Metabolic and morphological assessment of carbon ion radiotherapy response in sacral chordoma

Sacral chordoma is a rare and locally aggressive malignant tumor. Carbon ion radiotherapy (CIRT) has emerged as a promising therapeutic modality, but tumor size may not decrease immediately after treatment, presenting challenges in evaluating response. We aimed to evaluate tumor response after CIRT in sacral chordoma patients using quantitative parameters obtained from magnetic resonance imaging (MRI) multi-planar measurements and fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT).

A retrospective analysis was conducted on 35 sacral chordoma patients who underwent CIRT. Baseline follow-up images for up to 60 months post-CIRT were assessed. Dynamic changes in tumor size, long-axis diameter (LAD), short-axis diameter (SAD), and cranio-caudal diameter (CCD) were measured. Additionally, FDG-PET/CT parameters consisting of maximum standardized uptake value (SUV_max), metabolic tumor volume (MTV), and total lesion glycolysis (TLG) were evaluated.

A minor reduction in mean tumor size was first observed at six months post-CIRT (LAD: 1.2%; SAD: 3.1%; CCD: 3.4%). From 12 to 60 months post-CIRT, inter-measurement differences between LAD, SAD, and CCD were significant (p < 0.05), with the largest reduction in SAD (32.8%) and the smallest in LAD (11.0%) at 60 months. In a subgroup of 10 patients, who had FDG-PET/CT, the average monthly reduction was greater for metabolic parameters than for morphologic measurements (SUV_max: 2.0% monthly; LAD: 0.6% monthly).

Sacral chordomas treated with CIRT exhibited an unequal shrinkage pattern. Among MRI-based plane-specific measurements, SAD demonstrated the greatest reduction. Furthermore, FDG-PET/CT may predict or confirm treatment response before morphological changes become apparent.