likely to involve a larger extent of the WM. A more extensive WM involvement across all patients was associated with the chronic LE phenotype and, when observed at the end of therapy, was significantly associated with decreased academic performance years after treatment. Furthermore, decreasing neurocognitive performance from the end of therapy to a 2-year follow-up showed that a larger extent of LE early in therapy was significantly associated with decreasing processing speed, whereas a greater extent of LE at the end of therapy was significantly associated with increasing problems with attention and memory. T2 relaxation rates best differentiated NAWM from LE and were best appreciated using fluid-attenuated T2-wieghted sequences. Overall, asymptomatic LE during therapy can be seen in almost a quarter of patients during therapy, involve as much as 10% of WM volume, and is associated with decreasing neurocognitive performance in survivors.

ACKNOWLEDGMENTS

The authors wish to thank Rhonda Simmons and Chiquila Hull for their efforts in processing and analysis of the MR examinations. They would also like to thank Xingyu Li, assistant to Dr Li, and Dr Cheng Cheng, lead biostatistician for the clinical protocol, for their advice and assistance in statistical design and analysis.

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Keywords:

acute lymphoblastic leukemia; ALL; MRI; leukoencephalopathy; neurocognition; processing speed; attention; memory; conduct; learning

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