Objective To preliminarily explore the correlation between the quantitative parameters of spectral CT before proton radiotherapy and the changes in apparent diffusion coefficient （ADC） of MRI before and after radiotherapy in chordoma patients. Methods A retrospective analysis was conducted on imaging data from 28 patients with chordoma who underwent proton radiotherapy at Shandong Cancer Hospital and Institute from August 2022 to December 2024. Spectral CT images obtained prior to treatment were used to extract four quantitative parameters of the lesion area： relative iodine concentration （RIC），electron density （ED），effective atomic number （Zeff），and the slope of spectral attenuation curve （λHU）. In parallel，MRI-DWI scans before and after treatment were collected to compute the ADC difference （ΔADC）. The correlation between spectral CT parameters and ΔADC was assessed using scatter plots，simple linear regression，LOWESS curve fitting，correlation matrix，and bootstrap resampling methods. Results The ADC of 28 patients was 1 137.05 （921.07，1 643.91） before treatment，and 1 197.10 （994.75，1 785.57） after treatment，and the ΔADC was 133.18 （-36.46，253.04）. The RIC was 0.45 （0.12，0.67），the ED was 38.01±12.72，the Zeff was 8.40±0.64，and the λHU was -2.20±1.05. The scatter plots results indicated a positive distribution trend between RIC and ΔADC，and a negative trend between λHu and ΔADC. Simple linear regression showed that RIC had the highest goodness of fit with ΔADC （R²=0.75） and the largest regression coefficient （β=518.34），followed by λHU （R²=0.64，β=-121.94）. The goodness of fit between Zeff and ΔADC was low （R²=0.25）. No correlation was found between ED and ΔADC. LOWESS curve fitting showed a consistent trend with simple linear regression results，without significant deviation. The correlation matrix indicated that RIC was positively correlated with ΔADC （r=0.88，P<0.001），Zeff was moderately positively correlated with ΔADC （r=0.51，P=0.006），and λHU was negatively correlated with ΔADC （r=-0.84，P<0.001）. Bootstrap resampling analysis showed that the r values of RIC，Zeff，λHU and ΔADC were 0.87，0.50，-0.80，respectively. Conclusions The spectral CT parameter RIC of chordoma patients before proton radiotherapy is positively correlated with ΔADC，while λHU exhibits a negative correlation with ΔADC.