Purpose or Case Report: To measure and compare early (within 3 months) and late (beyond 3 months) changes in ADC of the clival marrow post-photon and proton therapy.
Methods & Materials: Photon therapy cohort
12 children with neurological tumours who had radiotherapy between 2006 and 2017 at our institution and with pre- and post-treatment DWI were identified.
Correlation with the DICOM-files of the radiotherapy plans was performed to check if the clivus was included in the field of treatment and to calculate the dose administered.

Proton therapy cohort
Between 2012 and 2017, 112 children with neurological tumours benefitted of the Proton Overseas Programme. By applying inclusion and exclusion criteria, ten eligible patients with pre- and post-treatment DWI were selected.
The DICOM-files with the pre-treatment plans were obtained from Jacksonville and Oklahoma City. The inclusion of the clivus in the field of treatment was confirmed and the dose which was administered was calculated.

Image acquisition and analysis
Children in both cohorts had been scanned at 1.5T including morphological sequences and DWI with b=0 and 1,000 s/mm².
A 4-6-mm circular region-of-interest was drawn in the clivus of the b=1000 s/mm² DWI using in-house software Adept®. Data from the entire volume of interest (2 or 3 contiguous slices) were obtained.
Aggregate values and descriptive statistics were obtained for each patient and for the entire cohort at baseline, early, and late time-points.
Results: The range of doses received with radiation therapy was much greater (39-54 Gy) with 7 patients receiving < 50 Gy. All patients in the proton beam therapy cohort received > 50 Gy, 8 of 10 received 54 Gy.

The median time interval between the end of treatment and the first post-treatment MR scan was < 2 months in both cohorts and equivalent between the baseline and the second post-treatment scan.

An early post-treatment rise in ADC occurred at 1-3 months in the photon therapy cohort and at 2 months after proton administration, which reduced by 3-6 and 4 months, respectively. Return to below baseline values was seen in 3 and 2 cases, respectively.
Conclusions: Longitudinal changes in ADC occur within normal skull marrow following radiotherapy and proton beam therapy to the brain as a result of acute injury to the tissue in which the energy is deposited. These changes are greater at an early time-point, but they gradually diminish. Slightly greater increases were seen early after proton therapy.