Participants will have a histiocytic disorder as determined by a corroborating constellation of histopathology, clinical, and/or radiologic findings.

Part A is a timed measure of visual scanning and graphomotor speed. Part B is a timed measure of cognitive flexibility.

Neurocognitive tests will yield raw scores that are converted to z-scores, normalized for participant's age, sex and educational level.The proportion of participants demonstrating cognitive impairment (as defined by 2 or more z-scores less than -1.5) will be summarized. Also, the proportion of participants with impairment (z < -1.5) for each cognitive test will be summarized. In this way the cognitive assessment yields a dichotomous outcome of impaired versus not impaired for each participant but also allow for more granular assessment of impairment in specific cognitive domains.

questionnaires will yield raw scores (total score for HADS and McGill QOL, as well as anxiety and depression subscores for the HADS). QOL scores will be analyzed as raw scores. Cognitive test scores will be transformed into z-sores (based on score distributions from established normative samples with a mean of 0 and standard deviation of 1) to define the presence/severity of cognitive dysfunction. Scores will be normalized for age, sex and education.

In a whole-brain analysis, first, cortical thickness will be compared between participants and controls and regions of statistically significant grey matter loss will be identified. This is done computionally with a whole-brain, vertex-by-vertex approach, as stated above. Also, as stated above (Section 7), statistical significance thresholding is at p<0.001, correcting for multiple comparisons using the False Discovery Rate (FDR) method and clustering thresholds, methods used in various high-throughput contexts, including MRI analysis.