Long-Term Memory Deficits and Early Onset Dementia in Aging Adult Survivors of Childhood Acute Lymphoblastic Leukemia Treated with Cranial Irradiation

Survivors of childhood acute lymphoblastic leukemia (ALL) who received prophylactic cranial radiation therapy (CRT) are at increased risk for deficits in neurocognitive skills, including attention, working memory and processing speed. As survivors age, global brain injury from CRT may reduce cognitive reserve, placing them at risk for early onset dementia or long-term memory deficits. The prevalence of dementia and memory abnormalities in adult survivors of childhood ALL has not previously been established.

Analyses were conducted on 265 of 445 (60%) eligible survivors of childhood ALL (median age 36 years, range 26–54 years; 52% female) treated with 18Gy (n=127) or 24Gy (n=138) CRT with a median time from CRT of 30 years (range 15–46 years). Participants completed the Wechsler Memory Scale IV, including the Brief Cognitive Status Exam (BSCE), and the Wechsler Abbreviated Scale of Intelligence (WASI). Age-adjusted standard scores were calculated and the BSCE was also adjusted for education level. Prevalence of memory impairment (<1 SD below age-expected mean), stratified by RT dose exposure, is reported and logistic regression used to identify risk factors for impairment. A subset of survivors (n=85) completed brain magnetic resonance imaging (MRI), including assessment of hippocampal volume, cortical thickness, white matter volume, diffusion tensor imaging, and functional MRI (fMRI) during a cued-recall memory task.

Survivors who received 24 Gy CRT had increased impairment on global measures of both short-term recall (33%; p<0.001) and long-term recall (30%; p<0.001), while no increase in impairment was seen after 18Gy. Impaired short-term recall was associated with smaller right (p=0.02) and left (p<0.01) temporal lobe volumes, while impaired long-term recall was associated with thinner parietal and frontal cortices. On subtests evaluating narrative memory (i.e. story recall) and design memory, increasing RT dose (24 vs. 18Gy) was associated with an increased prevalence of long-term memory impairment (narrative: 28% vs. 12%, p=0.001; designs: 13% vs. 3.2%, p=0.003). However, no CRT dose response was identified for short term narrative and design memory. Survivors with impaired long-term memory for designs demonstrated a compensatory increase in left hippocampal fMRI activation (p=0.005), and the effect was greater in the higher dose group (p = 0.04). The mean score for long-term narrative memory among survivors who received 24Gy was equivalent to the mean score of a 70–74 year old adult population. Neither young age (0–4 years) at CRT (Odds Ratio [OR] 1.4, 95% confidence interval [CI] 0.8–2.7), time from CRT (OR 1.0, 95% CI 0.9–1.1) nor intrathecal methotrexate exposure (OR 3.9, 95% CI 0.4–36.1) were significantly associated with long-term memory deficits. Reduced cognitive status (by BSCE) was identified after 18Gy (9%, p=0.11) and 24Gy (18%, p<0.001), suggesting a CRT dose-response effect. On diffusion tensor imaging, increased radial diffusivity in the frontal, parietal and temporal regions, an inverse measure of white matter integrity, was associated with reduced BSCE. Current employment rates were equivalent (63%) in both CRT dose groups, suggesting no difference in functional status.

Aging adult survivors of ALL who received 24Gy CRT have reduced cognitive status and significant impairment in short-term and long-term memory. There appears to be a dose response effect selective for long-term narrative and design memory, but not for short term narrative and design memory. These patterns are consistent with early onset of age-related (long-term) memory loss, and early stage dementia, yet at a median age of only 36 years. After 24Gy, survivors have the narrative memory equivalent to a 70 year-old in the general population. Survivors with memory impairment demonstrated reduced integrity on structural and functional neuroimaging in anatomical regions established as essential for memory formation and long-term recall. However, these memory impairments do not seem to affect functional status (employment rates) suggesting that, rather than frank dementia, deficits in middle adulthood are consistent with mild cognitive impairment (MCI). Longitudinal evaluation of this population is needed as MCI often progresses into early onset dementia with age.

No relevant conflicts of interest to declare.