119P Insight into early response of sacral chordomas (SC) to high-dose proton therapy (HDPT): First results of CHIPT study

Currently used Response Evaluation Criteria in Solid Tumors (RECIST) based on lesion diameter is insufficient for proper assessment of SC response to HDPT as despite relatively small volume changes durable HDPT response is observed. Functional magnetic resonance imaging (MRI) parameters showed good potential in better portraying these changes. The aim of this prospective study is to evaluate early changes of functional MRI parameters in SC occurring during and following HDPT. Inclusion criteria: histological diagnosis of SC, lesion size ≥ 1cm, no MRI contraindication, WHO performance score < 3, and referral to HollandPTC (Delft, Netherlands) for definitive 74GyE PT or combined PT (50GyE+resection+24GyE). Inclusion period: August 2020 - October 2023. The study was approved by the medical ethical committee. All patients signed the consent form. In each patient 3.0T MRI was made at the inclusion, mid-treatment (week 4), end of the treatment (week 8), and 6 months and 1 year following the first fraction. Functional MRI parameters were measured within 4 circular regions - each at a different scan slice. Differences in parameter values at different timepoints were analyzed with Mann-Whitney-U (α=0.05). Ten conventional SC patients (5 M; median age 70, range 39-78) were included; 8 received definitive HDPT only and 2 the combined treatment. Noticeable changes of parameters occurred at week 8 where ADC mean increased by 15.0% and Ktrans, Ve, and Vp decreased by 59.4%, 90.0% and 68.2%, respectively, compared to the inclusion (Table) One year later all functional parameters except ADC mean decreased. Table: 119PMean values of MRI parameters at different timepointsInclusionweek 4week 86 months1 yearADCmean [10−3mm2/s]1.5231.5481.7521.6681.525Ktrans [10−3/min]60.6878.6124.6143.0949.86Ve [10−3]689.58212.0669.20199.610.00Vp [10−3]3.3362.3311.0601.4570.160 Open table in a new tab Functional MRI parameters give a good insight into the response of SC to HDPT - already at week 8 of HDPT. The observed decrease of Ktrans, Ve, and Vp values most likely corresponds to necrosis; yet for a better understanding of microscopic processes portrayed by these parameters, they should be correlated with histopathology.