  Purpose: Proton vertebral body sparing craniospinal irradiation (VBS CSI)
treats the thecal sac while avoiding the anterior vertebral bodies in effort to
reduce myelosuppression and growth inhibition. However, robust treatment
planning needs to compensate proton range uncertainty, contributing unwanted
doses within the vertebral bodies. This work aims to develop an early in vivo
radiation damage quantification method using longitudinal magnetic resonance
(MR) scans to quantify dose effect during fractionated CSI. Materials and
methods: Ten pediatric patients were enrolled in a prospective clinical trial
of proton VBS CSI receiving 23.4-36 Gy. Monte Carlo robust planning was used
with spinal clinical target volumes defined as the thecal sac and neural
foramina. T1/T2-weighted MR scans were acquired before, during, and after
treatments to detect transition from hematopoietic to less metabolically active
fatty marrow. MR signal intensity histograms at each time point were analyzed
and fitted by multi-Gaussian models to quantify radiation damages. Results:
Fatty marrow filtration was observed on MR images as early as the fifth
fraction of treatment. Maximum radiation-induced marrow damage occurred 40-50
days from the treatment start, followed by marrow regeneration. The mean damage
ratios were 0.23, 0.41, 0.59, and 0.54 corresponding to 10, 20, 40, and 60 days
from the treatment start. Conclusions: We demonstrated a non-invasive method to
identify early vertebral marrow damage based on radiation-induced fatty marrow
replacement. The proposed method can be potentially used to quantify the
quality of CSI vertebral sparing to preserve metabolically active hematopoietic
bone marrow.
